How to select amplifier to enlarge the uA current ?

[vineyuan]

Hi team,

I need to calculate the phase shift between the current and the voltage. The tested cable should be applied with a voltage of 1-3V, 50kHz by our acquisition card. I need to obtain the current waveform.

The issue is: the current in this circuit is very small, which in range of ~10uA. I need an amplifier that can transfer this small current to a voltage with higher amplitude that can be the input of NI 6281 (at least in range of >10mV). 

It should not put a very large resistor to get the voltage because a very large resistor in series could change the phase shift of this circuit. Therefore, an amplifier is needed  to get the current amplitude and then I can calculate the phase shift between the current and the voltage. 

My question is : How to select an amplifier that could enlarge the current but not change the phase? Is there any evaluation kit recommended? Is there any better solution except the amplifier?

Great thanks to anyone's help!

Best Regards.

Liyuan

[Henrik_Volkers]

PAR, Ithaco, EG&G and other make nice amplifiers.

A DIY solution I would look at is a TIA (transimpedance amplifier), a nice OPamp with bipolar supply, + input to 'GND' , - input as signal(current) input and a feedback ressitor R_fb  in the range of 10k to 1M. 

Negitible burden voltage :), you measure against a virtual ground.

Beside the input bias current of the OP (should be in the pA to fA region, for 50kHz look for a high GBW) the output is:

V_out= - I_in*R_fb

Good cleaning of the PCB and clever routing/guarding is advised. 

A small adjustable compensation capacitor C_fb migth be needed (usually PTFE isolated twisted cable)

OP DEMO boards from TI, AD (Linear)  migth be available. look for the TI and linear(!) application notes.

Use differential DAQ input .. as noted in my signature: Ground is ....