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From: Tim Wescott on 27 Apr 2010 22:17
George Herold wrote:
> On Apr 27, 9:04 pm, Jamie
> <jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote:
>> George Herold wrote:
>>> 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�t 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�t observe.
>>> Is this the same for all opamps.. or is it a Burr-Brown (TI) thing?
>>> George H.
>> Yes they would, you need R and C as a by pass to the rails of the opamp
>> so they remain stable.
>>
>> jamie- Hide quoted text -
>>
>> - Show quoted text -
>
> The power supply rails are heavily filtered. The noise is at the 1nV/
> rtHz level.

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.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: whit3rd on 28 Apr 2010 17:54
On Apr 27, 5:06 pm, George Herold <ggher...(a)gmail.com> wrote:
> Do opamps have a blind spot when they are used to measure the noise on
> the power rails feeding them?
[using an op amp to measure its own power supply noise]
> The 1nV of supply noise should have caused a 10% increase, which I
> didn’t observe.

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.
From: George Herold on 28 Apr 2010 22:04
On Apr 28, 5:54 pm, whit3rd <whit...(a)gmail.com> wrote:
> On Apr 27, 5:06 pm, George Herold <ggher...(a)gmail.com> wrote:> Do opamps have a blind spot when they are used to measure the noise on
> > the power rails feeding them?
>
>  [using an op amp to measure its own power supply noise]
>
> > The 1nV of supply noise should have caused a 10% increase, which I
> > didn’t observe.
>
> 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.

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.
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