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Entry  Mon Mar 5 11:58:24 2018, gautam, Update, Electronics, Three opamps walked onto an AA board AA_TF.pdfAA_noise.pdf
    Reply  Wed Mar 7 12:04:14 2018, gautam, Update, Electronics, Three opamps walked onto an AA board AA_TF.pdfAA_noise.pdfD070081_LTspiceFiles.zip
Message ID: 13667     Entry time: Wed Mar 7 12:04:14 2018     In reply to: 13665
Author: gautam 
Type: Update 
Category: Electronics 
Subject: Three opamps walked onto an AA board 

Here are the plots. Comments:

  1. Measurement and model agree quite well yes.
  2. Of the 3 OpAmps, the ones installed seem to be the noisiest (per model)
  3. Despite #2, I don't think it is critical to replace the buffer opamps as we only win by ~10nV/rtHz in the 300-10kHz range.
  4. I don't understand the spec given in T070146. It says the noise everywhere between 10Hz-50kHz should be <75nV/rtHz. But even the model suggests that at 10Hz, the noise is ~250nV/rtHz for any choice of buffer opamp, so that's a factor of 3 difference which seems large. Maybe I made a mistake in the model but the agreement between measurement and model for the AD8682 choice gives me confidence in the simulation. LTSpice files used are in Attachment #3. Could also be an artefact of the way I made the measurement - between an output and ground instead of differentially...

I like LTspice for such modeling - the GUI is nice to have (though I personally think that typing out a nodal file a la LISO is faster), and compared to LISO, I think that the LTspice infrastructure is a bit more versatile in terms of effects that can be modeled. We can also easily download SPICE models for OpAmps from manufacturers and simply add them to the library, rather than manually type out parameters in opamp.lib for LISO. But the version available for Mac is somewhat pared down in terms of the UI, so I had to struggle a bit to find the correct syntax for the various simulation commands. The format of the exported data is also not as amenable to python plotting as LISO output files, but i'm nitpicking...

Quote:

I've gotten the LTSpice model working now, will post the comparison of modelled output noise for various combinations here shortly.

 

Attachment 1: AA_TF.pdf  181 kB  | Hide | Hide all
AA_TF.pdf
Attachment 2: AA_noise.pdf  464 kB  | Hide | Hide all
AA_noise.pdf
Attachment 3: D070081_LTspiceFiles.zip  20 kB
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