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Instrumentation: Liquid and Gas Sensing - VE2013


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Liquid Sensing: Visible light absorption spectroscopy and colorimetry are two fundamental tools used in chemical analysis. Most of these light-based systems use photodiodes as the light sensor, and require similar high input impedance signal chains. This session examines the different components of a photodiode amplifier signal chain, including a programmable gain transimpedance amplifier, a hardware lock-in amplifier, and a Σ-Δ ADC that can measure a sample and reference channel to greatly reduce any measurement error due to variations in intensity of the light source.

Gas Sensing: Many industrial processes involve toxic compounds, and it is important to know when dangerous concentrations exist. Electrochemical sensors offer several advantages for instruments that detect or measure the concentration of toxic gases. This session will describe a portable toxic gas detector using an electrochemical sensor. The system presented here includes a potentiostat circuit to drive the sensor, as well as a transimpedance amplifier to take the very small output current from the sensor and translate it to a voltage that can take advantage of the full-scale input of an ADC.

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Instrumentation: Liquid and Gas Sensing - VE2013

  1. 1. Instrumentation: Liquid and Gas Sensing Reference Designs and System Applications Walt Kester, Applications Engineer, Greensboro, NC, US
  2. 2. Today's Agenda Understand challenges of precision high impedance sensing applications Electrochemical gas detection (CN0234) Spectroscopy application using transimpedance amplifiers for photodiode preamplifiers (CN0312)  Design problems  Low current measurement  Noise  Maintaining required bandwidth Applications selected to illustrate important design principles applicable to a variety of high impedance sensor conditioning circuits See tested and verified Circuits from the Lab® signal chain solutions chosen to illustrate design principles  Low cost evaluation hardware and software available  Complete documentation packages:  Schematics, BOM, layout, Gerber files, assemblies 3
  3. 3. Circuits from the Lab Circuits from the Lab® reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. 4  Complete Design Files on CD and Downloadable ■ Windows Evaluation Software ■ Schematic ■ Bill of Material ■ PADs Layout ■ Gerber Files ■ Assembly Drawing ■ Product Device Drivers Evaluation Board Hardware
  4. 4. System Demonstration Platform (SDP-B, SDP-S)  The SDP (System Demonstration Platform) boards provides intelligent USB communications between many Analog Devices Evaluation Boards and Circuits from the Lab boards and PCs running the evaluation software. 5 EVALUATION BOARD SDP-B USB POWER USB SDP-S EVALUATION BOARD POWER  SDP-S (USB to serial engine based)  One 120-pin small footprint connector.  Supported peripherals:  I2C  SPI  GPIO  SDP-B (ADSP-BF527 Blackfin® based)  Two 120-pin small footprint connectors  Supported peripherals:  I2C  SPI  SPORT  Asynchronous Parallel Port  PPI (Parallel Pixel Interface)  Timers
  5. 5. Gas Detectors Commonly used for industrial safety  Area monitors permanently mounted near potential gas sources  Portable detectors worn on worker’s clothing Capable of detecting sub-ppm levels of toxic gases Use infrared light, electrochemical sensors, heat, or a combination  Multiple-gas detectors will typically have one sensor per target gas 6
  6. 6. Gas Detection Using Electrochemical Sensors Typically used as toxic gas detectors  Carbon monoxide, chlorine, hydrogen sulfide and other nasty industrial chemicals  Can detect down to sub-ppm levels of gas concentration  Could have VERY long settling times (10s or minutes) A potentiostat circuit is used to keep the reference electrode and working electrode at the same voltage by controlling the voltage at the counter electrode A transimpedance amplifier converts the current in/out of the working electrode into a voltage 7 + − …To make the voltage between RE and WE 0V… ...and this current is proportional to gas concentration… 200µA FS typical Inject current here…
  7. 7. CN0234: Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor Circuit Features  Low power gas detection  110 µA total current  Buck-boost regulator for high efficiency Circuit Benefits  Detects dangerous levels of gas  Low power, battery operated 8 Target Applications Key Parts Used Interface/Connectivity Industrial Medical Consumer ADA4505-2 ADR291 ADP2503 AD7798 SPI (AD7798) SDP(EVAL-CN0234-SDPZ) USB (EVAL-SDP-CB1Z) EVAL-CN0234-SDPZ ADAPTER BOARD TO EVAL-SDP-CB1Z Industry-Standard Footprint
  8. 8. 5V, AVCC 3.3V AIN1(+) AIN1(−) AVDD VIN VOUT REFIN(+) DVDD REFIN(−)GND DOUT/RDY DIN SCLK CS AD7798 TO SDP 2.5V 4 6 5 C6 10µF C11 0.1µF C13 2.2µF C12 0.1µFC10 22µF R5 100kΩ R8 11.5kΩ R7 330kΩ R6 36.5kΩ R4 33Ω AVCC R3 1MΩ R2 11kΩ R1 11kΩ R6 1kΩ G D Q1 MMBFJ177 S G D Q2 NTR2101PT1GOSCT S C5 0.02µF C9 22µF 4 5 8 7 SW1 PVIN VIN EN AGND SYNC/ MODE SW2 VOUT FB PGND 2 1 10 3 6 9 C4 0.02µF C3 0.02µF C2 0.1µF C1 0.1µF 2.5V GND VREF L1 1.5µH ADP2503ACPZ 1 1 CE RE WE 2 3 U3 CO-AX 2 AGND U2-B ADA4505-2U2-A ADA4505-2 U1 ADR291GR AVCC 832 6 4 J2-1 J2-2 DGND 1 2 B2 1 2 B1 2 1 AVCC 2.5V TO 5.5V EXTERNAL INPUT L2 1k AT 100MHz + + 5V VCC 5V AVCC CN0234: Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor 9 Total current consumption is 110 μA for normal operation (not including ADC). P-Channel JFET keeps RE and WE shorted when circuit is powered off. ADP2503 buck-boost regulates battery input or external power to 5 V ADR291 generates 2.5 V to offset circuit for single supply operation Efficient reverse voltage protection ADA4505-2 has 2 pA max Input bias current and 10 μA quiescent current per amp AD7798 16-bit sigma-delta ADC provides differential input, and allows full evaluation of front end circuit. Can be in power down mode most of time @ 1 µA 0.16Hz BW
  9. 9. Gas Detection Using Electrochemical Sensors Most instruments are portable, battery powered. Low power consumption is absolute highest priority.  Impractical to power down analog circuitry due to long sensor settling times.  Bandwidth is less than 1 Hz, so micropower op amps are a good fit. Typical accuracy of 1% to 5% is required. 10
  10. 10. CN0234 Features and Hints Provides a convenient platform to experiment with electrochemical sensors Sensor can measure up to 2000 ppm of carbon monoxide  2000 ppm of carbon monoxide will kill you, so test with less than 100 ppm unless using a fume hood. Electrochemical sensors’ offset is very sensitive to temperature and humidity  Best practice is to calibrate with a known gas concentration periodically. On-board 16-bit ADC allows evaluation of entire sensor circuit  Using a 16-bit ADC results in high dynamic range without the need for programmable gains. 10-pin header allows easy access to ADC’s serial port  Easy to interface to your own microcontroller or Analog Devices' SDP board using adapter board. 11
  11. 11. CN0234 Circuit Evaluation Board EVAL-CN0234-SDPZ 12 SDP CONNECTOR 10-PIN FEMALE CONNECTOR 10-PIN MALE CONNECTOR ON BOTTOM OF PCBSOFTWARE DISPLAY Complete Design Files ■ Schematic ■ Bill of Material ■ PADs Layout ■ Gerber Files ■ Assembly Drawing EVAL-CN0234-SDPZ ADAPTER BOARD TO EVAL-SDP-CB1Z Industry-Standard Footprint
  12. 12. Spectroscopy and Colorimetry 13 Fundamentals of Spectroscopy Signal Conditioning Synchronous Detection Photodiode Fundamentals Photodiode Preamp Design Challenges and Solutions ■ Bias Current ■ Stability ■ Noise Programmable Gain Transimpedance Amplifiers (PGTIA) CN0312 Dual Channel Spectroscopy/Colorimetry Demo Board Illustrates a System Solution
  13. 13. Quick Intro to Spectroscopy Spectroscopy is the study of the interaction of matter and radiated energy.  Matter = liquids and gases  Radiated energy = light 14 We can use spectroscopy techniques to answer two questions about an unknown sample: ■ What is it? ■ How much is there? Light after passing through a prism
  14. 14. What Is It? (Absorption Spectra) All atoms and molecules have unique and well known spectra  By measuring a material’s spectra, we can determine the chemical composition, concentration, etc.  No need to look at the entire spectrum—measuring a subset of wavelengths may be sufficient Absorption spectrum  A sample absorbs light at specific wavelengths according to the compounds or molecules present in it  After obtaining the absorption spectrum of a sample, we can refer to libraries containing thousands of spectrums for known substances 15 Absorption Spectra for Hydrogen
  15. 15. How Much Is There? (Beer-Lambert Law) Measure the Concentration “ The [light] absorbed is directly proportional to the path length through the medium and the concentration of the absorbing species.”  This works for gases or liquids.  c = Concentration  l = Path length  ε = Molar absorptivity  (Known constant for a given compound) 16
  16. 16. Beer-Lambert Law in the Real World … In real life, whatever we are measuring needs to be in a container of some sort.  The container walls will cause reflections, extra absorption, and light scattering, making it impossible to apply the simple Beer-Lambert Equation. To compensate for the effects of the container, we can compare the absorption between two containers.  One container holds the sample, while the other container holds a known substance (such as water, air, or whatever solvent was used to prepare the sample) Instead of looking at the difference between transmitted and received light, we look at the ratio of light received through the sample cell, and light received through the reference cell. 17
  17. 17. So Where Is This Stuff Used Anyway? 18 Chromatography  Gas  Liquid Spectroscopy  Ultraviolet (UV)  Visible (VIS)  Near infrared (IR)  Fourier Transform IR (FT-IR)  Raman  Fluorescence  Atomic Absorption Particle Analysis Nondispersive Infrared (NDIR) Gas Detection Colorimetry Water Quality Flame Detection
  18. 18. UV-VIS Spectroscope Sensor Signal Chain 19 Programmable gain transimpedance amp AC coupling buffering Synchronous detector (full- wave rectifier) 24-bit sigma-delta ADC Signal bandwidths tend to be < 5 kHz, but front-end op amp may have very high gain. Liquid
  19. 19. Synchronous Detection in the Frequency Domain (Similar to RF Demodulation or Full- Wave Rectification) It is equivalent to having a band-pass filter around the modulation frequency  Unlike a discrete component band-pass filter, it can easily be made very narrow at the expense of response time. Using a square wave makes modulation very simple  Noise at harmonics of the fundamental does not get rejected, so select modulation frequency carefully! 20
  20. 20. Ultraviolet-Visible (UV-VIS) Sensor: “Large Area” Silicon Photodiode Modeled as a light-dependent current source Cj can be 50 pF to 5000pF depending on the size of the diode Rsh can be from 500 MΩ to 5 GΩ at 25°C for different diodes Rs is typically a few ohms and can be ignored for most calculations Dark current is the amount of current generated when no light hits the photodiode  Should ideally be zero, but increases with reverse bias voltage 21 CjRsh Id Rs
  21. 21. Photodiode Transfer Function Operating the photodiode with zero reverse bias results in the lowest dark current (photovoltaic mode)  Manufacturers typically spec dark current at Vr = 10 mV 22 (a) (b) PHOTODIODE CURRENT DARK CURRENT PHOTODIODE VOLTAGE SHORT CIRCUIT CURRENT SHORT CIRCUIT VOLTAGE LIGHT INTENSITY idark 10mV
  22. 22. Measuring Photodiode Output Photodiode voltage is very nonlinear with light input Photodiode current is linear with light input  Need to convert photodiode current to an output voltage Transimpedance amplifier  Current-to-voltage converter  Transimpedance "gain" = Rf  In dB: 20log(Rf/1Ω) 23
  23. 23. Transimpedance Amplifier Looks like a short to the photodiode Photodiode current flows through the feedback resistor and is converted to a voltage Ideally, ALL of the photodiode current goes through Rf  In reality, all op amps have input bias current that introduces error to the output Op amp offset voltage causes offset due to itself and to increased dark current Op amps with pA-class Ib and low input offset voltages are typically preferred (usually FET inputs)  AD8605 (1 pA Ib, 300 μV Vos), AD8615 (1 pA, 60 μV Vos), ADA4817 (20 pA Ib, 2 mV Vos)  AD549 (0.06 pA Ib, 500 μV Vos) 24
  24. 24. Transimpedance Amplifier Stability Example Photodiode:  Cs = 150 pF, Rsh = 600 MΩ Op Amp: AD8615  Ib = 1 pA max (200 fA typical!), Cin = 9.2 pF, 24 MHz unity gain frequency Assume Rf = 1 MΩ so 5 V out when Id = 5 μA Rf and Cin form a pole in the open-loop transfer function 25 Don’t forget op amp’s differential and common-mode input capacitance! Ci = CDIFF + CCM 1MΩ 150pF 9.2pF
  25. 25. Transimpedance Amplifier Stability The amplifier has no phase margin  It’s an oscillator, not an amplifier The phase must be ‘a healthy distance’ away from 180° when the unity gain crosses 0 dB To guarantee stability, design for 45° of phase margin  Unless you KNOW you need less phase margin, consider this a minimum  60° or more makes it easier to sleep at night. 26 120dB 80dB 60dB 40dB 20dB 0dB 100dB 100Hz 1kHz 10kHz 100kHz 0° 180° 90° p1f p2f cf
  26. 26. Transimpedance Amplifier Stability Adding a capacitor in parallel with Rf introduces a zero to the open- loop transfer function and stabilizes the amplifier  We want to guarantee at least 45° of phase margin  Using a larger Cf results in more phase margin  But also lowers the signal bandwidth.  For now, select Cf = 4.7 pF • Could go as low as 1 pF, but parasitic capacitances start to dominate 27
  27. 27. Compensated Open-Loop Gain 28 Phase Margin ≈ 85° Zero
  28. 28. Closed-Loop Bandwidth and Gain 29 f3db signal ≈ 34kHz
  29. 29. Transimpedance Amplifier Noise Sources Major Contributors:  Resistor Johnson Noise  Current Noise  Voltage Noise 30 1MΩ 4.7pF
  30. 30. Transimpedance Amplifier Resistor Noise Feedback Resistor Johnson Noise  Appears on the output unamplified 31 4.7pF 1MΩ
  31. 31. Transimpedance Amplifier Op Amp Current Noise Op Amp Current Noise  Appears on the output as a voltage  Multiplied by Rf 33 1MΩ 4.7pF AD8615 50fA/√Hz
  32. 32. Transimpedance Amplifier Voltage Noise-2 Op Amp Voltage Noise  Modeled as a voltage source on the + input  Vout = Input Noise × Noise Gain  In a ‘DC’ circuit, the noise gain is equal to the noninverting gain.  …actually, the noise gain is still simply the noninverting gain, it’s just that the noninverting gain is a function of frequency! 35 1MΩ 4.7pF AD8615
  33. 33. Noise Gain vs. Signal Gain Unlike other amplifier configurations, the noise gain is very different from the signal gain. The op amp’s noise appears at the output multiplied by this gain (~35× at the peak) 36 1MΩ 4.7pF 150pF+ 9.2pF AD8615 7nV/√Hz, 24MHz GBW 24MHz
  34. 34. Op Amp Output Noise To get the output noise in V rms, integrate the square of the noise density over frequency and take the square root. Or take a shortcut! Approximation: 254 µV rms Using Integration: 266 µV rms (I dare you to do it by hand!) 37 38MHz
  36. 36. TIA Output Noise The three main noise contributors are all Gaussian and independent of each other, so we can RSS them together This is just transimpedance amplifier noise  Johnson noise of photodiode shunt resistor, Rsh, is integrated over the signal noise bandwidth: 1.57 × (1/2πRfCf). Negligible if Rsh >> Rf  Shot noise of photodiode is negligible 39 Contributor Output Noise Feedback Resistor 30 µV rms Op amp Current Noise 12 µV rms Op amp Voltage Noise 254 µV rms
  37. 37. Add Filter after Amplifier to Reduce Noise Op Amp noise over large noise gain bandwidth dominates… But the signal bandwidth is much lower  Signal Bandwidth = 34 kHz What if we simply add an RC low pass filter after the amplifier?  Cut-off frequency similar to the signal bandwidth Reduce RMS noise from 256 µV rms to 49 µV rms with simple 34 kHz RC filter  For the cost of about US$0.03 (assuming you use expensive C0G caps!)  If the output is going to an ADC, you may also need to buffer it. 40 34kHz BW 1MΩ 4.7pF
  38. 38. The Need for Programmable Gain The same equipment may need to test samples with very different light absorption.  Almost-clear liquids like water or alcohol-based solutions  Very opaque liquids like petroleum- based compounds  Sometimes simultaneously  Concentration ratios Programmable gain amplifiers help increase the system’s dynamic range 41 VS.
  39. 39. System Output Noise A good PGA will contribute very little noise when G = 1 When G = 10, the TIA noise is also amplified 10× Limit the PGA bandwidth to reduce noise 42
  40. 40. Two Alternatives: TIA + PGA vs. PGTIA TIA + PGA  Traditional Photodiode Amplifier  Programmable Gain Amp  Possibly Followed by ADC Driver PGTIA  Programmable Gain Transimpedance Amplifier  Lower Noise 43
  41. 41. An Alternative Architecture: PGTIA For G = 1 MΩ and the same bandwidth, the noise remains the same For G = 10 MΩ and the same bandwidth, the noise goes up about 3× (not 10×)  Cf = 0.47 pF Further noise reduction by adding a low-pass filter at the output  Attenuate everything beyond the signal bandwidth Do not have to consider additional errors due to a second amplifier 44
  42. 42. So, How Do You Build a PGTIA? The basic idea: Gain and frequency response depends on switch on and off impedance  Changes with temperature, supply voltage, and signal voltage 45 C lp R lp Rf Cf Rf Cf − +
  43. 43. Improved PGTIA Kelvin switching  Twice as many switches, but switch resistance does not matter very much.  Looks like an op amp output with slightly higher output resistance 46 Rf2 Cf2 Rf1 Cf1 - + CpCp
  44. 44. PGTIA: Frequency Domain Effects-1 Cp is typically less than 1 pF  In our G = 10 MΩ example, Cf is only 0.47 pF  Even Cp = 0.5 pF can make a big difference! 47 Rf2 Cf2 Rf1 Cf1 - + CpCp
  45. 45. PGTIA: Frequency Domain Effects-2 Cp is typically less than 1 pF  In our G = 10 MΩ example, Cf is only 0.47 pF  Even Cp = 0.5 pF can make a big difference! 48 Rf2 Cf2 Rf1 Cf1 - + 2*Cp Total Feedback Capacitance 2*CpCf1 2*Cp+ Cf1 Cf2 +=
  46. 46. PGTIA: Adding More Switches-1 Adding a set of switches in series reduces Cp by half Better, but what if you need more? 49
  47. 47. CN-0312 PGTIA Switch Configuration 52 ADG633 Ron ~ 50Ω
  48. 48. CN0312: Dual-Channel Colorimeter with Programmable Gain Transimpedance Amplifiers and Synchronous Detectors Circuit Features  Three modulated LED drivers  Two photodiode receive channels  Programmable gain Circuit Benefits  Ease of use  Self contained solution  Dual channel, 16-bit ADC for data analysis 53 Target Applications Key Parts Used Interface/Connectivity Industrial Medical Consumer AD8615/AD8618 AD8271 ADG633, ADG733 ADR4525 AD7798 SPI (AD7798) SDP (EVAL-CN0312-SDPZ) USB (EVAL-SDP-CB1Z) EVAL-SDP-CB1Z EVAL-CN0312-SDPZ
  49. 49. CN0312 Dual Channel Spectroscopy/ Colorimetry Demo Board 54 AD8615 AD8615 AD8615 AD8615 ADG733 ADG733 AD8271 AD8271 AD7798 ADR4525
  50. 50. CN0312 Addresses Challenges of Precision Photometry Convenient platform for exploring programmable gain TIAs Features  Three square-wave modulated LEDs  Two photodiode channels with selectable gain  Hardware lock-in amplifiers  AD7798 16-bit sigma-delta ADCs 55 J2 - J2 + 1 2 0 P IN SD P LEDs Beam- splitter Reference Container Sample Container D2 D3 Photodiodes (Notice correct orientation of anode tab) External 6-12VDC 1 2 0 P IN SD P EVAL-SDP-CB1Z CON A OR CONB EVAL-CN0312-SDPZ USB PC USB EVAL-SDP-CB1Z EVAL-CN0312-SDPZ
  51. 51. Summary Many chemical analyzer applications are based on light and photodiodes. Designing with photodiodes presents unique challenges:  Photodiode’s large shunt capacitance makes the amplifier unstable, requiring compensation  Compensation reduces the signal bandwidth  Reduced signal bandwidth may not be so bad (if you don’t need it!), since it also implies lower noise gain  Signal bandwidth is dominated by Rf and Cf  Noise gain bandwidth can be much higher than the signal bandwidth, and its magnitude is mainly determined by the ratio of the diode’s shunt capacitance to Cf. ADI’s amplifier portfolio allows you to customize a solution for very low input bias currents, low noise, and/or low drift, depending on each specific application! 56
  52. 52. Tweet it out! @ADI_News #ADIDC13 What We Covered Gas Detection Using Electrochemical Sensors (CN0234)  Gas detection fundamentals  Electrical equivalent circuit  Conditioning circuits Spectroscopy and Colorimetry (CN0312)  Fundamentals of spectroscopy  Modulated laser light sources  Photodiode receivers  Synchronous demodulation  Transimpedance amplifiers  Gain  Stability  Noise  Programmable gain transimpedance amplifiers 57
  53. 53. Tweet it out! @ADI_News #ADIDC13 Visit the Single Supply, Micropower Gas Detector Demo in the Exhibition Room 58 SDP CONNECTOR 10-PIN FEMALE CONNECTOR 10-PIN MALE CONNECTOR ON BOTTOM OF PCBSOFTWARE DISPLAY Complete Design Files ■ Schematic ■ Bill of Material ■ PADs Layout ■ Gerber Files ■ Assembly Drawing EVAL-CN0234-SDPZ ADAPTER BOARD TO EVAL-SDP-CB1Z Industry-Standard Footprint This demo board is available for purchase:
  54. 54. Tweet it out! @ADI_News #ADIDC13 Visit the Dual Channel Spectroscopy/Colorimetry Demo Board in the Exhibition Room 59 Circuit Features  Three modulated LED drivers  Two photodiode receive channels  Programmable gain Circuit Benefits  Ease of use  Self contained solution  Dual channel 16-bit ADC for data analysis Complete Design Files ■ Schematic ■ Bill of Material ■ PADs Layout ■ Gerber Files ■ Assembly Drawing EVAL-SDP-CB1Z EVAL-CN0312-SDPZ This demo board is available for purchase: