Opamp blind spot?

Question

Do opamps have a blind spot when they are used to measure the noise on the power rails feeding them?

I was using the following circuit to measure the supply noise which I know to be about 1nV/rtHz.

|\ Vsupply----C1C1--+-----+ \ | | >-+--->out..more gain.. R1 +-- / | R1 | |/ | | | | | +-R3R3-+ | R2 | R2 +---+ | GND

C1 was a 1uF metal film R1 was 10k R2 was 100 ohms R3 was 1k

The opamp was an opa134, though I latter tried an opa228.

When I tied C1 to ground or shorted the (+) input to ground through a

1 ohm resistor I measured about 8nV/rtHz of noise... as expected. When I hooked C1 to either supply rail the noise not only didn=92t go up, it went down about 10% (in power). (7.7nV/rtHz.)

When I replaced the FET opa134 with the bipolar opa228, I measured only 3nV/rtHz as expected with the input grounded and basically the same, (it was a wee bit less.), when hooked onto either supply rail. The 1nV of supply noise should have caused a 10% increase, which I didn=92t observe.

Is this the same for all opamps.. or is it a Burr-Brown (TI) thing?

George H.

Tim Wescott · Accepted Answer

Try it with a real signal on the supply rail, instead of just what ever noise you count on being there.-- Tim WescottControl system and signal processing consulting

Jamie · Answer

Yes they would, you need  R and C as a by pass to the rails of the opamp so they remain stable.jamie

George Herold · Answer

..e quoted text -Yeah, tomorrow I thought I should short out the filters on the powersupply and see how much 'stuff' I can couple through.George H.

George Herold · Answer

..pThe power supply rails are heavily filtered.  The noise is at the 1nV/rtHz level.George H.

Tim Wescott · Answer

Still, doing R-C isolation of the power rails right at the op amp never hurts. Certainly you should always bypass the chip right at the package.

whit3rd · Answer

[using an op amp to measure its own power supply noise]

Your common mode feedthrough and the power supply 'noise' are interacting. What you need, to get noise 'addition' to work, is for the phase of the power-pin noise and the input noise to be different (90 degrees different , or completely uncorrelated).

I'd think in terms of a pi filter (C-L-C best, but C-R-C is good, too) on the power pins before I'd trust the op amp to have uncorrelated power and input signals.

George Herold · Answer

s have a blind spot when they are used to measure the noise on

Thanks Whit3rd, It seemed like it must be some weird coherence thing. I'll try some CRC filters at the opamp supply pins. BTW I shorted the transitor on the power supply capacitor multiplier.. this let about 150uV p-p of mostly AC 60Hz crud onto the supply rail. Which I could measure just fine with the above circuit.

So there's not any blind spot...

George H.

Opamp blind spot?

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