Very low current measurement


Thread Starter


I am deciding whether a sensor I want to purchase is compatible with my data acquisition system. The input impedance of the daq terminal is 40MOhm. The maximum output of the sensor is 50e-9 Amperes (i.e. 50 nanoAmps). The lowest voltage range for the daq board is +/- 2.5 mV. With a max voltage reading of 2.5 mV and using R=V/i, then the shunt resistor I should use is 50kOhm.

Does this seem to be a reasonable solution to be able to use this sensor with my current daq system, given the shunt resistor I've calculated?

Many thanks for the insight - Mike

Robert Scott

The input impedance of your DAQ may be 40MOhm, but in order to realize that impedance you will need to sacrifice speed, in case you were thinking of making fast measurements. With a source impedance of 50KOhm, in combination with the input capacitance of the DAQ sample-and-hold (look it up), you will have a fairly long time constant. You will need to leave the multiplexer (if there is one) in one place for 5 or 6 time constants at least, and changes in the current will be filtered accordingly. Also, when using such a low voltage range, you will need to pay special attention to noise and offset sources.

-Robert Scott
Real-Time Specialties
Embedded Systems Consulting
have u solved ur problems? as i've to build a pico ammeter here (1e^-12), so probably we r facing the same problem. hope u can help me in some way.