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  1. 1. Analog and Interface Product Solutions Signal Chain Design Guide Devices For Use With SensorsDesign ideas in this guide use the following devices. A complete device list and corresponding data sheets for theseproducts can be found at Comparators Analog-to-Digital Temperature Voltage Digital Digital-to-AnalogAmplifiers Converters Sensors References Potentiometers ConvertersMCP6XX MCP654X MCP3421 MCP9800 MCP1525 MCP40XX MCP4725MCP6XXX MCP656X MCP3422/3/4 MCP9804 MCP1541 MCP40D1X MCP4728MCP6V01/2/3 MCP355X MCP9700/A MCP41XX MCP482XMCP6V06/7/8 MCP3901 MCP9701/A MCP42XX MCP492X MCP43XX MCP45XX MCP46XX MCP41XXX MCP42XXX
  2. 2. Signal Chain OverviewTypical sensor applications involve the monitoring of sensor The MCU controls the actuators and maintains the operationparameters and controlling of actuators. The sensor signal of the sensor signal conditioning circuits based on thechain, as shown below, consists of analog and digital condition of the signal detection. For the digital to analogdomains. Typical sensors output very low amplitude analog feedback path, the digital-to-analog converter (DAC), digitalsignals. These weak analog signals are amplified and filtered, potentiometer and Pulse-Width-Modulator (PWM) devices areand converted to digital using op amps, an analog-to-digital most commonly used. The MOSFET driver is commonly usedor voltage-to-frequency converter, and is processed at the for the interface between the feedback circuit and actuatorsMCU. The analog sensor output typically needs proper signal such as motors and valves. Microchip offers a large portfolioconditioning before it gets converted to a digital signal. of devices for signal chain applications.Typical Sensor Signal Chain Control Loop Analog Domain Digital Domain Sensors Indicator (LCD, LED) Reference Voltage ADC/ Filter V-to-Freq MUX Op Amp PIC® MCU or dsPIC® DSC Digital Potentiometer Actuators Motors, Valves, Driver Relays, Switches, (MOSFET) DAC/PWM Speakers, Horns, Op Amp LEDs2 Signal Chain Design Guide
  3. 3. Sensor OverviewMany system applications require the measurement of a There are sensors that respond to these phenomena byphysical or electrical condition, or the presence or absence producing the following electrical properties:of a known physical, electrical or chemical quantity. Analog ■ Voltagesensors are typically used to indicate the magnitude or ■ Currentchange in the environmental condition, by reacting to the ■ Resistancecondition and generating a change in an electrical property ■ Capacitanceas a result. ■ ChargeTypical phenomena that are measured are: This electrical property is then conditioned by an analog■ Electrical Signal and Properties circuit before being converted to a digital circuit. In this way,■ Magnetic Signal and Properties the environmental condition can be “measured” and the■ Temperature system can make decisions based on the result.■ Humidity The table below provides an overview of typical phenomena,■ Force, Weight, Torque and Pressure the type of sensor commonly used to measure the■ Motion and Vibration phenomena and electrical output of the sensor.■ Flow■ For additional information, please refer to Application Note Fluid Level and Volume■ AN990. Light and Infrared■ Chemistry/GasSummary Of Common Physical Conditions and Related Sensor Types Phenomena Sensor Electrical Output Magnetic Hall Effect Voltage Magneto-Resistive Resistance Temperature Thermocouple Voltage RTD Resistance Thermistor Resistance IC Voltage Infrared Current Thermopile Voltage Humidity Capacitive Capacitance Infrared Current Force, Weight, Torque, Pressure Strain Gauge Resistance/Voltage Load Cell Resistance Piezo-electric Voltage or Charge Mechanical Transducer Resistance, Voltage, Capacitance Motion and Vibration LVDT AC Voltage Piezo-electric Voltage or Charge Microphone Voltage Ultrasonic Voltage, Resistive, Current Accelerometer Voltage Flow Magnetic Flowmeter Voltage Mass Flowmeter Resistance/Voltage Ultrasound/Doppler Frequency Hot-wire Anemometer Resistance Mechanical Transducer (turbine) Voltage Fluid Level and Volume Ultrasound Time Delay Mechanical Transducer Resistance, Voltage Capacitor Capacitance Switch On/Off Thermal Voltage Light Photodiode Current Chemical pH Electrode Voltage Solution Conductivity Resistance/Current CO Sensor Voltage or Charge Photodiode (turbidity, colorimeter) Current Signal Chain Design Guide 3
  4. 4. Product OverviewsOperational Amplifiers (Op Amps) Analog-to-Digital Converters (ADC)Microchip Technology offers a broad portfolio of op amp Microchip offers a broad portfolio of high-precisionfamilies built on advanced CMOS technology. These families Delta-Sigma, SAR and Dual Slope A/D Converters. Theare offered in single, dual and quad configurations, which MCP3550/1/3 Delta-Sigma ADCs offer up to 22-bitare available in space saving packages. resolution with only 120 μA typical current consumption inThese op amp families include devices with Quiescent a small 8-pin MSOP package. The MCP3421 is a singleCurrent (IQ) per amplifier between 0.6 μA and 6 mA, with channel Delta-Sigma ADC and is available in a small 6-pina Gain Bandwidth Product (GBWP) between 10 kHz and SOT-23 package. It includes a voltage reference and PGA.60 MHz, respectively. The op amp with lowest supply voltage The user can select the conversion resolution up to 18 bits.(VDD) operates between 1.4V and 6.0V, while the op amp The MCP3422/3 and the MCP3424 are two channel andwith highest VDD operates between 6.5V and 16.0V. four channel versions, respectively, of the MCP3421 device. The MCP300X (10-bit), MCP320X (12-bit) and MCP330XThese op amp families fall into the following categories: (13-bit) SAR ADCs combine high performance and lowGeneral Purpose, Low Offset, Auto-zeroed, High Speed, Low power consumption in a small package, making them idealNoise and mCal (self calibrating input offset voltage (VOS)). for embedded control applications. The TC5XX Dual SlopeComparators ADC devices offer another alternative with up to 17-bits of conversion resolution.The MCP6541 and MCP6561 family of comparatorsprovide ultra low power, 600 nA typical, and higher speed The “Analog-to-Digital Converter Design Guide” (Microchipwith 40 ns propagation delay, respectively. The MCP6541 Document No. 21841) shows various application examplesfamily low operating current is suitable for battery powered of the ADC devices.application and the output drive capability is ideal for alert Microchip also offers many high accuracy energy meteringbuzzer driver applications. The MCP6561 family with greater devices which are based on the Delta-Sigma ADC cores.than 4 MHz toggle frequency is ideal for higher speed The “Complete Utility Metering Solution Guide” (Microchipembedded system applications where sinusoidal output Document No: 24930) offers detailed solutions for meteringfrom sensors to square wave conversion is needed. The applications.47 ns typical propagation delay also makes this device idealfor microprocessor interface. Both families of comparators Voltage Referencesare available with single, dual and quad as well as with Microchip offers the MCP15XX family of low power and lowpush-pull and open-drain output options (MCP6546 and dropout precision Voltage References. The family includesMCP6566). the MCP1525 with an output voltage of 2.5V and the MCP1541 with an output voltage of 4.096V. Microchip’sProgrammable Gain Amplifier (PGA) voltage references are offered in SOT23-3 and TO-92The MCP6S21/2/6/8 and MCP6S91/2/3 PGA families packages.give the designer digital control over an amplifier usinga serial interface (SPI bus). An input analog multiplexerwith 1, 2, 6 or 8 inputs can be set to the desired inputsignal. The gain can be set to one of eight non-invertinggains: +1, 2, 4, 5, 8, 10, 16 and 32 V/V. In addition, asoftware shutdown mode offers significant power savingsfor portable embedded designs. This is all achieved in onesimple integrated part that allows for considerably greaterbandwidth, while maintaining a low supply current. Systemswith multiple sensors are significantly simplified.The MCP6G01 family are analog Selectable Gain Amplifiers(SGA). The Gain Select input pin(s) set a gain of +1 V/V,+10 V/V and +50 V/V. The Chip Select pin on theMCP6G03 puts it into shutdown to conserve power.4 Signal Chain Design Guide
  5. 5. Product OverviewsDigital Potentiometers Digital-to-Analog Converters (DAC)Microchip’s family of digital potentiometers offer a wide Microchip has a number of Digital-to-Analog Converters thatrange of options. These devices support the 6-bit through range from high performance 12-bit devices to cost effective8-bit applications. Offering both volatile and non-volatile 8-bit devices.options, with digital interfaces from the simple Up/ The MCP4725 is a single channel 12-bit DAC with non-Down interface to the standard SPI and I2C™ interfaces. volatile memory (EEPROM). The user can store DAC inputThese devices are offered in small packages such as code and configuration register bits into the EEPROM. This6-lead SC70 and 8-lead DFN for the single potentiometer non-volatile memory feature enables the device to holddevices, 14-lead TSSOP and 16-lead QFN packages for the the DAC input code during power-off time allowing the DACdual potentiometer devices, and 20-lead TSSOP and QFN output to be available immediately after power-up. Thispackages for the quad potentiometer devices. Non-volatile feature is very useful when the DAC is used as a supportingdevices offer a Wiperlock™ Technology feature, while volatile device for other device operations in systems. Thedevices will operate down to 1.8V. Resistances are offered MCP4725 is availabe in a tiny SOT23-6 package.from 2.1 kΩ to 100 kΩ. Over 50 device configurations arecurrently available. The MCP4728 is a 12-bit DAC with four analog outputs. This device also has non-volatile memory (EEPROM) for each DACThe “Digital Potentiometer Design Guide” (Microchip channel. The user can select internal reference or VDD asDocument No. 22017), shows various application examples reference individually for each channel. The MCP4725 andof the digital potentiometer devices. MCP4728 are available with I2C serial interface. The MCP4821/2 family of 12-bit DACs combines high performance with an internal reference voltage and SPI interface. The MCP4921/2 family is similar and allows for an external reference. These DAC devices provide high accuracy and low noise, and are ideal for industrial applications where calibration or compensation of signals (such as temperature, pressure and humidity) is required. The TC1320/1 family of DACs has 8- and 10-bit precision that uses the 2-wire SMBus/I2C serial interface protocol. Signal Chain Design Guide 5
  6. 6. Local SensorsLocal Sensing Sensors and ApplicationsLocal sensors are located relatively close to their signal Single Sensorsconditioning circuits, and the noise environment is not ■ Thermistors for battery chargers and power supplysevere; most of these sensors are single ended (not temperature protectiondifferential). Non-inverting amplifiers are a good choice for ■ Humidity Sensors for process controlamplifying most of these sensors’ output because they have ■ Pyroelectric infrared intrusion alarms, motion detectionhigh input impedance, and require a minimal amount of and garage door openersdiscrete components. ■ Smoke and fire sensors for home and officeKey Amplifier Features ■ Charge amplifier for Piezoelectric Transducer detection ■ Thermistor for battery chargers and home thermostats■Low Cost ■ LVDT position and rotation sensors for industrial control – General Purpose Op Amps ■ Hall effect sensors for engine speed sensing and door■High Precision openers – Low Offset Op Amps ■ Photoelectric infrared detector – Auto-zeroed Op Amps ■ Photoelectric motion detectors, flame detectors, intrusion – Low Noise Op Amps alarms■Rail-to-Rail Input/Output – Most Op Amp families Multiple Local Sensors■High Input Impedance ■Temperature measurement at multiple points on a Printed – Almost all Op Amp families Circuit Board (PCB)■Low Power and Portable Applications ■Sensors that require temperature correction – Low Power Op Amps ■Weather measurements (temperature, pressure, humidity,■High Bandwidth and Slew Rate light) – High Speed Op Amps■Load Drive Capacitive Humidity Sensor Circuit – High Output Drive Op Amps (PIC16F690DM-PCTLHS)Classic Gain Amplifier VDD_DIG C1 100 nF U1 RINT R6.65 MΩ PIC16F690 VINT P1 – IINT P4 CSEN + VSEN VOUT Timer1 P2 VCM + P3 CCG MCP6XX, VDD - MCP6XXX C2 PH Monitor SR Latch 100 nF – RCM1 20 kΩ Comparator U2 MCP6291 + VREF RCM2 20 kΩ CCM 100 nF6 Signal Chain Design Guide
  7. 7. Remote SensorsRemote Sensing ProductsAll sensors in a high noise environment should be ■High Precisionconsidered as remote sensors. Also, sensors not located – Low Offset Op Ampson the same PCB as the signal conditioning circuitry – Auto-zeroed Op Ampsare remote. Remote sensing applications typically use a – Low Noise Op Ampsdifferential amplifier or an instrumentation amplifier. Sensors and ApplicationsKey Amplifier Features ■High temperature sensors■Differential Input – Thermocouples for stoves, engines and process control■Large CMR – RTDs for ovens and process control■Small VOS ■Wheatstone Bridges – Pressure Sensors for automotive and industrial control – Strain gauges for engines ■Low side current monitors for motors and batteriesDifferential Amplifier VREF EMI – EMI + VOUT MCP6V02 MCP6V07 MCP617MCP6V01 Thermocouple Auto-zeroed Reference Design (MCP6V01RD-TCPL) PC (Thermal Management Software) Using USB PIC18F2550 (USB) Microcontroller I2C™ Port CVREF 10-bit ADC Module VOUT2 x1 2nd Order RC MCP1541 4.1V 3 Low-Pass Filter Voltage Reference VSHIFT VOUT1 SDA Difference VREF SCLK Amplifier ALERT Cold Junction Type K Thermocouple + Compensation Weld Bead MCP9800 Temp Sensor – Connector Signal Chain Design Guide 7
  8. 8. Oscillator Circuits For SensorsRC Operational Amplifier Oscillators The block diagram below shows a typical systemFor Sensor Applications level design, including the state-variable oscillator,Op Amp or state-variable oscillators can be used to PIC microcontroller and temperature sensor (used foraccurately measure resistive and capacitive sensors. temperature correction).Oscillators do not require an analog-to-digital converter and Sensors and Applicationsprovide a sensor measurement whose conversion to digital Resistive Sensorshas an accuracy limited only by the reference clock signal. ■RTDsState-variable oscillators are often used in sensor ■Humidityconditioning applications because they have a reliable ■Thermistorsstart-up and a low sensitivity to stray capacitance. Capacitive SensorsAbsolute quartz pressure sensors and humidity sensors ■Humidityare examples of capacitive sensors that can use the ■Pressurestate-variable oscillator. Also, this circuit can be used with ■ Level Oilresistive sensors, such as RTDs, to provide temperature-to-frequency conversion. Related Application Notes: AN895: Oscillator Circuits for RTD Temperature Sensors AN866: Designing Operational Amplifier Oscillator Circuits for Sensor Applications Available on the Microchip web site at: www.microchip.comOscillator Circuits for Sensors C1 C4 C2 R1 = RTDA R4 – R2 = RTDB R8 – R3 + – R7 VDD/2 + MCP6294 VDD/2 + + 1/4 MCP6234 MCP6294 VDD/2 VOUT MCP6004 2/4 MCP6234 VDD/2 – MCP6004 3/4 MCP6294 MCP6234 MCP6541 MCP6004 MCP6561 VDD Attributes: R5 VDD • Precision dual Element RTD Sensor Circuit + • Reliable Oscillation Startup VDD/2 R6 C5 – • Freq. ∝ (R1 x R2)1/2 4/4 MCP6294 MCP6234 MCP6004Oscillator Circuits for Sensors R1 = RTD VDD Attributes: C1 – • Low Cost Solution + A1 VOUT • Single Comparator Circuit MCP6541 • Square Wave Output R2 MCP6561 • Freq. = 1/ (1.386 x R1 x C1) VDD R3 R48 Signal Chain Design Guide
  9. 9. Wheatstone BridgeBridge Sensor Circuit steps (if any) should be used to increase the overall systemSensors for temperature, pressure, load or other phys- resolution when using the MCP355X. In many situations, theical excitation quantities are most often configured in a MCP355X devices can be used to directly digitize the sensorWheatstone bridge configuration. The bridge can have output, eliminating any need for external signal conditioninganywhere from one to all four elements reacting to the circuitry.physical excitation, and should be used in a ratiometeric Using the absolute pressure sensor as our Wheatstoneconfiguration when possible, with the system reference bridge example, the NPP-301 device has a typical fulldriving both the sensor and the ADC voltage reference. By scale output of 60 mV when excited with a 3V battery. Theusing the same reference for both the sensor excitation and pressure range for this device is 100 kPa. The MCP3551ADC, the variation in the reference can be cancelled out. has a output noise specification of 2.5 μVRMS.Furthermore, the output voltage from the bridge sensor is The following equation is a first order approximation of theproportional to the excitation voltage. Therefore, the ADC relationship between pressure in pascals (P) and altitude (h),that is using the external reference is more popular than in meters.the ADC that is using internal reference, for the ratiometric hconfiguration. log(P) ≈ 5 – 15500One example sensor from GE NovaSensor is an absolute Using 60 mV as the full scale range and 2.5 μV as thepressure sensor, shown below, a four element varying resolution, the resulting resolution from direct digitization inbridge. This example uses the MCP355X family of delta meters is 0.64 meters or approximately 2 feet.sigma ADCs. When designing with the MCP355X familyof 22-bit delta-sigma ADCs, the initial step should be to It should be noted that this is only used as an example forevaluate the sensor performance and then determine what discussion; temperature effects and the error from a first order approximation must be included in final system design.Example of Wheatstone Bridge Sensor Configuration with High Resolution Delta-Sigma ADC 0.1 μF 1.0 μF To VDD NPP-301 1 8 2 VDD VREF R2 VIN+ SCK SPI MCP3551 SDO MCU R1 R3 VIN- CS 5, 6, 7 3 VSS 4 R4 Altimeter Watch ∆V ~ [(∆R2+ ∆R4) - (∆R1+∆R3)]/4R * VDD With R1 = R2 = R3 = R4 = R Signal Chain Design Guide 9
  10. 10. Delta-Sigma ADCsVoltage and Current Measurment Using MCP3422 Analog-to-Digital Converter (ADC) Feature SummaryDelta-Sigma ADCs 18 bits (User can select 18-, 16-, Bit ResolutionThe MCP342X family devices are easy-to-use high precision 14- or 12-bit options)delta sigma ADC devices from Microchip. These devices have Number of Differential Input 2an internal reference (2.048V) with a user programmable ChannelsPGA (x1, x2, x4, x8). The ADC resolution is programmable as Internal Programmable Gain x1, x2, x4 or x8 (user option)12-bit, 14-bit, 16-bit or 18-bit by the user. AmplifierThis ADC family offers single, dual and four differential INL Error (Typical) 10 PPM of full scale rangeinput channels. Because of its simplicity and low price, this Offset Error (Typical) 15 μVdevice family can be used for various applications from Internal Reference Voltage 2.048Vsimple voltage and current measurement to high precisiontemperature measurement. Output Noise (Typical) 1.5 μVrms Gain Error (Typical) 0.05% of full scale range Interface I2C™Voltage Measurement Using MCP3421 Device R1 + + VBAT ADC ADC VBAT – – R2 MCP3421 MCP3421 MCU MCU (a) If VREF < VBAT (b) If VREF > VBAT ________ R2 VIN = ( R1 + R2 ) ● (VBAT) (R1 + R2) 1 VMeasured = ADC Output Codes ● LSB ● _________ ● _____ R2 PGA Reference Voltage LSB = ________________ 2 N –1 Reference Voltage 2.048V LSB of 18-bit ADC = ________________ = ______ = 15.625 μV 2 N –1 217Current Measurement Using MCP3421 Device Wireless Temperature Monitoring Solution Current Sensor To Load 2.4 GHz Charging Discharging Current Current + – Heat 18-bit ∆∑ ADC Battery ADC MCP3421 + MCP3421 MRF24J40 – (Thermocouple) MCP9804 MCU Temp Sensor PIC® MCU ±1°C Current = (Measured Voltage)/(Known Resistance Value of Current Sensor) Direction of current is determined by sign bit (MSB bit) of the ADC output code.10 Signal Chain Design Guide
  11. 11. Delta-Sigma ADCsTemperature Measurements Using 4 Channel ADC (MCP3424)See Thermocouple Reference Design (TMPSNSRD-TCPL1) Thermocouple Sensor Isothermal Block Isothermal Block (Cold Junction) (Cold Junction) MCP3424 MCP9804 Delta-Sigma ADC MCP9804 1 CH1+ CH4- 14 SCL 2 CH1- CH4+ 13 SDA SDA 3 CH2+ CH3- 12 SCL 4 CH2- CH3+ 11 5 VSS Adr1 10 VDD 0.1 μF 6 VDD Adr0 9 7 SDA SCL 8 MCP9804 MCP9804 10 μF Heat SDA SCL SCL SCL SDA To MCU SDA 5 kΩ 5 kΩ VDD Signal Chain Design Guide 11
  12. 12. Thermistor and RTD SolutionsThermistor Solutions RTD SolutionTypically, thermistors require an external resistor for biasing. Resistive Temperature Detectors (RTDs) are highly accurateIn addition, the inherent non-linearity of a thermistor is and repeatable temperature sensing elements. Whenimproved by biasing the thermistor in a resistive ladder using these sensors a robust instrumentation circuit iscircuit to linearize the temperature-to-voltage conversion. required and it is typically used in high performance thermalTypically, the thermistor voltage is directly connected to an management applications such as medical instrumentation.ADC to digitize the voltage measurement. The measured This solution uses a high performance Delta-Sigma Analog-voltage is converted to temperature using a lookup table. to-Digital converter, and two resistors to measure RTDHowever, at hot and cold temperature extremes the non- resistance ratiometrically. A ±0.1°C accuracy and ±0.01°Clinearity of this approach is much greater with reduced measurement resolution can be achieved across the RTDchange in voltage, which results in lower accuracy. This temperature range of -200°C to +800°C with a single pointrequires higher resolution and a more costly ADC. calibration.The solution is to use Microchip’s Linear Active Thermistors, This solution uses a common reference voltage to biasthe MCP9700 and MCP9701. These are low-cost voltage the RTD and the ADC which provides a ratio-metric relationoutput temperature sensors that replace almost any between the ADC resolution and the RTD temperatureThermistor application solutions. Unlike resistive type resolution. Only one biasing resistor, RA, is needed to set thesensors such as Thermistors, the signal conditioning measurement resolution ratio (shown in equation below).and noise immunity circuit development overhead can beavoided by using the low-cost Linear Active Thermistors. RTD ResistanceThese sensors output voltage is proportional to ambienttemperature with temperature coefficient of 10 mV/°C and19.5 mV/°C. Unlike thermistors, these devices do not require RRTD = RA (2 n – CodeCode) 1–additional computation for temperature measurement. Where:The factory set coefficients provide linear interface tomeasure ambient temperatures (refer to AN1001 for sensor Code = ADC output codeoptimization). RA = Biasing resistorMCP9700 and MCP9701 Key Features n = ADC number of bits■SC70, TO92 packages (22 bits with sign, MCP3551)■Operating temperature range: -40°C to +150°C■Temperature Coefficient: 10 mV/°C (MCP9700) For instance, a 2V ADC reference voltage (VREF) results in■Temperature Coefficient: 19.5 mV/°C (MCP9701) a 1 μV/LSb (Least Significant Bit) resolution. Setting RA =■Low power: 6 μA (typ.) RB = 6.8 kΩ provides 111.6 μV/°C temperature coefficientApplications (PT100 RTD with 0.385Ω/°C temperature coefficient). This■ Refrigeration equipment provides 0.008°C/LSb temperature measurement resolution■ Power supply over temperature protection for the entire range of 20Ω to 320Ω or -200°C to +800°C. A■ General purpose temperature monitoring single point calibration with a 0.1% 100Ω resistor provides ±0.1°C accuracy as shown in the figure below. This approach provides a plug-and-play solution with minimum adjustment. However, the system accuracy depends on several factors such as the RTD type, biasing circuit tolerance and stability, error due to power dissipation or self-heat, and RTD non-linear characteristics.RTD Instrumentation Circuit Block Diagram and Output Performance (see Application Note AN1154) VDD VLDO LDO 0.1 C* C* Measured Accuracy (°C) RB 5% 0.05 VREF 1 μF 0 RA 1% VREF -0.05 VDD + MCP3551 RTD PIC® 3 -0.1 MCU – SPI -200 0 200 400 600 800 Temperature (°C) *See LDO Data Sheet at: Signal Chain Design Guide
  13. 13. Programmable Gain Using Digital PotentiometersProgrammable Amplifier Gain Using a Equation 1-1 shows how to calcultate the gain for theDigital Potentiometer simplified circuit (Figure 1-1b). The gain is the ratio of theMany sensors require their signal to be amplified before digital potentiometers wiper position on the RAB resistorbeing converted to a digital representation. This signal gain ladder. As the wiper moves away from the midscale value,may be done with and operational amplifier. Since all sensors the gain will either become greater then one (as wiper moveswill have some variation in their operational characteristics, towards Terminal A), or less then one (as wiper movesit may be desireable to calibrate the gain of the operational towards Terminal B).amplifier to ensure an optimal output voltage range. The device’s wiper resistance (RW) is ignored for first orderThe figure below shows two inverting amplifier with calculations. This is due to it being in series with the op ampprogrammable gain circuits. The generic circuit (a) where R1, input resistance and the op amp input impedence is veryR2, and Pot1 can be used to tune the gain of the inverting large.amplifier, and the simplified circuit (b) which removesresistors R1 and R2 and just uses the digital potentiometers Circuit Gain EquationRAW and RBW ratio to control the gain.The simplified circuit reduces the cost and board area but VOUT = – RBW x VIN RAWthere are trade-offs (for the same resistance and resolution),Using the R1 and R2 resistors allows the range of the gain RABto be limited and therefore each digital potentiometer step RBW = x Wiper Code # of Resistorsis a fine adjust within that range. While in the simplifiedcircuit, the range is not limited and therefore each digitalpotentiometer step causes a larger variation in the gain. RAW = # of Resistors – Wiper Code x RABThe feedback capacitor (CF) is used for circuit stability. # of ResistorsInverting Amplifier with Programmable Gain Circuits Generic Circuit (a) Pot1 R1 R2 A B VIN W CF – Op Amp(1) + VOUT Simplified Circuit (b) Pot1 A B VIN W CF – Op Amp(1) + VOUTNote 1: A general purpose op amp, such as the MCP6001. Signal Chain Design Guide 13
  14. 14. Programmable Gain Using Digital PotentiometersThe table below shows a comparison of the gain between the For devices with an odd number of RS resistors have a mid-circuits (a and b) in Figure 1-1 when using the same Digital scale wiper value that is near unity gain. The MCP4261 is anPotentiometer (10 kΩ, 7-bit). What you also see is that example of a device that has an even number of RS resistorswhen R1 = R2 = 10 kΩ, the circuit’s gain range is between 1 in the RAB string, while the MCP4011 is an example of aand 3. While when the simplified circuit is used (effectively device that has an odd number of RS resistors in the RABhaving R1 = R2 = 0Ω) the circuit’s gain range is between ~0 string.and >127. Therefore the capability for finer calibration of For devices with an odd number of RS resistors in the RABthe circuit is capable with the generic circuit, albeit with a string to be able to have an exact unity gain, the devicenarrower range. would need to be used in the generic circuit configurationSome devices have an even number of step resistors (RS) (Figure 1-1), and the components would need to be selectedin the RAB string, while others have an odd number. In so R1 + RAW could equal R2 + RBW.the simplified circuit, devices with an even number of RSresistors have a mid-scale wiper value that is unity gain.Amplifier Gain vs. Wiper Code and RW # of # of Wiper 10 kΩ Gain Comment Taps Resistors Code Simplified Circuit(1) Generic Circuit(1, 2) 0 0.0 1.000000 Zero Scale 1 0.007874 1.007843 2 0.015873 1.015748 3 0.024000 1.023715 4 0.032258 1.031746 . . . . . . . . . . . . 62 0.939394 1.639175 63 0.969231 1.652850 129 128 64 1.000000 1.666667 Mid Scale 65 1.031746 1.680628 66 1.064516 1.694737 . . . . . . . . . . . . 124 31.000000 2.878788 125 41.666667 2.908397 126 63.000000 2.938462 127 127.000000 2.968992 128 Divide Error(3) 3.000000 Full ScaleNote 1: Gain = ( (RAB / # of Resistors) * Wiper Code ) /( ( (# of Resistors - Wiper Code) / # of Resistors) * RAB) 2: Uses R1 = R2 = 10 kΩ. 3: Theoretical calculations. At full scale in the simplified circuit a divide by 0 error results.14 Signal Chain Design Guide
  15. 15. Sensor Circuit Calibration using a DACSetting the DC Set Point for Sensor Circuit selected, the internal 2.048 VREF would produce 1 mV ofA common DAC application is digitally controlling the set resolution. If a smaller output step size is desired, the outputpoint and/or calibration of parameters in a signal chain. The range would need to be reduced. So, using gain of 1 is afigure below shows controlling the DC set point of a light better choice than using gain of 2 configuration option fordetector sensor using the MCP4728 12-bit quad DAC device. smaller step size, but its full-scale range is one half of thatThe DAC provides 4096 output steps. If G = 1 and internal of the gain of 2. Using a voltage divider at the DAC output isreference voltage options are selected, then the internal another method for obtaining a smaller step size.2.048 VREF would produce 500 μV of resolution. If G = 2 isSetting the DC Set Point Light VDD Comparator 1 RSENSE – R1 VTRIP1 + R2 MCP6544(1/4) 0.1 μF Light VDD VDD Comparator 2 RSENSE – 0.1 μF 10 μF R1 VTRIP2 + R3 MCP6544(2/4) R2 0.1 μF R4 VDD 1 10 VSS R5 SCL 2 9 VOUT D R6 SDA 3 MCP4728 8 VOUT C LDAC 4 7 VOUT B VDD Light RDY/BSY 5 6 VOUT A Analog Outputs Quad DAC Comparator 3 ⎩ RSENSE – ⎨ R1 VTRIP3 To MCU + ⎧ R2 MCP6544(3/4) 0.1 μF VOUT = VREF x Dn GX Light VDD 4096 R2 Comparator 4 VTRIP = VOUT x R1 + R2 RSENSE – Where Dn = Input Code (0 to 4095) R1 VTRIP4 + GX = Gain Selection (x1 or x2) MCP6544(4/4) R2 0.1 μF Signal Chain Design Guide 15
  16. 16. Mindi™ Amplifier Designer & SimulatorMindi™ Amplifier Designer & Simulator Mindi™ Active Filter Designer & Simulator The Mindi Active Filter Designer & Simulator is an Application Circuit within the Mindi Circuit Designer & Simulator that provides full schematic diagrams of the active filter circuit with recommended component values and displays the signal responses in frequency and time domains. The Mindi Active Filter Designer & Simulator allows the design of low-pass filters up to an 8th order filter with Chebychev, Bessel or Butterworth approximations from frequencies of 0.1 Hz to 1 MHz. It also can be used to design band-pass and high-pass filters with Chebychev and Butterworth approximations. The circuit topologies supported by the tool are the Sallen Key and Multiple Feedback (MFB). The low-pass filters can use either the Sallen Key or MFB, the band-pass filter is available with the MFB and the high-pass filter uses the Sallen Key.The Mindi Amplifier Designer & Simulator is an ApplicationCircuit that generates full schematic diagrams of an amplifiercircuit with recommended component values and displaysthe signal responses in frequency and time domains.This application circuit allows the following designs:■ Inverting Amplifier■ Non-inverting Amplifier■ Voltage Follower■ Difference Amplifier■ Inverting Summing Amplifier■ Inverting Comparator■ Inverting Differentiator■ Inverting IntegratorOnce the amplifier characteristics have been identified,the Mindi Amplifier Designer & Simulator can generate andsimulate the schematic of the amplifier circuit. For maximumdesign flexibility, changes in resistor and capacitor valuescan be implemented to fit the demands of the application.The tool also generates a Design Summary of the designed Users can select a flat pass-band or sharp transition fromamplifier, including Design Requirements, Application pass-band to stop-band. Other options, such as minimumSchematic, Result Plot and Bill of Materials (BOM). Users ripple factor, sharp transition and linear phase delays arecan directly download the schematic, BOM and Mindi offline available. Once the filter characteristics have been identified,version. the Mindi Active Filter Designer & Simulator can generate and simulate the schematic of filter circuit. For maximum design flexibility, changes in resistor and capacitor values can be implemented to fit the demands of the application. The tool will recalculate all values to meet the desired response, allowing real-world values to be substituted or changed as part of the design process. The tool also generates a Bill of Materials (BOM) of the designed filter. Both of these tools are available on the Microchip web site ( under “Design & Simulation Tools” or on the Mindi home page ( The op amps and evaluation boards can also be ordered from the Microchip web site.16 Signal Chain Design Guide
  17. 17. Development ToolsThese following development boards support the the DACsdevelopment of signal chain applications. These productfamilies may have other demonstration and evaluation MCP4725 PICtail™ Plus Daughter Boardboards that may also be useful. For more information visit (MCP4725DM-PTPLS) This daughter board demonstrates the MCP4725 (12 bit DAC with non-volatileADCs memory) features using the Explorer 16MCP3421 Battery Fuel Gauge Demo Development Board and the PICkit Serial(MCP3421DM-BFG) Analyzer. The MCP3421 Battery Fuel Gauge Demo Board demonstrates how to measure the MCP4725 SOT-23-6 Evaluation Board (MCP4725EV) battery voltage and discharging current The MCP4725 SOT-23-6 Evaluation Board using the MCP3421. The MCU algorithm is a quick and easy evaluation tool for calculates the battery fuel being used. the MCP4725 12-bit DAC device. It worksThis demo board is shipped with 1.5V AAA non-rechargeable with Microchip’s popular PICkit™ Serialbattery. The board can also charge a single-cell 4.2V Li-Ion Analyzer or independently with the customer’s applicationsbattery. board. The PICkit Serial Analyzer is sold separately.MCP3551 Tiny Application (Pressure) Sensor Demo(MCP355XDM-TAS) MCP4728 Evaluation Board (MCP4728EV) This 1˝ x 1˝ board is designed to The MCP4728 Evaluation Board is a tool for quick demonstrate the performance of the and easy evaluation of the MCP4728 4-channel MCP3550/1/3 devices in a simple low-cost 12-bit DAC device. It contains the MCP4728 application. The circuit uses a ratiometric device and connection pins for the Microchip’s sensor configuration and uses the system popular PICkit™ Serial Analyzer. The PICkit Serial power supply as the voltage reference. The Analyzer is sold separately.extreme common mode rejection capability of the MCP355Xdevices, along with their excellent normal mode powersupply rejection at 50 and 60 Hz, allows for excellent system Op Ampsperformance. MCP6031 Photodiode PICtail™ Plus Demo BoardMCP3551 Sensor Application Developer’s Board (MCP6031DM-PTPLS)(MCP355XDV-MS1) The MCP6031 Photodiode PICtail Plus The MCP355X Sensor Developer’s Board Demo Board demonstrates how to use a allows for easy system design of high transimpedance amplifier, which consists of resolution systems such as weigh scale, MCP6031 high precision op amp and external temperature sensing, or other small resistors, to convert photo-current to voltage. signal systems requiring precise signalconditioning circuits. The reference design includes LCD MCP651 Input Offset Evaluation Boarddisplay firmware that performs all the necessary functions (MCP651EV-VOS)including ADC sampling, USB communication for PC dataanalysis, LCD display output, zero cancellation, full scale The MCP651 Input Offset Evaluationcalibration, and units display in gram (g), kilogram (kg) or Board is intended to provide a simpleADC output units. means to measure the MCP651 Input Offset Evaluation Board op amp’s inputMCP3901 ADC Evaluation Board for 16-bit MCUs offset voltage under a variety of operating(MCP3901EV-MCU16) conditions. The measured input offset voltage (VOST) includes The MCP3901 ADC Evaluation Board for the input offset voltage specified in the data sheet (VOS) 16-bit MCUs system provides the ability to plus changes due to: power supply voltage (PSRR), common evaluate the performance of the MCP3901 mode voltage (CMRR), output voltage (AOL), input offset dual channel ADC. It also provides a voltage drift over temperature (ΔVOS/ΔTA) and 1/f noise. development platform for 16-bit PIC based applications, using existing 100-pin PIM systems. Signal Chain Design Guide 17
  18. 18. Development ToolsMCP6V01 Input Offset Demo Board Thermocouple Reference Design (TMPSNSRD-TCPL1)(MCP6V01DM-VOS) The Thermocouple Reference The MCP6V01 Input Offset Demo Board Design demonstrates how to is intended to provide a simple means to instrument a Thermocouple and measure the MCP6V01/2/3 op amps input accurately sense temperature over offset voltage (VOS) under a variety of bias the entire Thermocouple measurement range. This solution conditions. This VOS includes the specified uses the MCP3421 18-bit Analog-to-Digital Converter (ADC) input offset voltage value found in the data to measure voltage across the Thermocouple.sheet plus changes due to power supply voltage (PSRR),common mode voltage (CMRR), output voltage (AOL) and MCP6V01 Thermocouple Auto-Zero Reference Designtemperature (IVOS/ITA). (MCP6V01RD-TCPL) The MCP6V01 Thermocouple Auto-ZeroedMCP661 Line Driver Demo Board (MCP661DM-LD) Reference Design demonstrates how to use This demo board uses the MCP661 in a a difference amplifier system to measure very basic application for high speed op electromotive force (EMF) voltage at the amps; a 50Ω line (coax) driver. The board cold junction of thermocouple in order to accurately measure offers a 30 MHz solution, high speed PCB temperature at the hot junction. This can be done by usinglayout techniques and a means to test AC response, step the MCP6V01 auto-zeroed op amp because of its ultra lowresponse and distortion. Both the input and the output are offset voltage (VOS) and high common mode rejection ratioconnected to lab equipment with 50Ω BNC cables. There (CMRR).are 50Ω terminating resistors and transmission lines on theboard. The op amp is set to a gain of 2V/V to overcome the MCP6XXX Active Filter Demo (MCP6XXXDM-FLTR)loss at its output caused by the 50Ω resistor at that point. This kit supports Mindi™ Active FilterConnecting lab supplies to the board is simple; there are Designer & Simulator and active filtersthree surface mount test points provided for this purpose. designed by FilterLab® V2.0. These filters are all pole and are built by cascadingAmplifier Evaluation Board 1 first and second order sections.(MCP6XXXEV-AMP1) The MCP6XXX Amplifier Evaluation Board 1 is designed to support inverting/ Temp Sensors non-inverting amplifiers, voltage followers, inverting/non-inverting comparators, MCP9800 Temp Sensor Demo Board inverting/non-inverting differentiators. (MCP9800DM-TS1) The MCP9800 Temperature SensorAmplifier Evaluation Board 2 Demo Board demonstrates the sensor’s(MCP6XXXEV-AMP2) features. Users can connect the demo The MCP6XXX Amplifier Evaluation Board 2 board to a PC with USB interface and supports inverting summing amplifiers and evaluate the sensor performance. The non-inverting summing amplifiers. 7-Segment LED displays temperature in degrees Celsius or degrees Fahrenheit; the temperature alert feature can be set by the users using an on board potentiometer. An alert LED is used to indicate an over temperature condition. InAmplifier Evaluation Board 3 addition, temperature can be datalogged using the Microchip(MCP6XXXEV-AMP3) Thermal Management Software Graphical User Interface (GUI). The sensor registers can also be programmed using The MCP6XXX Amplifier Evaluation Board the GUI. 3 is designed to support the difference amplifier circuits which are generated by the MCP6S26 PT100 RTD Evaluation Board Mindi™ Amplifier Designer. (TMPSNS-RTD1) The PT100 RTD Evaluation BoardAmplifier Evaluation Board 4 demonstrates how to bias a Resistive(MCP6XXXEV-AMP4) Temperature Detector (RTD) and The MCP6XXX Amplifier Evaluation Board accurately measure temperature. Up 4 is designed to support the inverting to two RTDs can be connected. The integrator circuit. RTDs are biased using constant current source and the output voltage is scaled using a difference amplifier. In addition to the difference amplifier, a multiple input channel Programmable Gain Amplifier (PGA) MCP6S26 is used to digitally switch between RTDs and increase the scale up to 32 times.18 Signal Chain Design Guide
  19. 19. Development ToolsRTD Reference Design Board (TMPSNSRD-RTD2) Thermocouple Reference Design BoardThe RTD Reference Design demonstrates how to implement (TMPSNSRD-TCPL1)Resistive Temperature Detector (RTD) and accurately The Thermocouple Reference Design demonstrates howmeasure temperature. This solution uses the MCP3551 to implement a thermocouple and accurately sense22-bit Analog-to-Digital Converter (ADC) to measure voltage temperature over the entire thermocouple measurementacross the RTD. The ADC and the RTD are referenced using range. This solution uses the MCP3421 18-bit Analog-an onboard reference voltage and the ADC inputs are directly to-Digital Converter (ADC) to measure voltage across theconnected to the RTD terminals. This provides a ratio metric Thermocouple. The ADC has an internal 2.048V referencetemperature measurement. The solution uses a current voltage and a Programmable Gain Amplifier with 1, 2, 4,limiting resistor to bias the RTD. It provides a reliable and 8 V/V. At a Gain of 8 V/V the PGA effectively adds 3 LSbaccurate RTD instrumentation without the need for extensive to the ADC. This increases the ADC resolution to 21-bitcircuit compensation and calibration routines. or 2 μV/LSb. Therefore, the Thermocouple EMF voltage isIn addition, the this reference design includes a silicon measured with 2 μV resolution. For K-type thermocouple,temperature sensor, MCP9804. This sensor is used for measurement system provides a ±0.05°C resolution. Thecomparison only, it is not needed to instrument an RTD. The cold-junction compensation is done using a ±1°C accurateMCP3551 and MCP9804 outputs are read using a USB PIC 0.0625°C resolution silicon temperature sensor, themicrocontroller. This controller is also connected to a PC MCP9804. This solution provides a reliable and accurateusing USB interface. The thermal management software is Thermocouple instrumentation without the need forused plot the RTD temperature data in stripchart format. extensive circuit compensation and calibration routines. Signal Chain Design Guide 19
  20. 20. Related Support MaterialThe following literature is available on the Microchip web SAR ADCssite: There are additionalapplication notes that may be useful. AN246: Driving the Analog Inputs of a SAR A/D Converter This application note delves into the issues surrounding theSensor Conditioning Circuits Overview SAR converter’s input and conversion nuances to insure that the converter is handled properly from the beginning of theAN866: Designing Operational Amplifier Oscillator Circuits design phase. For Sensor ApplicationsOperational amplifier (op amp) oscillators can be used AN688: Layout Tips for 12-Bit A/D Converter Applicationto accurately measure resistive and capacitive sensors. This application note provides basic 12-bit layout guidelines,Oscillator design can be simplified by using the procedure ending with a review of issues to be aware of. Examples ofdiscussed in this application note. The derivation of the good layout and bad layout implementations are presenteddesign equations provides a method to select the passive throughout.components and determine the influence of each component AN693: Understanding A/D Converter Performanceon the frequency of oscillation. The procedure will be Specificationsdemonstrated by analyzing two state-variable RC op-amp This application note describes the specifications used tooscillator circuits. quantify the performance of A/D converters and give theAN895: Oscillator Circuits for RTD Temperature Sensors reader a better understanding of the significance of thoseThis application note shows how to design a temperature specifications in an application.sensor oscillator circuit using Microchip’s low-cost AN842: Differential ADC Biasing Techniques, Tips andMCP6001 operational amplifier (op amp) and the MCP6541 Trickscomparator. Oscillator circuits can be used to provide True differential converters can offer many advantages overan accurate temperature measurement with a Resistive single-ended input A/D Converters (ADC). In addition to theirTemperature Detector (RTD) sensor. Oscillators provide a common mode rejection ability, these converters can also befrequency output that is proportional to temperature and are used to overcome many DC biasing limitations of commoneasily integrated into a microcontroller system. signal conditioning circuits.AN990: Analog Sensor Conditioning Circuits – An Overview AN845: Communicating With The MCP3221 UsingAnalog sensors produce a change in an electrical property PIC Microcontrollersto indicate a change in its environment. this change in This application note will cover communications between theelectrical property needs to be conditioned by an analog MCP3221 12-bit A/D Converter and a PIC microcontroller.circuit before conversion to digital. Further processing occurs The code supplied with this application note is written asin the digital domain but is not addressed in this application relocatable assembly code.note.Delta-Sigma ADCs Passive Keyless Entry (PKE) TB090: MCP2030 Three-Channel Analog Front-End DeviceAN1007: Designing with the MCP3551 Delta-Sigma ADC OverviewThe MCP3551 delta-sigma ADC is a high-resolution This tech brief summarizes the technical features of theconverter. This application note discusses various design MCP2030 and describes how the three channel stand-alonetechniques to follow when using this device. Typical analog front-end device can be used for various bidirectionalapplication circuits are discussed first, followed by a section communication applications.on noise analysis. AN1024: PKE System Design Using the PIC16F639AN1030: Weigh Scale Applications for the MCP3551 This application note described how to make hands-freeThis application note focusses specifically on load cells, reliable passive keyless entry applications using thea type of strain gauge that is typically used for measuring PIC16F639 – a dual die solution device that includes bothweight. Even more specifically, the focus is on fully active, MCP2030 and PIC16F636.temperature compensated load cells whose change indifferential output voltage with a rated load is 2 mV to Utility Metering4 mV per volt of excitation (the excitation voltage being thedifference between the +Input and the –Input terminals of Refer to DS01008: Utility Metering Solutionsthe load cell).AN1156: Battery Fuel Measurement Using Delta-Sigma ADC DevicesThis application note reviews the battery fuel measurementusing the MCU and ADC devices. Developing battery fuelmeasurement in this manner provides flexible solutions andenables economic management.DS21841: Analog-to-Digital Converter Design Guide20 Signal Chain Design Guide
  21. 21. Related Support MaterialDigital Potentiometers AN866: Designing Operational Amplifier Oscillator Circuits For Sensor ApplicationsAN691: Optimizing the Digital Potentiometer in Precision Gives simple design procedures for op amp oscillators. Circuits These circuits are used to accurately measure resistive andIn this application note, circuit ideas are presented that capacitive sensors.use the necessary design techniques to mitigate errors,consequently optimizing the performance of the digital AN884: Driving Capacitive Loads With Op Ampspotentiometer. Explains why all op amps tend to have problems driving large capacitive loads. A simple, one resistor compensationAN692: Using a Digital Potentiometer to Optimize a scheme is given that gives much better performance. Precision Single Supply Photo DetectThis application note shows how the adjustability of the AN951: Amplifying High-Impedance Sensors –digital potentiometer can be used to an advantage in Photodiode Examplephotosensing circuits. Shows how to condition the current out of a high-impedance sensor. A photodiode detector illustrates the theory.AN1080: Understanding Digital Potentiometer Resistance AN990: Analog Sensor Conditioning Circuits – An Overview Variations Gives an overview of the many sensor types, applicationsThis application note discusses how process, voltage and and conditioning circuits.temperature effect the resistor network’s characteristics,specifications and techniques to improve system AN1014: Measuring Small Changes in Capacitive Sensorsperformance. This application note shows a switched capacitor circuit that uses a PIC microcontroller, and minimal external passiveOp Amps components, to measure small changes in capacitance. TheAN679: Temperature Sensing Technologies values are very repeatable under constant environmental conditions.Covers the most popular temperature sensor technologiesand helps determine the most appropriate sensor for an AN1016: Detecting Small Capacitive Sensors Using theapplication. MCP6291 and PIC16F690 Devices The circuit discussed here uses an op amp and aAN681: Reading and Using Fast Fourier Transformation microcontroller to implement a dual slope integrator and (FFT) timer. It gives accurate results, and is appropriate for smallDiscusses the use of frequency analysis (FFTs), time capacitive sensors, such as capacitive humidity sensors.analysis and DC analysis techniques. It emphasizes Analog-to-Digital converter applications. AN1177: Op Amp Precision Design: DC Errors This application note covers the essential backgroundAN684: Single Supply Temperature Sensing with information and design theory needed to design a precision Thermocouples DC circuit using op amps.Focuses on thermocouple circuit solutions. It builds fromsignal conditioning components to complete application AN1228: Op Amp Precision Design: Random Noisecircuits. This application note covers the essential background information and design theory needed to design low noise,AN695: Interfacing Pressure Sensors to Microchip’s precision op amp circuits. The focus is on simple, results Analog Peripherals oriented methods and approximations useful for circuits withShows how to condition a Wheatstone bridge sensor using a low-pass response.simple circuits. A piezoresistive pressure sensor applicationis used to illustrate the theory. AN1258: Op Amp Precision Design: PCB Layout Techniques This application note covers Printed Circuit Board (PCB)AN699: Anti-Aliasing, Analog Filters for Data Acquisition effects encountered in high (DC) precision op amp circuits. Systems It provides techniques for improving the performance,A tutorial on active analog filters and their most common giving more flexibility in solving a given design problem. Itapplications. demonstrates one important factor necessary to convert aAN722: Operational Amplifier Topologies and good schematic into a working precision design. DC Specifications AN1297: Microchip’s Op Amp SPICE Macro ModelsDefines op amp DC specifications found in a data sheet. It This application note covers the function and use ofshows where these specifications are critical in application Microchip’s op amp SPICE macro models. It does not explaincircuits. how to use the circuit simulator but will give the user aAN723: Operational Amplifier AC Specifications and better understanding how the model behaves and tips on Applications convergence issues.Defines op amp AC specifications found in a data sheet. Itshows where these specifications are critical in applicationcircuits. Signal Chain Design Guide 21