CN0357 Datasheet, PDF (1/5 Pages) Analog Devices – Devices Connected

Circuit Note
CN-0357
Devices Connected/Referenced
ADA4528-2
5.0 V, Ultralow Noise, Zero Drift, RRIO,
Dual Op Amp
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.
AD5270-20
ADR3412
1024-Position,1% Resistor Tolerance Error,
50-TP Memory Digital Rheostat
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
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.
VREF
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
AVCC
U4
ADR3412
1.2V
V_IN V_OUT
AVCC
R4
12.4kΩ
U2-A
ADA4528-2
U1
CO-AX
R3
12.4kΩ
C10
GND
C8
0.1µF
0.1µF
C3
CE WE
RE
C4
R2
33Ω
0.02µF
0.02µF
Q1
MMBFJ270
D
S
R5
12.4kΩ
R6
12.4kΩ
3.3V
G
R1
1MΩ
U2-B
ADA4528-2
R10
3.3kΩ
R8
100kΩ
C2
0.02µF
C9
10µF
W
A
U3-B
AD5270-20
AGND
U5
AD8500
REF(+)
V_DD
1.2V _AIN1(–)
DOUT/RDY
R9
3.3kΩ
R12
150Ω
C14
5.6nF
AIN1(+)
DIN
U8 SCLK
AD7790
CS
GND REF(–)
AGND
Figure 1. Low Noise Gas Detector Circuit (Simplified Schematic: all Connections and Decoupling not Shown)
TO
PROCESSOR
Rev. 0
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