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Circuit Note
CN-0357
Circuits from the Lab® reference designs are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, 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
Circuits from the Lab® reference designs from Analog Devices have been designed and built by Analog
Devices engineers. Standard engineering practices have been employed in the design and
construction of each circ
uit, and their function and performance have been tested and verified in a lab
environment at room temperature. However, you are solely responsible for testing the circuit and
determining its suitability and applicability for your use and application. Accordingly, in no event shall
Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due
to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©2014 Analog Devices, Inc. All rights reserved.
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(–)
V
IN
V
OUT
REF(+)
V
DD
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
12332-001
Figure 1. Low Noise Gas Detector Circuit (Simplified Schematic: all Connections and Decoupling not Shown)
Verzeichnis
- ・ Blockdiagramm on Seite 1 Seite 3
- ・ Technische Daten on Seite 2
