How to read wide range of small current - 20ua through 200ua

Guest · Posted: Wed Aug 06, 2008 8:35 am

Hi guys. Thanks for your input over the past couple of years.

I need to measure the output from a constant current device that outputs an ac sine wave ranging from 20 microamps to about 200 microamps at its highest setting. This is for a simple go/nogo tester for the device.

ADC and frequency counter on the 18f887 work pretty well when output is rectified then sent to a voltage divider setup for high current mode, or manually re-setup for low current mode.

I am struggling in my newbie attempt to compress this output range, and get the measurements over the entire range without frying any components or the pic due to overvoltage.

Sorry for the hardware question. I have struggled(in a fun way, I guess) for a couple weeks with this one. Any thoughts?

Thanks!

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Is either end of your constant current source grounded? Are we allowed to ground either end? Ideally I would tie one end of the current source to the A/D midscale voltage and put a 5k load resistor on it. That would give a midscale +/- 1.4V signal for the A/D.

Tell us more.
_________________
The search for better is endless. Instead simply find very good and get the job done.

mkent · Joined: 09 Sep 2003 Posts: 37 Location: TN, USA

You could use a op amp in the current to voltage mode to change the 20 - 200 ua to .4 - 4 volts; then feed the A/D.
Google "op amp current voltage"

newguy · Joined: 24 Jun 2004 Posts: 1908

When I was doing my PhD I had to build an I-V converter. It had a switchable gain (rotary switch), with gains of 10^5 to 10^9 V/A. The converter was created with double-ended (+/-) supplies for the opamps.