AN4348

Maintaining the sensor biased

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Maintaining the sensor biased

If the sensor is left unbiased it becomes polarized. Due to its capacitive behavior, it then

requires a long time to stabilize again (from a few minutes up to a day). For sensors

sensor can be kept biased during power-off. Note that the power-off state can exist when

the batteries are being replaced. In portable applications, power-off may sometimes exist

in-between measurements (to minimize consumption). The solution to keep the sensor

biased is to use a P-JFET (such as J177) connected between the WE and the RE. In the

case of a two-electrode sensor the connection is between the WE and the CE as its gate is

connected to the power line via a resistor. When in power-off mode, the P-JFET shorts the

two electrodes to maintain the correct biasing of the sensor (see Figure 10).

Figure 10: Carbon monoxide sensing with TSU102

While the power is on, the P-JFET is in the pinch-off region (i.e. there is no current between

the drain and the source) and it has no impact on the application. In this case, the input

flowing through the transistor. This is because this current would be amplified by the trans-

impedance stage (like the current generated by the sensor) which would result in an offset

error. Particular attention should be paid to low power supply values. However, since the

user wishes to benefit from the widest ADC range possible, it is recommended to use the

op-amp with a low Vicm value (e.g. for carbon monoxide sensing) which results in a large

sensors), an additional battery may be used to continue biasing the sensor while the

application is switched off.

In the case of a two-electrode galvanic sensor with a voltage gain configuration as shown

even during power-off mode.

Note that the R, C network around the driving op-amp (IC1B) may be changed for stability

reasons depending on the sensor used. However, the TSU102 was tested with different

sensors, and good performances were obtained even with a simple direct connection

(R = 0 Ω, C = 0 F).

The reference voltage is set to 300 mV for a power supply of 3.3 V.