AC resistance bridge

P.S. The primary problem with the SRS product is the (lack of) stability. Excitation will be a relatively healthy 10uA or so, resistance is in the 1K range.

That's not hard with any sufficient op-amp, as you already know, so, I'm confused.

I was checking out the manual to a piece of equipment at work, an HP high resistance/low current meter (I forget the number), it uses a parametric method -- essentially, the capacitive unbalance (and therefore change in oscillating frequency) of some small voltage across some very special diodes (probably controlled capacitance more than anything else). Frequency being easier to read than microvolts and nanoamps.

If you're serious about nanovolts, that sounds like SQUID territory to me. Isn't thermal noise on a chunk of copper about that much?

Tim

-- Deep Fryer: A very philos> > Does anyone know of a SMALL and HIGH PERFORMANCE commercial resistance

What's +/-0.5ppm of that?

For 1K at 300K it's about 4nV/Hz^0.5 Don't need much bandwidth. There are interesting issues with stray capacitance, even at ~1Hz excitation. It would be nice to find something off-the-shelf, but if I must...

Best regards, Spehro Pefhany

Not that hard to build yourself. An AD630 does this job. The datashhet has a few application notes.

Rene

Thanks, Rene, I'll look at that in detail. I think I'll need a transformer at the front end though. Most of the commercial designs seem to use multiplying DACs fed with quadrature signals to deal with the phase shift.

Best regards, Spehro Pefhany