CN0357

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 V_REF to provide sufficient headroom for operation with different types of sensors (Alphasense Application Note AAN-105-03, Designing a Potentiostatic Circuit, Alphasense, Ltd.). 

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.

The output voltage of the transimpedance amplifier is

where I_WE is the current into the WE terminal, and R_F 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.

Applying 1.2 V to V_REF 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.

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.

The input noise of the transimpedance amplifier appears at the output, amplified by the noise gain. For this circuit, only low frequency noise is of interest because the frequency of operation of the sensor is very low. The ADA4528-2 has a 0.1 Hz to 10 Hz input voltage noise of 97 nV p-p; therefore, the noise at the output is 18 μV p-p, as shown in Equation 5.

Because this is a very low frequency 1/f noise, the noise is very hard to filter. However, because the sensor response is also very slow, it is possible to use a very low frequency, low-pass filter (R5 and C6) with a cutoff frequency of 0.16 Hz. Even with such a low frequency filter, its effect on the sensor response time is negligible, when compared to the 30 second response time of the sensor.

The noise free code resolution of the system is determined from the peak-to-peak output noise. The maximum output of the ADA4528-2 is 2.4 V, so the noise free number of counts is

The noise free code resolution becomes

To take advantage of the entire ADC range available (±1.2 V), the AD8500 micropower, rail-to-rail input/output amplifier is chosen to drive the input of the AD7790. If the entire range is not necessary, the AD8500 can be removed and the internal buffer of the AD7790 can be used in its place.

One important characteristic of electrochemical sensors is their very long time constant. When first powered up, it can take several minutes for the output to settle to its final value. When exposed to a midscale step in concentration of the target gas, the time required for the sensor output to reach 90% of its final value can be in the order of 25 seconds to 40 seconds. If the voltage between the RE terminal and the WE terminal has a sudden change in magnitude, it can take several minutes for the output current of the sensor to settle. This long time constant also applies when cycling power to the sensor. To avoid very long start-up times, P-channel JFET Q1 shorts the RE terminal to the WE terminal when the supply voltage drops below the gate-to-source threshold voltage (~2.0 V) of the JFET.