INA111: Picoamp current measurement using instrumentation/operational amplifier

SK,

To start, you cannot directly measure 1pA current with 50ohm shunt resistor as this would require you to be able to measure 50pV voltage (50*1e-12) - this is impossibe considering that the integrated noise of any IC across a reasonable bandwidth would be much higher.  Also, in order to measure 1pA current with an instrumentation amplifier (INA), as you pointed out, you would have to have INA with an input bias current of 10fA or less, which we do not have.  However, we have operational amplifiers like LMP7721 with the typical IB of +/-3fA, and maximum of +/-20fA, that could be used in the transimpedance configuration as shown below.

Using RF of 20Mohm, the first stage converts 1pA current into VF1 of 20uV, which then gets gained up by gain of G=101 of the second stage into 20mV.  Assuming required range of the IG1 current of 1nA, this would result in the output of -2V for IG1=1nA - see below.  CF and C1 are used to filter out resistor thermal noise before the signal in fed into second stage. If needed, the circuit may be re-configured to give you a positve voltage. I used here +/-2.5V supplies for convenience.

Attached please find the Tina-TI schematic for your own simulation.

SK Low-Side current measurement - LMP7721.TSC

Hi Sk,

I fully agree with Marek. If I'm understanding him correctly, you could open the ground connection of your 50R shunt and connect the ground side of 50R shunt to the input of his TIA, as shown below:

marek_lmp7721.TSC

And by using the good old chopping method (periodically turning-on and -off your your decive and by this turning-on and -off the device current to be measured), you can easily correct for any offset voltage error and even offset voltage drift error at the output of second OPAmp. You measuring signal would be the Vpp value of output voltage, or by other words 27.88mV - 25.81mV = 2.07mV in the above example.

Kai

@Marek Lis Thanks for your solution. I Will certainly look into the solutions carefully and will update soon.

@Kai was thinking of the same configuration as you have mentioned as a backup option. Will simulate and study the 2 circuits closely, analyse them, and update the observations in the same thread. Thank you Kai

Hi. I forgot to include the most important design requirement, the current to be measured in the range of 1pA to 60 μA. I mentioned (my fault) only the minimum limit for the setup in my design requirement descriptions as I was using the minimum current value for zeroing on the amplifier (since the minimum current and input bias are almost the same for many amp) and forgot the max limit.

On studying the circuits provided, stage 1 is designed as a trans-impedance amplifier, and I used this as my reference guide for designing/understanding. Following it, the second stage is an amplifier.

So if the measurement current range is such a wide variation, Can I still use the same configuration?  Or I should go for a separate similar circuit for each range, say 1pA to 100nA and 101nA to 60 μA? I am now studying closely the circuits will update soon of my inferences on the design suitability.

Thanks,

SK