Precise small AC current measurement

Hi

I am trying to look for a current measurement circuit that is able to measure an AC current signal whose peak-to-peak is in the range of 10 uA to 100 uA. Basically, I have a circuit whose average supply current is about 50 mA, and the small AC current signal sits on top of it (i.e. the supply current is 50 mA DC + about 50 uA AC). The supply voltage is 12 V (so if a sense resistor is to be used, it should be really small). Note that I want to be able to see the signal on the oscilloscope.

Assume the frequency of the AC current signal is controlled (right now it is 250 kHz, but I can reduce/increase it).

Anybody knows a useful circuit I can buy?

PS: I have been looking at a couple of possible useful products, but not sure if they will solve the problem:
LT1167: http://cds.linear.com/docs/Datasheet/1167fc.pdf
Ultraprecise Current Sense Amplifier Dramatically Enhances Efficiency and Dynamic Range – Design Note 423: http://cds.linear.com/docs/Design%20Note/dn423f.pdf
Sub-uA RMS Current Measurement for Quartz Crystals: http://cds.linear.com/docs/LT%20Journal/LTMag-V17N02-15-SubMicroAmpCurrentMeasurement-Williams.pdf

I agree with the "Sub-uA.." application note, that a current transformer is a convenient way for a selective AC measurement. Of course you don't need an expensive GHz current transformer like Tektronix CT-1, a small ferrite toroid or similar small high frequency transformer will do. An amplifier is most likely required for µA sensitivity.

FvM said:

I agree with the "Sub-uA.." application note, that a current transformer is a convenient way for a selective AC measurement. Of course you don't need an expensive GHz current transformer like Tektronix CT-1, a small ferrite toroid or similar small high frequency transformer will do. An amplifier is most likely required for µA sensitivity.

Click to expand...

Thank you. Would you please give me specific examples of the small high frequency transformer you mentioned? Also, what kind of amplifier would be used?

Transformers can be easily calculated in terms of main, leakage inductance and saturation current. For higher frequency, parallel capcitance should be considered.

A small torroid (6 - 10 mm) with a few windings can basically do. The current transformer principle involves termination of the secondary with a low impedance or a transimpedance amplifier.

OP amplifiers should do.

Note that I want to be able to see the signal on the oscilloscope.

Click to expand...

Is it a DC component that you wish to eliminate? If so then you can switch the scope to 'AC input' mode. This will insert a series capacitor, and eliminate the DC component.

Then turn up the sensitivity setting on your scope. You may be able to observe the AC signal without it needing amplification.

FvM said:

Transformers can be easily calculated in terms of main, leakage inductance and saturation current. For higher frequency, parallel capcitance should be considered.

A small torroid (6 - 10 mm) with a few windings can basically do. The current transformer principle involves termination of the secondary with a low impedance or a transimpedance amplifier.

OP amplifiers should do.

Click to expand...

So do I place a transimpedance amplifier after the secondary of the current transformer, and then place another op amp after that?


BradtheRad: I am aware of the 'AC coupling' mode of oscilloscopes and how to change their sensitivity. The problem here is that the scope by itself is incapable of detecting such a small AC current. This is why I asked if anyone is aware of a nice current measurement circuit I can buy so that I can eventually be able to view the signal on the scope.