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Converting AC current to DC current

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Newbie level 6
Feb 7, 2013
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Hi all,

I am doing a project based on scanning kelvin probe and required to design a circuit to convert AC current to an AC voltage which will be read by a DAQ.

The magnitude of the AC is in the range of 1nA to 1pA. Frequency is below 200hz.
Can a transimpedance amplifier work here?
Most appreicate if someone can guide me and what type of op amp do I need?


Yes, signal is to be amplified in order to be read by DAQ.

Things that i have forgot to mention
- A 0.02% thin film 10Mohm resistor is to be used as the feedback resistor of the transimpedance amplifier to give a gain of 10^7.
- 10Mohm resistor is selected to be used as it is suppose to be low noise and resistors that are greater than 10Mohm are usually more noisy because of its compostion?
- Is a feedback capacitor required in transimpedance opamp too?

The resolution of the DAQ is 32uV with a range of -1 to 1v and a voltage amplifier would be required as 2nd-stage gain to amplify the signal to a value which is large enough to be read by the DAQ.

In addition, noises often predominate in the measurment of such a small signal.
Is there anything that I should take note of? Such as prototyping on a stripboard instead of breadboard?
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Yes, a transimpedance amp will work, but be careful of stability. Too high a feedback resistor and/or too much capacitance on input node causes significant phase delay on feedback that may make the circuit oscillate.

Any guides that I can read regarding transimpedance amplifier and which op amp should I use?
Do I need feedback capicator in this case?

I seem to be having a lot of noises at the output of my transimpedance amp using TL082

What parameters should I be looking out for the op amp?
Am I correct to say that low input bias current is not important for my case as I am interested in converting AC current?

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