CN0357 Datasheet, PDF (2/5 Pages) Analog Devices – Devices Connected

CN-0357
CIRCUIT DESCRIPTION
Figure 2 shows a simplified schematic of an electrochemical
sensor measurement circuit. Electrochemical sensors work by
allowing gas to diffuse into the sensor through a membrane and
by interacting with the working electrode (WE). The sensor
reference electrode (RE) provides feedback to Amplifier U2-A,
which maintains a constant potential with the WE terminal by
varying the voltage at the counter electrode (CE). The direction
of the current at the WE terminal depends on whether the
reaction occurring within the sensor is oxidation or reduction.
In the case of a carbon monoxide sensor, oxidation takes place;
therefore, the current flows into the working electrode, which
requires the counter electrode to be at a negative voltage (typically
300 mV to 400 mV) with respect to the working electrode. The
op amp driving the CE terminal should have an output voltage
range of approximately ±1 V with respect to VREF to provide
sufficient headroom for operation with different types of sensors
(Alphasense Application Note AAN-105-03, Designing a
Potentiostatic Circuit, Alphasense, Ltd.).
I_WE
V_REF
RF
+ ^VREF
RE
– _{CE WE}
I_WE
I_WE
SENSOR
V_OUT
Figure 2. Simplified Electrochemical Sensor Circuit
The current into the WE terminal is less than 100 nA per ppm
of gas concentration; therefore, converting this current into an
output voltage requires a transimpedance amplifier with a very
low input bias current. The ADA4528-2 op amp has CMOS inputs
with a maximum input bias current of 220 pA at room
temperature, making it a very good fit for this application.
The ADR3412 establishes the pseudo ground reference for the
circuit, which allows for single-supply operation while consuming
very little quiescent current (100 µA maximum).
Amplifier U2-A sinks enough current from the CE terminal to
maintain a 0 V potential between the WE terminal and the
RE terminal on the sensor. The RE terminal is connected to the
inverting input of Amplifier U2-A; therefore, no current flows
in or out of it. This means that the current comes from the
WE terminal and it changes linearly with gas concentration.
Transimpedance Amplifier U2-B converts the sensor current into
a voltage proportional to the gas concentration.
The sensor selected for this circuit is an Alphasense CO-AX
carbon monoxide sensor. Table 1 shows the typical
specifications associated with carbon monoxide sensors of this
general type.
Warning: carbon monoxide is a toxic gas, and concentrations
higher than 250 ppm can be dangerous; therefore, take extreme
care when testing this circuit.
Circuit Note
Table 1. Typical Carbon Monoxide Sensor Specifications
Parameter
Value
Sensitivity
55 nA/ppm to
100 nA/ppm
(65 nA/ppm
typical)
Response Time (t90 from 0 ppm to 400 ppm CO) <30 seconds
Range (ppm) CO, Guaranteed Performance)
0 ppm to
2,000 ppm
Overrange Limit (Specifications Not
Guaranteed)
4,000 ppm
The output voltage of the transimpedance amplifier is
VO = 1.2 V + IWE × RF
(1)
where IWE is the current into the WE terminal, and RF is the
transimpedance feedback resistor (shown as the AD5270-20
U3-B rheostat in Figure 1).
The maximum response of the CO-AX sensor is 100 nA/ppm,
and its maximum input range is 2000 ppm of carbon monoxide.
These values result in a maximum output current of 200 μA and
a maximum output voltage determined by the transimpedance
resistor, as shown in Equation 2.
V_O
= 1.2
V + 2000
ppm ×100
nA
ppm
× R_F
VO = 1.2 V + 200 µA × RF
(2)
Applying 1.2 V to VREF of the AD7790 allows a usable range of
±1.2 V at the output of the transimpedance amplifier, U2-B.
Selecting a 6.0 kΩ resistor for the transimpedance feedback
resistor gives a maximum output voltage of 2.4 V.
Equation 3 shows the circuit output voltage as a function of
ppm of carbon monoxide, using the typical response of the
sensor of 65 nA/ppm.
V_O
= 1.2 V + 390
μV
ppm
(3)
The AD5270-20 has a nominal resistance value of 20 kΩ. There
are 1024 resistance positions, resulting in resistance step sizes of
19.5 Ω. The 5 ppm/°C resistance temperature coefficient of the
AD5270-20 is better than that of most discrete resistors, and its
1 µA of supply current is a very small contributor to the overall
power consumption of the system.
Resistor R4 keeps the noise gain at a reasonable level. Selecting
the value of this resistor is a compromise between the magnitude of
the noise gain and the sensor settling time errors, when exposed
to high concentrations of gas. For the example shown in
Equation 4, R4 = 33 Ω, which results in a noise gain of 183.
6.0 kΩ
NG = 1+
= 183
(4)
33 Ω
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