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Selection of op-amp to sense nA current.

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moeedmughal

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Hi all,

I want to measure ultra low current (couple of nano ampere). Is there any special technique to measure such low currents or differntial voltage measurement across a series resistor (Rsense) will do ?

Suggestions to select appropriate operational amplifier for low current measurment are required. Thank you in advance.

Regards,
Moeed

- - - Updated - - -

I want to measure AC current. Maximum frequency is few hundred hertz.
 

Couple of nA isn't particularly low. Good "electrometer" OPs like OP129 have < 100 fA input current. Measurement "across a series resistor" involves a differential amplifier and respective common mode errors. Better to use a ground referred I/V converter OP (transimpedance amplifier) circuit, if ever possible.

Leakage currents can be a problem, so high quality isolator (e.g. Teflon) and shielding/guarding is strongly suggested.
 
for such low level measurement , better to use standard meters available.

one such is keithley instruments (part of tektronix now)
 

Hi,

I agree with FvM.

I´ve used the transimpedance amplifier and ADC solution to safely detect signals with 30pA threshold for an XRAY dosemeter.
Careful desgin, well desgined amplifer circuit, well selected amplifiers, clean power supply, shielding (maybe guarding, more important at DC signals), proper PCB layout.....is essential.

Especially for the transimpedance amplifier oyu need careful selection of OPAMP according current noise and voltage noise.
Current noise is more important. But high frequency voltage noise is amplified because of (stray) capacitance of your input signal.
Shielding of input signal wires adds capacitance..Here a series resistor at the input can be used to act as a low pass filter.

Klaus
 
Thank you all for suggestions. I would go for TIA. I have LMP7721 in stock. Soon i will update about the progress.

Cheers !
Moeed
 

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