Instrumentation: Liquid and Gas Sensing Reference Designs and System Applications

Walt Kester, Applications Engineer, Greensboro, NC, US

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

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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.

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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

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.

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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

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

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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

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+

−

…To make the voltage between RE and WE 0V…

...and this current is proportional to gas concentration… 200µA FS typical

Inject current here…

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

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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

5V, AVCC 3.3V

AIN1(+)

AIN1(−)

AVDD

VIN VOUT

REFIN(+) DVDD

REFIN(−)GND

DOUT/RDY

DIN

SCLK

CS

AD7798

TOSDP

2.5V

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5

C610µF

C110.1µF

C132.2µF

C120.1µFC10

22µF

R5100kΩ

R811.5kΩ

R7330kΩ

R636.5kΩ

R433Ω

AVCC

R31MΩ

R211kΩ

R111kΩ

R61kΩ

G

D

Q1MMBFJ177

S

G

D

Q2NTR2101PT1GOSCT

S

C50.02µF

C922µF

4587

SW1PVINVIN

EN AGND

SYNC/MODE

SW2VOUT

FBPGND

21103

6 9

C40.02µF

C30.02µF

C20.1µF

C10.1µF

2.5V

GND

VREF

L11.5µH

ADP2503ACPZ

1

1 CERE

WE 2

3

U3

CO-AX

2

AGND

U2-BADA4505-2U2-A

ADA4505-2

U1ADR291GR AVCC

832 6

4

J2-1J2-2

DGND 1

2B2

1

2B1

21

AVCC

2.5V TO 5.5VEXTERNAL

INPUT

L21k AT 100MHz

+

+

5VVCC

5VAVCC

CN0234: Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor

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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

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.

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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.

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CN0234 Circuit Evaluation Board EVAL-CN0234-SDPZ

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SDP CONNECTOR

10-PIN FEMALE CONNECTOR

10-PIN MALE CONNECTOR ON BOTTOM OF PCB SOFTWARE 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

Spectroscopy and Colorimetry

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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

Quick Intro to Spectroscopy

Spectroscopy is the study of the interaction of matter and radiated energy. Matter = liquids and gases Radiated energy = light

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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

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

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Absorption Spectra for Hydrogen

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)

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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.

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So Where Is This Stuff Used Anyway?

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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

UV-VIS Spectroscope Sensor Signal Chain

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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

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!

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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

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CjRshId

Rs

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

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( a ) ( b )

PHOTODIODE CURRENT

DARK CURRENT

PHOTODIODE VOLTAGE

SHORT CIRCUIT CURRENT

SHORT CIRCUIT

VOLTAGE

LIGHT INTENSITY

idark

10mV

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)

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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)

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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

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Don’t forget op amp’s differential and common-mode input capacitance!

Ci = CDIFF + CCM

1MΩ 150pF

9.2pF

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.

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120dB

80dB

60dB

40dB

20dB

0dB

100dB

100Hz 1kHz 10kHz 100kHz

0°

180°

90°

p1f p2fcf

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

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Compensated Open-Loop Gain

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Phase Margin ≈ 85° Zero

Closed-Loop Bandwidth and Gain

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f3db signal ≈ 34kHz

Transimpedance Amplifier Noise Sources

Major Contributors: Resistor Johnson Noise Current Noise Voltage Noise

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1MΩ

4.7pF

Transimpedance Amplifier Resistor Noise

Feedback Resistor Johnson Noise Appears on the output unamplified

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4.7pF

1MΩ

Transimpedance Amplifier Op Amp Current Noise Op Amp Current Noise Appears on the output as a voltage Multiplied by Rf

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1MΩ

4.7pF

AD8615

50fA/√Hz

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!

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1MΩ 4.7pF

AD8615

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)

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1MΩ

4.7pF

150pF+9.2pF

AD8615 7nV/√Hz, 24MHz GBW

24MHz

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!)

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38MHz

By the Way… Are FET Input Op Amps Always the Best Choice?

AD8615

FET

AD8671

Bipolar

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In=50fA/√Hz

In=300fA/√Hz

LESS DRIFT

LOWER 1/F NOISE LOWER VOLTAGE NOISE

HIGHER CURRENT NOISE

7nV/√Hz

2.5nV/√hz

INPUT VOLTAGE NOISE INPUT BIAS CURRENT

INPU

T B

IAS

CU

RR

ENT

(pA)

INPU

T B

IAS

CU

RR

ENT

(nA)

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

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Contributor Output Noise Feedback Resistor 30 µV rms Op amp Current Noise 12 µV rms Op amp Voltage Noise 254 µV rms

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.

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34kHz BW

1MΩ

4.7pF

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

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VS.

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

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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

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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

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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

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C lp

Rlp

Rf

Cf

Rf

Cf

− +

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

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Rf2

Cf2

Rf1

Cf1

-+

CpCp

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!

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Rf2

Cf2

Rf1

Cf1

-+

CpCp

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!

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Rf2

Cf2

Rf1

Cf1

-+

2*C p

Total Feedback Capacitance

2*C pCf12*C p+ Cf1

Cf2 +=

PGTIA: Adding More Switches-1

Adding a set of switches in series reduces Cp by half

Better, but what if you need more?

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CN-0312 PGTIA Switch Configuration

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ADG633 Ron ~ 50Ω

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

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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

CN0312 Dual Channel Spectroscopy/ Colorimetry Demo Board

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AD8615 AD8615

AD8615 AD8615

ADG733

ADG733

AD8271

AD8271

AD7798

ADR4525

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

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J2 -J2 +

120

PIN SDP

LEDs

Beam-splitter

Reference Container

SampleContainer

D2

D3Photodiodes

(Notice correct orientation of

anode tab)

External6-12VDC12

0

PIN SDP

EVAL-SDP-CB1Z

CON A OR

CONB

EVAL-CN0312-SDPZ

USB

PC

USB

EVAL-SDP-CB1Z

EVAL-CN0312-SDPZ

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!

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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

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Tweet it out! @ADI_News #ADIDC13

Visit the Single Supply, Micropower Gas Detector Demo in the Exhibition Room

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SDP CONNECTOR

10-PIN FEMALE CONNECTOR

10-PIN MALE CONNECTOR ON BOTTOM OF PCB SOFTWARE 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: www.analog.com/DC13-hardware

Tweet it out! @ADI_News #ADIDC13

Visit the Dual Channel Spectroscopy/Colorimetry Demo Board in the Exhibition Room

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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: www.analog.com/DC13-hardware