Electronics > Projects, Designs, and Technical Stuff
PICs with Op Amps: Non-inverting? Off-piste?
NivagSwerdna:
I'm thinking about using a DSPIC33EV256GM006-I/PT for a small project and one of my requirements is to sense current through a device...
... my plan is to have a 1 0.22 ohm sense resistor and then a x10 x100 gain Op Amp into the ADC. I would like to utilise on onboard OP1 with Rf 18k Rin 2k in a non-inverting configuration.
But... all the datasheets and tech notes seem to always show inverting configurations? Am I missing something? Will non-inverting work?
fourtytwo42:
Shouldn't be a problem, I use them that way in PIC24EP256MC202 and others.
As you are doing current sensing beware Vcmr includes AVss but not below it.
David Hess:
The reason only inverting configurations are shown is revealed in the specifications if you know where to look. The common mode rejection is a horrible 45dB *typical* or 1:178 so for every volt of change in the non-inverting input, the input offset voltage changes by 5.6 millivolts, and it could be *worse* than that. (1) The poor common mode rejection comes from having a rail-to-rail input and being produced on a digital CMOS process. The other DC specifications are equally poor as expected.
(1) This cannot be treated as a gain error because it is non-linear.
NivagSwerdna:
--- Quote from: David Hess on March 31, 2020, 01:47:12 am ---The reason only inverting configurations are shown is revealed in the specifications if you know where to look.
--- End quote ---
That's really interesting.
CM40 VCMR Common-Mode Input Voltage Range AVSS — AVDD V
CM41 CMRR Common-Mode Rejection Ratio — 45 — db VCM = AVDD/2
... and in inverting mode there is less CMRR error since both inputs held to ground (or virtual ground) and hence no CM dynamic voltage. Thanks for that!
Zero999:
Other than what's already been mentioned above, the only thing I can add is non-inverting configuration has a higher noise gain, than inverting, so it will be less stable, but if the op-amp is unity gain stable, it won't be a problem.
A couple of threads discussing noise gain and inverting/non-inverting configuration.
https://www.eevblog.com/forum/beginners/op-amp-spec-noise-gain-configuration/
https://www.eevblog.com/forum/projects/inverting-vs-non-inverting-pros-and-cons/
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