Voltage pulse frequency is between 1 Hz to 5 kHz and expected
current is in between 10 to 300 uA. (Cell impedance is between
10kOhms and 100 kOhns approximately) Voltage signal is applied as
like this (using two analog channels in NI card): First pulse :
Ch0 = 1 V and Ch1 = 3 V. Second pulse Ch0 = 3 V and Ch1 = 1 V
Third pulse : Ch0 = 1 V and Ch1 = 3 V. Fourth pulse Ch0 = 3 V and
Ch1 = 1 V Fifth pulse : Ch0 = 1 V and Ch1 = 3 V so on . . . . . .
If those pulses are square pulses then the common mode voltage
could have significant components up to at least 5x5KHz.
Getting a good common mode rejection at high frequency can be
difficult and it is always worth while investigating the
possibility of not having a CMV in the first place.
It looks like you have a two-terminal cell, which needs a +/-
2V drive and have perhaps chosen those +1V to +3V differential
signals because your NI card does not do bipolar voltages.
The circuit below will provide a +/- 2V drive to the cell
from your NI card, and produce a grounded I-V conversion.
3 _ R1 R2
_| |_---/\/\---+-/\/\--+ All R's = 10k, 0.1% (say).
1 | |
V1 +--|-\ | OP amps powered from +/-
| >-+ supply rails.
0v--|+/ |
+------------------------>ADC-3
R3 | R7
V2 +-/\/\--+- --/\/\-- -+
3_ _ R4 | R5 | R6
|_| ---/\/\---+-/\/\--+----[CELL]---+-/\/\--+-->ADC-1
1 | | | |
+--|-\ | +--|-\ |
| >-+<+/-2V | >-+
0v--|+/ | 0v--|+/
+------------------------>ADC-2
The left hand end of the cell has the +/- 2V drive on it.
The right hand is at 0V, into the virtual earth of an I-V
converter. R6=10k gives ADC-1 a scaling of 3V/300uA.
That is a bipolar output. R7 is an optional resistor
that biasses ADC-1 into being positive-only. The amount
of bias varies according to where V1 is, but this can
be easily dealt with in the software. An ADC-3 input
is useful for measuring the bias.
ADC-2 is an optional measurement that gives the exact
voltage on the cell during each pulse-polarity. Useful
for (say) software-adjusting V1/V2 for zero dc-bias
on electrochemical cells.
