Measuring AC current with an oscilloscope.

[FM79]

I have a technical question, for research purposes.

I have LED that we probe in AC mode. To measure the applied voltage in AC and the light-response in AC is no problem.

However I would like to measure the current going through the device as well and I would like to link it to an oscilloscope. Now OSC usually measure potential (and have limited range) and I'd like to know how to have a signal directly related to the current on the OSC.

If you know of any particular instrumentation please let me know.

 

[Paul Colby]

Put a resistor in series with the LED and measure the voltage across it with a scope. That's how I've done it in the past. Ohms law rocks.

 

[FM79]

Yes, that's basically trivial.

I was looking for a more detailed and reliable solution. Currents will be in the range from nano to micro ampere (or even pico-ampere for some other devices) and frequencies above 100 kHz.

I think that, amplifier aside, I need a bit more than that.

 

[Paul Colby]

By AC mode I assume you are using a time dependent driver for the LED. Why are the currents you expect so small?

 

[FM79]

Because I am working at low voltages and new materials.

 

[Svein]

The standard current-to-voltage converter for very low current is the classic Op-Amp circuit:

Depending on your requirements, the circuit can be tweaked to compensate for temperature and bias current variations. Google "current voltage converter".

 

[Rive]

Yes, that's basically trivial.

I was looking for a more detailed and reliable solution. Currents will be in the range from nano to micro ampere (or even pico-ampere for some other devices) and frequencies above 100 kHz.

I think that, amplifier aside, I need a bit more than that.

Check for some 'current sense amplifier' with programmable gain. Pretty reliable stuff, and usually there is a ton of AppNote too.

 

[FM79]

Thank you Svein and Rive.