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Sanity-check + end-result: a x 1000 measurement preamplifier
Sanity-check + end-result: a x 1000 measurement preamplifier
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Old 18th January 2019, 09:23 PM   #31
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by syn08 View Post
Not sure why you think the PSRR potential issue depends on the op amp only.
It doesn't of course, but getting meaningful results either means a global simulation using the right models for all the critical components, or a way to disentangle the contributions of each one to be able to compare the two options (the ground-referenced one could superior, but it also contributes to the leakage of the PS noise)

Quote:
Originally Posted by gerhard View Post
With this construction you can check how noise on VCC impairs your
input noise density. 60 Ohms is 1nV/rtHz; the VCVS adds it to the clean VCC
with a scale factor of your choice. The 0.1uV offset avoids a LTspice hickup.
Setting E1 = 10 adds 10 nV/rtHz to the clean VCC.

To see the contribution of the VCC noise, just click on the 60 Ohm resistor
when plotting noise density.
Thanks for the tip, it is neat and simple.
I already thought about it, but the solutions I had found were more clunky and far less convenient.
I tested it, and to get a significant impact at 1Hz, I need a factor of 10,000 or 10µV/rtHz. At 1 kHz, it remains relatively insignificant.
That is valid for the AD823 only, but the LF357 would need to be 10's of dB worse to make a difference with more realistic levels.

Sanity-check + end-result: a x 1000 measurement preamplifier-lna21-png

In addition, the passive supply filter is not present.
When it is, the noise at 1Hz remains unchanged, logically, but between 10Hz and the rest, it is essentially wiped out (not that it actually matters, but it is good to know)

Sanity-check + end-result: a x 1000 measurement preamplifier-lna22-png

Edit:
I just checked, and in fact the 823 is significantly worse on PSRR than the 357, something I never imagined, but it certainly puts the issue to rest
Attached Images
File Type: png LNA21.png (121.3 KB, 117 views)
File Type: png LNA22.png (119.1 KB, 121 views)
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Last edited by Elvee; 18th January 2019 at 09:27 PM.
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Old 18th January 2019, 09:44 PM   #32
gerhard is online now gerhard  Germany
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Methinks that should be V(onoise) / gain to see the noise density effect at the input.
gain is predeclared.
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Old 18th January 2019, 09:52 PM   #33
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Elvee View Post
I just checked, and in fact the 823 is significantly worse on PSRR than the 357, something I never imagined, but it certainly puts the issue to rest
Not sure why you cling so hard on the op amp PSRR performance. The noise source due to the imbalanced op amp inputs contribution is outside of the op amp and PSRR of the op amp is largely irrelevant. It's the op amp CMRR that matters much more. Ideally, only the imbalances to the op amp inputs will be amplified.
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Old 18th January 2019, 10:48 PM   #34
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by gerhard View Post
Methinks that should be V(onoise) / gain to see the noise density effect at the input.
gain is predeclared.
Indeed (I am a bit tired), and LTspice can in fact plot it directly, but the individual contributions are always referred to V(onoise), meaning that to make comparisons, one still needs to enter the gain for other traces:

Sanity-check + end-result: a x 1000 measurement preamplifier-lna23-png

It brings the contribution of the supply noise to 1.6pV/rtHz@1kHz for 10µV/rt/Hz, which makes it even more irrelevant, but it matters even less.
Quote:
Originally Posted by syn08 View Post
Not sure why you cling so hard on the op amp PSRR performance. The noise source due to the imbalanced op amp inputs contribution is outside of the op amp and PSRR of the op amp is largely irrelevant. It's the op amp CMRR that matters much more. Ideally, only the imbalances to the op amp inputs will be amplified.
My mistake: I meant CMRR (I am decidedly tired), which is much worse for the 823 than for the 357.
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File Type: png LNA23.png (123.0 KB, 107 views)
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Old 2nd February 2019, 04:26 PM   #35
jackinnj is offline jackinnj  United States
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Sanity-check + end-result: a x 1000 measurement preamplifier
I changed out the opamps to OPA637 and simmed in TINA:
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