Re: Final final 1kHz oscillator

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