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From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Final final 1kHz oscillator
Date: Wed, 27 Nov 2024 19:38:09 +0000
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On 20/11/2024 17:36, Phil Hobbs wrote:
> john larkin <JL@gct.com> wrote:
>> On Sun, 17 Nov 2024 02:01:25 +0000, JM
>> <sunaecoNoChoppedPork@gmail.com> wrote:
>>
>>> On Thu, 14 Nov 2024 11:14:28 -0500, "Edward Rawde"
>>> <invalid@invalid.invalid> wrote:
>>>
>> <snnnniiiiiipppp>
> 
>>> Because of common mode (non)linearity.  In most op-amps (especially
>>> older designs) the distortion generated by the common mode signal is
>>> typically one to two orders of magnitude larger than the transfer
>>> distortion.  If it is not clear from the data sheet, it is best to
>>> measure the CM performance or just assume that it's 40dB worse than
>>> the specified figure.
>>
>> To quote Jim Williams, "always invert."
>>
> “Except when you can’t.”
> 
>> Just the ESD diodes are a distortion hazard. I've seen people
>> bootstrap the power supplies to improve CMRR.

"Always invert" except when its better not to.
For example, see Fig 6-7 of the OPA1656 data sheet.  The distortion
is around 6dB lower for the non-inverting configuration.  This chip
was designed to have very low variation in common-mode capacitance
with voltage.
John


>>
> Pease has an article somewhere about how to measure CMR and PSR
> correctly—as he says, most of the obvious methods actually measure
> open-loop gain.
> 
> Cheers
> 
> Phil Hobbs
> 
> 
>