Very low current measurement with SCXI-1121

[syrinxstar]

I'm trying to figure our whether a sensor I want to purchase is compatible with our SCXI-1121. The sensor will deliver a maximum output of 50e-9 Amperes (i.e. 50 nanoAmps). The measurement frequency is essentially static. If I use the 2000 gain setting, then the maximum voltage I could sense is 2.5 mV. With R=V/I, then the shunt resistor I should use is 50kOhm. Will I be able to get reasonably accurate (better than 10%) readings with this setup? My concern is that shunt resistors are usually smaller than this but this seems to be a sound solution.

[AVR]

It would seem that the measurement should be possible given you are only expecting 10% accuracy - typical accuracy of a generic resistor. Now, for your input, you can either (A)Set the gain at 2000 for an input signal of 5.0 VDC, or, (B) Set the gain to 1.0 and set the input range for your DAQ device to, say, -.1/+.1 for an input resolution of approximately 48.8 micro-volts/bit on a 12-bit DAQ. I actually favor option (B) as gain settings of much higher than 500 can introduce a significant amount of noise and jitter on the signal. Given the level of the signal you are trying to acquire, I strongly recommend that you ground the unused inputs on the 1121 to avoid any cross-talk that will distort the input signal from your sensor. If, however, you are using the other t
hree inputs, be sure that they are of the same level ( 2 mV ). On a multiplexed (MUX) input circuit, you do not want to ever have a significant difference in the input voltages from channel-to-channel. It causes genuine accuracy problems due to the settling time requirements on the input amplifier. Good luck.