Circuit Note

CN-0357

Circuits from the Lab® reference designs are engineered and

tested for quick and easysystemintegrationto helpsolve today’s

analog, mixed-signal, and RF design challenges. For more

informationand/orsupport,visit www.analog.com/CN0357.

Devices Connected/Referenced

ADA4528-2

5.0 V, Ultralow Noise, Zero Drift, RRIO,

Dual Op Amp

AD5270-20

1024-Position,1% Resistor Tolerance Error,

50-TP Memory Digital Rheostat

ADR3412

Micropower, 0.1% Accurate,1.2 Voltage

Reference

AD8500

Micropower, RRIO, Op Amp

AD7790

Low Power, 16-Bit Sigma-Delta, ADC

Low Noise, Single-Supply, Toxic Gas Detector, Using an Electrochemical Sensor with

Programmable Gain TIA for Rapid Prototyping

Rev. 0

CircuitsfromtheLab®referencedesignsfromAnalogDeviceshavebeendesignedandbuiltbyAnalog

Devices engineers. Standard engineering practices have been employed in the design and

constructionofeachcircuit,andtheirfunctionandperformancehavebeentestedandverifiedinalab

environment at room temperature. However, you are solely responsible for testing the circuit and

determiningitssuitabilityandapplicabilityforyouruseandapplication.Accordingly,innoeventshall

AnalogDevicesbeliablefordirect,indirect,special,incidental,consequentialorpunitivedamagesdue

toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)

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EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN-0357 Circuit Evaluation Board (EVAL-CN0357-PMDZ)

SDP to Pmod Interposer Board (PMD-SDP-IB1Z)

System Demonstration Platform (EVAL-SDP-CB1Z)

Design and Integration Files

Schematics, Layout Files, and Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 is a single-supply, low noise,

portable gas detector, using an electrochemical sensor. The

Alphasense CO-AX carbon monoxide sensor is used in this

example.

Electrochemical sensors offer several advantages for

instruments that detect or measure the concentration of many

toxic gases. Most sensors are gas specific and have usable

resolutions under one part per million (ppm) of gas

concentration.

The circuit shown in Figure 1 uses the ADA4528-2, dual auto

zero amplifier, which has a maximum offset voltage of 2.5 µV at

room temperature and an industry leading 5.6 µV/√Hz of

voltage noise density. In addition, the AD5270-20 programmable

rheostat is used rather than a fixed transimpedance resistor,

allowing for rapid prototyping of different gas sensor systems,

without changing the bill of materials.

The ADR3412 precision, low noise, micropower reference

establishes the 1.2 V common-mode, pseudo ground reference

voltage with 0.1% accuracy and 8 ppm/°C drift.

For applications where measuring fractions of ppm gas

concentration is important, using the ADA4528-2 and the

ADR3412 makes the circuit performance suitable for interfacing

with a 16-bit ADC, such as the AD7790.

3.3V

AIN1(+)

AIN1(–)

REF(+)

REF(–)

GND

DOUT/RDY

DIN

SCLK

CS

AD7790

TO

PROCESSOR

1.2V

C9

10µF

R8

100kΩ

R10

3.3kΩ

R9

3.3kΩ

U3-B

AD5270-20

R2

33Ω

3.3V

R1

1MΩ

R6

12.4kΩ

R5

12.4kΩ

R3

12.4kΩ

R4

12.4kΩ

G

D

Q1

MMBFJ270

S

C2

0.02µF

W

A

C4

0.02µF

C3

0.02µF

C8

0.1µF

C10

0.1µF

1.2V

GND

VREF

CE

RE

WE

U1

U8

CO-AX

AGND

U2-B

ADA4528-2

U5

AD8500

U2-A

ADA4528-2

U4

ADR3412

AVCC

C14

5.6nF

R12

150Ω

AGND

AVCC

Figure 1. Low Noise Gas Detector Circuit (Simplified Schematic: all Connections and Decoupling not Shown)