# Constant current source, 100uA, up to 100kHz, single supply

#### Col

Joined Apr 19, 2012
44
Hi,

I want to measure an impedance of up to 100kOhm (with a capacitive part that should also not exceed j*100kOhm at 100kHz). Maybe I could go down to 1kOhm or 100 ohm, quite a range I know. I'll be doing a 4 point with instrumentation amplifier that I can easily switch the gain (but suffer BW towards 100kHz) The next challenge is that I'm aiming at a battery powered very small and simple device. So single supply is preferable.

I've looked at current source devices like REF200, but these are DC right? I though also about using an opamp and but something about stability of these things driving these kinds of loads makes me nervous.

Any ideas or suggestion would be most welcome!

Cheers!

#### Marley

Joined Apr 4, 2016
502
You want to measure at 100kHz? Will your instrumentation amplifier work at this frequency?
Are you looking for an AC current source or a DC source that you can superimpose 100kHz?

#### AnalogKid

Joined Aug 1, 2013
11,027
First, why? To get a decent answer, fill us in on the overall nature of the requirements.
Do you want to measure only the magnitude of the impedance at 100 kHz, or both the magnitude and phase?

ak

#### Col

Joined Apr 19, 2012
44
Yea, AD623 works at 100kHz, at least for unity gain (or even 10 gain with few dB reduction).

I'm not really sure the AC/DC difference you mention here, I guess AC means generate on two supplies and level shift? I would prefer to assemble from a single supply so I guess a DC source of 50uA with superimposed 100kHz would be best, but wouldnt this simply be the same as using an AC with a reference of 50uA?

Yea, I would like both magnitude and phase

#### dannyf

Joined Sep 13, 2015
2,197
It is dicy if you have to do it at 100ua - still doable but not easy.

If you just want to measure impedance, put a known resistor in parallel with your dut and measure the voltage across the known resistor plus dut, both magnitude and phase, to obtain the impedance vector of your dut.

#### Tesla23

Joined May 10, 2009
542