CN-0357

Circuit Note

Rev. 0 | Page 2 of 5

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

sufficient headroom for operation with different types of sensors

(Alphasense Application Note AAN-105-03, Designing a

Potentiostatic Circuit, Alphasense, Ltd.).

CE

–

+

SENSOR

RE

WE

RF

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.

Table 1. Typical Carbon Monoxide Sensor Specifications

Parameter

Value

Sensitivity

55 nA/ppm to

100 nA/ppm

(65 nA/ppm

typical)

<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

(1)

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.

F

O

R

V

×

×

+

=

ppm

nA

100

ppm

2000

V

2

.

1

(2)

±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.

ppm

μV

390

V

2

.

1

+

=

O

V

(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.

183

Ω

33

k

Ω

6.0

1

=

+

=

NG

(4)