Bi-Potentiostat

Hi Chris,

The LMP91000 + LMP2231 can do this job.

See the example below wiht some calculations.

The main equation of a 4lead gase sensor is the following (1)

(1) I_signal = I_sensing - (R_{TIA_AUX}/R_TIA)*I_auxiliary

then yuo can write the classicla transimpedance equation for the 2 TIAs

(2) VOUT = INT_ZERO + R_TIA*I_sensing

(3) VAUX= INT_ZERO+R_{TIA_AUX}*I_auxiliary

The previous equations together with equation (1) give the following result

(4) I_signal = (VOUT-VAUX)/R_TIA

 The previous calculation refers to the ideal case, in the real world the external amplifier usually has an offset and a bias current that could impact on the measurements.

The complete equation which takes in account these effects is listed below:

(5) VAUX= INT_ZERO+ R_{TIA_AUX} *(I_auxiliary -I_{bias_LMP2231})+V_{OS_LMP2231}

When  equations (5) and (2)  are replaced in equation (1) , the signal current is the following:

(6) I_signal = (VOUT-VAUX)/R_TIA + V_{OS_LMP2231}/R_TIA – R_{TIA_AUX}/R_TIA * I_{bias_LMP2231}

The LMP2231 has a very low input bias current (less than 1pA @25°C and less than 50pA @125°C) that is negligible if compared to the auxiliary and sensing current of a 4-lead gas sensor. Moreover the gas sensor manufacturers suggest an R_{TIA_AUX}/R_TIA << 10, so the term R_{TIA_AUX}/R_TIA * I_{bias_LMP2231} is negligible. The equation (6) becomes:

(7) I_signal = (VOUT-VAUX)/R_TIA + V_{OS_LMP2231}/R_TIA

The offset voltage of the LMP2231 is less than 150uV @25°C and less than 230uV@ 125°C, moreover its drift is less than 0.4uV/°C.

Regards,

Domenico