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.