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Class 3: The Fundamentals of Designing with Semiconductors

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Beyond the Op Amp

Beyond the Op Amp

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  • In the first seminar, we discussed the issues of capturing physical variables, which are not electronic, into electronic format through sensors. Sensors provide weak, difficult to handle signals. Amplifiers pick those signals off the sensors and amplify them to make it easier to use them in the whole system.
  • In this session and the next, we will cover amplifiers. Today we will talk about op amps, the fundamental building block for most of analog circuitry. In the next session, we will build up these op amps into more complex amplifiers and other devices for processing signals in the analog domain. These amplifiers will prepare the signals for digitizing in the ADC stage.
  • As you increase the gain, the noise gain line moves up and the usable bandwidth goes down.
  • A series of thermocouple amplifiers is available from Analog Devices so you can find the right accuracy and operating temperature range for your needs. These products use very little power, less than 1mW, and are in a small MSOP package. Two of the important things to look at when using a thermocouple amplifier are the initial accuracy and the temperature range where the amplifier is accurate. This is the temperature of the board that the amplifier is on, not the temperature of the thermocouple itself.
  • As you increase the gain, the noise gain line moves up and the usable bandwidth goes down.
  • I also want to remind you that every month Analog Devices presents a webcast on a current Hot Topic in designing with Semiconductors. Next month we’ll be presenting a webcast on Challenges in Embedded Design for Motor Control systems, in April we’ll look at MEMs solutions for Instrumentation applications, and in May we’ll tackle multi-parameter vital signs patient monitors. Registration will be available for each about a month before broadcast at www.analog.com slash webcast, where you can also access our library of archived webcasts that you can view anytime, on demand.
  • Transcript

    • 1. The World Leader in High Performance Signal Processing SolutionsTHE FUNDAMENTALS OF DESIGNING WITH SEMICONDUCTORS FOR SIGNAL PROCESSING APPLICATIONS Class 3 - BEYOND THE OP AMP Presented by David Kress
    • 2. Analog to Electronic signal processing Sensor Amp Converter Digital Processor (INPUT) Actuator Amp Converter (OUTPUT)
    • 3. Analog to Electronic signal processing Sensor Amp Converter Digital Processor (INPUT) Actuator Amp Converter (OUTPUT)
    • 4. Amplifiers and Operational Amplifiers Amplifiers  Make a low-level, high-source impedance signal into a high- level, low-source impedance signal  Op amps, power amps, RF amps, instrumentation amps, etc.  Most complex amplifiers built up from combinations of op amps Operational amplifiers  Three-terminal device (plus power supplies)  Amplify a small signal at the input terminals to a very, very large one at the output terminal
    • 5. Specialty Amplifiers Specialty Amplifiers  Designed for a specific signal type  Extract and amplify only the signal of interest  Pick off a small differential signal from a large common-mode voltage  Capture and demodulate a low-level AC signal  Compress a high-dynamic range signal  Provide automatic or controlled gain-changing  Send and receive precision signals  Provide high-speed low-impedance power output  Use the analog domain to its best advantage to prepare a clean signal for the data converter
    • 6. Specialty Amplifier Types General Inst. Amps. Differential Amps. Current-sense Amps. Programmable gain Demodulating amps (AD630 and AD698) Thermocouple amps Logarithmic amps with time-gain-control ADC drivers Clamp amps Funnel amplifier Line drivers/receivers Isolation amps
    • 7. Single-ended vs. Differential Signals Single-ended signals  Signalis measured referred to ground  When signals are bipolar (+ and-), negative supplies needed  AC signals are typically bipolar or need special ‘floating’, or capacitive coupling  Ground often carries high noise from other signals or power, compromising the signal Differential signals  Both sides of the signal float ‘off ground’  Signals are separated from ground and other signals  High frequency and accuracy usually need differential handling  Common mode (average) can be set for single supply  Specialized differential/difference amplifiers are needed
    • 8. Instrumentation, Difference andDifferential Amplifiers Instrumentation amplifiers  Amplify differential inputs to a single-ended output  Normally both amplifier inputs are high impedance  Provide high gain (up to 10,000) and low noise  Normally handle low-level signals from sensors Difference amplifiers  Amplify differential inputs from high common mode voltage levels  Often include input attenuator to allow operation outside supplies  High common model rejection even at high frequencies Differential amplifiers  High frequency amplifiers with differential input and output  Handle higher-level signals at lower gains  Typically used for line driving/receiving and ADC driving
    • 9. The Generic Instrumentation Amplifier(In Amp) RS/2 RS RG + COMMON MODE VOLTAGE ~ VSIG + _ 2 VCM IN-AMP GAIN = G + VSIG _ VOUT VREF ~ ~ _ 2 ~ RS/2 VCM COMMON MODE ERROR (RTI) = CMRR
    • 10. Op Amp Subtractor or Difference Amplifier R1 R2 V1 _ VOUT R2 1+ R1 CMR = 20 log10 + Kr R1 R2 REF V2 Where Kr = Total Fractional Mismatch of R1/ R2 TO R2 R1/R2 VOUT = (V2 – V1) R1 R2 R2 = CRITICAL FOR HIGH CMR R1 R1 EXTREMELY SENSITIVE TO SOURCE IMPEDANCE IMBALANCE 0.1% TOTAL MISMATCH YIELDS  66dB CMR FOR R1 = R2
    • 11. The Three Op Amp In Amp + + R2 R3 VSIG A1 ~ 2 _ _ _ VCM R1 VOUT RG A3 R1 ~ + + _ VSIG R2 R3 ~ VREF 2 A2 _ 2R1 + VOUT = VSIG • R3 1 + + VREF R2 RG GAIN × 100 2R1 CMR 20log IF R2 = R3, G = 1 + RG % MISMATCH
    • 12. GENERALIZED BRIDGE AMPLIFIER USINGAN IN-AMP VBR+R +VS R–R R VOUT = VB GAIN R - RG IN AMP REF VOUT +R–R R+R -VS
    • 13. AD620B Bridge Amplifier DC Error Budget +10V 499 MAXIMUM ERROR CONTRIBUTION, +25°C VCM = 5V FULLSCALE: VIN = 100mV, VOUT = 10V RG + VOS 55µV ÷ 100mV 550ppm AD620B IOS 350 × 0.5nA ÷ 100mV 1.8ppm Gain Error 0.15% 1500ppm – REF Gain 40ppm 40ppm G = 100 Nonlinearity 350100mV FS LOAD CELL CMR Error 120dB 1ppm × 5V ÷ 100mV 50ppm AD620B SPECS @ +25°C, ±15V 0.1Hz to 10Hz 280nV ÷ 100mV 2.8ppm VOSI + VOSO/G = 55µV max 1/f Noise IOS = 0.5nA max Total Gain Error = 0.15% Unadjusted  9 Bits Accurate 2145ppm Gain Nonlinearity = 40ppm Error 0.1Hz to 10Hz Noise = 280nVp-p Resolution  14 Bits Accurate 42.8ppm CMR = 120dB @ 60Hz Error
    • 14. SINGLE-SUPPLY DATA ACQUISITIONSYSTEM +2V  1V VCM = +2.5V G = 100 +2V
    • 15. High Common-Mode Current SensingUsing the AD629 Difference Amplifier VCM = 270V for VS = 15V
    • 16. AD8251/53 Digitally Programmable GainInstrumentation Amplifier (PGIA) AD8251 Fine Gain Setting of 1,2,4,8 AD8253 Coarse Gain Setting of 1,10,100,1000 Low noise and low offset with 10MHz bandwidth +VS DGND WR A1 A0 2 6 5 4 -IN LOGIC –IN 1 A1 - 7 OUT A1 Gain Logic A3 OUT A2 + +IN 10 A2 +IN AD8253 06983-001 8 3 9 -VS REF +VS –VS REF AD8251 AD8253
    • 17. Demodulating Amplifiers AC demodulation  Low-level low-frequency AC signal processing can be used for capturing low-level signals  A modulated signal bypasses issues of offset and noise in amplifiers  Useful for transformer-coupled position detectors  Lock-in amplifier can find narrow band signal 100db below the interfering noise
    • 18. IMPROVED LVDT OUTPUT SIGNAL PROCESSING ABSOLUTE + FILTER VALUE + VOUT AC ~ _SOURCE ABSOLUTE _ FILTER VALUE LVDT + VOUT _ POSITION + _
    • 19. Lock-in Amplifier AD630 demodulates 400Hz signal 100dB below noise CLIPPED C BAND-LIMITED WHITENOISE AD630 B 16 5k 100R 15 10k AD542 1 2.5k AD542 20 A 13 R 19 17 2.5k B 100dB 100R ATTENUATION 14 10k C OUTPUT A 10 LOW-PASS 0.1Hz 9 FILTER MODULATED 400Hz CARRIER CARRIER PHASE REFERENCE
    • 20. Thermocouple Amplifiers Cold junction compensation  Thermocouples use two different metals that develop a voltage varying with temperature  The temperature effect also occurs at the point where the thermocouple wires connect to the instrument  This ‘cold junction’ effect must be compensated for to get accurate measurements  Various techniques have been used including ice baths  Modern thermocouple amplifiers include accurate compensation circuitry
    • 21. Using a Temperature Sensor for Cold-Junction Compensations V(OUT) TEMPERATURE V(COMP) COMPENSATION CIRCUIT COPPER COPPER METAL A SAME METAL A TEMP TEMP SENSOR T1 V(T1) V(T2) T2 METAL B V(COMP) = f(T2) ISOTHERMAL BLOCK V(OUT) = V(T1) – V(T2) + V(COMP) IF V(COMP) = V(T2) – V(0°C), THEN V(OUT) = V(T1) – V(0°C)
    • 22. AD594/AD595 Monolithic ThermocoupleAmplifier with Cold-Junction Compensation +5V 0.1µF BROKEN 4.7k THERMOCOUPLE VOUT ALARM 10mV/°C OVERLOAD TYPE J: AD594 DETECT TYPE K: AD595 THERMOCOUPLE AD594/AD595 +A – – –TC ICE G + G POINT + + COMP +TC
    • 23. Log Amplifiers Signal compression  Many applications must capture signals over a very wide dynamic range  Radio antennas capturing broadcast signals  Photomultipliers and photodiodes capture light signals over a very wide range  To process and use these signals, they need to be compressed to a much smaller range Logarithmic amplifiers  Log amplifiers compress signals over ranges of as much as 120db – a million to one -- to a normal range of 1 to 10 volts  Accuracy is typically 0.1 to 0.5 dB -- 1 to 5%
    • 24. Log Amp Transfer Function VYLOG (VIN/VX) IDEAL ACTUAL2VY SLOPE = VY VY VIN VOUT = VY log10 VX + 0 ACTUAL VIN=10VX VIN=100VX INPUT ON VIN=VX - IDEAL LOG SCALE
    • 25. Log Amplifier Accuracy AD8307 covers 80dB with 0.5dB accuracy 5 4 3 500MHz 2 ERROR (dB) 1 0 10MHz –1 100MHz –2 –3 –4 –5 –80 –70 –60 –50 –40 –30 –20 –10 0 10 20 INPUT LEVEL (dBm)
    • 26. AD8307 six-decade RF powermeasurement TO ANTENNA 100kΩ 0.1µF 1/2W VP 22Ω 51pF +5V NC 8 7 6 5 VR1 LEAD- INP VPS ENB INT THROUGH 2kΩ AD8307 CAPACITORS, INT ±3dB 1nF 50Ω INPUT INM COM OFS OUT FROM P.A. 604Ω 1 2 3 4 1µW TO 2kΩ 1kW NC VOUT 51pF 1nF OUTPUT NC = NO CONNECT
    • 27. Time-gain-control with AD8335 Ultrasound processor changes AD8335 gain to account for changes in signal strength with tissue depth TX HV AMPs TX BEAMFORMER BEAMFORMER CENTRAL CONTROL MULTICHANNEL TGC USES MANY VGAs HV AD8335 VGAs T/R Rx BEAMFORMER MUX/ SWITCHES LNAs DEMUX (B AND F MODES) TRANSDUCER TGC ARRAY TIME GAIN COMPENSATION 128, 256 ETC. ELEMENTS CW (ANALOG) BIDIRECTIONAL BEAMFORMER SPECTRAL IMAGE AND COLOR CABLE DOPPLER MOTION DOPPLER (PW) PROCESSING PROCESSING PROCESSING MODE (B MODE) (F MODE) AUDIO DISPLAY OUTPUT
    • 28. ADC driver amplifiers High performance ADCs  Recent high performance ADCs have 16-bits and more at 200MSPS and higher  Such performance requires a differential input signal Differential amplifiers  Differential or single-ended input converted to differential output  Low impedance output stage rejects ADC switching spikes  Common mode level set and gain setting allow optimum match to ADC range
    • 29. ADC driver ADA4932 differential output drives differential input of 16-bit 10MSPS AD7626 ADC +5V 0.1µF 0.1µF +2.048V AD8031 +7.25V 0.1µF 5 6 7 8 +5V +2.5V +2.5V R6 499Ω 1 +VS 0.1µF 0.1µF 0.1µF FROM –FB 50Ω +4.096V SIGNAL TO 0V R3 R8 SOURCE 499Ω 33Ω VCM VDD1 VDD2 VIO 2.4MHz 2 +IN 11 BPF IN– R2 –OUT C5 53.6Ω 9 VOCM 56pF ADA4932-1 AD7626 0.1µF R5 499Ω 3 –IN R9 +OUT 10 33Ω IN+ C1 GND R1 2.2nF C6 R7 53.6Ω R4 499Ω 4 56pF +FB 0V TO 39Ω –VS +4.096V PAD 16 15 14 13 0.1µF –2.5V
    • 30. ADC Input Clamp Amplifiers Imaging systems  Ultrasound and imaging systems often exhibit high-level transients in practice  Input signals can easily exceed supply and ADC input range  Long recovery times can impair image stability Clamp amplifiers  Clamp amplifiers capture and suppress input transients  Amplifier output does not exceed ADC range  Transient recovery takes a few nanoseconds
    • 31. AD8036/AD8037 Clamp Amplifier EquivalentCircuit RF 14 0  -V IN + A1 A2 VOUT A - +1 +VIN +1 S1 VH B +1 VL C S1 A B C +1 V IN > V H 0 1 0 + CH V L  V IN  V H 1 0 0 - V IN < V L 0 0 1 + CL -
    • 32. Comparison of Input and Output Clamping
    • 33. AD8475:Differential Funnel Amp & ADC DriverKEY FEATURES BENEFITS Active precision attenuation  Connect industrial sensors to high  (0.4x or 0.8x) precision differential ADC’s Level-translating  Simplify design  VOCM pin sets output common mode  Enable quick development Single-ended to differential conversion  Reduce PCB size Differential rail-to-rail output  Reduce cost Input range beyond the rail APPLICATIONSKEY SPECIFICATIONS  Process control modules 150 MHz bandwidth  Data acquisition systems 10 nV/√Hz output noise  Medical monitoring devices 50 V/μS slew rate  ADC driver -112dB THD+N 1 ppm/°C max gain drift 500 μV max output offset 3 mA supply current Large Low Voltage Input ADC Inputs Signal
    • 34. AD8475 : Funnel Amplifier + ADC Driver +5V SNR=97dB 4V 2.5V THD=-113dB 0.5V – 4.5V +IN 0.4x VOUT(DIFF) ±4V ±10V 0V -IN 20Ω 270pF 1.35nF AD8475 AD7982 270pF 20Ω +IN VOCM 0.5V – 4.5V -IN 0.4x VOUT(DIFF) ±4V REF 4V 2.5V +5V ADR435 10kΩ 0.1µF 10kΩ Interface ±10V or ±5V signal on a single-supply amplifier Integrate 4 Steps in 1  Attenuate  Single-Ended-to-Differential Conversion  Level-Shift  Drive ADC Drive differential 18-bit SAR ADC up to 4MSPS with few external components
    • 35. Balanced Audio Transmission System
    • 36. Video Difference Amplifier with VariableCommon AD830 allows different input and output common mode voltage for matching ADC input range VP 0.1µF V1 1 AD830 8 INPUT GM SIGNAL VOUT V2 2 7 INPUT A=1 COMMON 3 6 GM C 0.1µF 4 5 VN V3 VOUT = V1 – V2 + V3 OUTPUT COMMON
    • 37. APPLICATIONS FOR ISOLATIONAMPLIFIERS Sensor is at a High Potential Relative to Other Circuitry (or may become so under Fault Conditions) Sensor May Not Carry Dangerous Voltages, Irrespective of Faults in Other Circuitry (e.g. Patient Monitoring and Intrinsically Safe Equipment for use with Explosive Gases) To Break Ground Loops
    • 38. AD210 3-PORT ISOLATION AMPLIFIER FB INPUT OUTPUT T1 –IN _ DEMOD _ MOD VO + FILTER + +INICOM OCOM T2 POWER T3+VISS INPUT OUTPUT +VOSS POWER POWER–VISS SUPPLY SUPPLY –VOSS POWER OSCILLATOR PWR PWR COM
    • 39. AD210 ISOLATION AMPLIFIER KEYFEATURES  Transformer Coupled  High Common Mode Voltage Isolation:  2500V RMS Continuous  ±3500V Peak Continuous  Wide Bandwidth: 20kHz (Full Power)  0.012% Maximum Linearity Error  Input Amplifier: Gain 1 to 100  Isolated Input and Output Power Supplies, ±15V, ±5mA
    • 40. MOTOR CONTROL CURRENT SENSING HIGH VOLAGE AC INPUT < 2500V RMS +15V FB INPUT OUTPUT T1 + –IN _ _ OUTPUT DEMOD MOD VO +IN + FILTER AD620 +0.01 RG OCOM REF ICOM _ –15V +VISS T2 POWER T3 INPUT OUTPUT +VOSS –VISS POWER POWER SUPPLY SUPPLY –VOSS RG = 499 POWER M FOR G = 100 AD210 OSCILLATOR PWR PWR COM +15V
    • 41. Fundamentals Webcasts 2011 January Introduction and Fundamentals of Sensors February The Op Amp March Beyond the Op Amp April Converters, Part 1, Understanding Sampled Data Systems May Converters, Part 2, Digital-to-Analog Converters June Converters, Part 3, Analog-to-Digital Converters July Powering your circuit August RF: Making your circuit mobile September Fundamentals of DSP/Embedded System design October Challenges in Industrial Design November Tips and Tricks for laying out your PC board December Final Exam, Ask Analog Devices www.analog.com/webcast
    • 42. Next webcasts Challenges in Embedded Design for Motor Control systems  April 13th at Noon (EDT) MEMs solutions for Instrumentation applications  May 18th at Noon (EDT) Multi-Parameter Vital Signs Patient Monitors  June 22nd at Noon (EDT) www.analog.com/webcast
    • 43. The World Leader in High Performance Signal Processing SolutionsThank You