[SOLVED] Dual supply instr op amps - how to use them with single supply?

[ikorman]

Hi all

I'm working on some simple, yet precise measurement circuits which incorporates instrumentation op amps. As I'm looking for op amps with low noise and voltage offset, I found out that op amps that would suite my need have one drawback: they require dual supply voltage. As I'm mostly working in micro controller environment where I have single supply voltages, I'm curious is there some simple way to generate negative voltage (like single chip solution)? I've heard about things like LMC7660, but I'm not sure how switching will influence op amp circuit in sense of precision and stability.

I now that there are precision, single supply, rail-to-rail op amps, but it is not so easy to find one with certain characteristics.

Thx.
Ivan

 

[keith1200rs]

I would have thought you would be able to find a suitable instrumentation amplifier which works off a single rail, but if not you can use things like the LMC7660 or LM828 - just make sure you filter it well.

Most rail-rail opamps don't quite manage the rails - the outputs fall up to 100mV short depending on the device and the load. Check the data sheets carefully.

Keith.

 

[dick_freebird]

The front end amplifier input range still has to accommodate
the worst negative common mode offset, and even "rail to
rail" might not do it for you; "beyond rail" amps are fewer,
and still limited to maybe a FET VT or so beyond (if you
need a nice input bias current spec).

Charge pump is how I'd go. Since the inst amp is pretty
low current you could double-filter the supply to help
out the PSRR@frequency. You can also probably RC
filter the output to someplace between pump chop and
maximum frequency of interest.

Have a look at appnotes about input and supply ranges.
Often there is one node inside running at A=2 (for a unity
gain inst amp) so the supply needs to be at least
Vin(max)*2+headroom - and this should be figured for
both ends of the worst case input band. If you are using
the inst amp for higher gains with pin-strap gain options
you want to see where the gain is being made (front or
back) and make sure the figuring is right.