Distortion and Magic Resistor Ratios

jhaible jhaible at debitel.net
Mon Mar 13 13:38:34 CET 2000

> Magic Numbers: The 100K/220 ohm input structure
> for the CA3080 is practically religious. But simple
> math shows a +22 DC mv excursion for a +10DCV input.
> Or 44 mv p-p for a +/-10p-p for 20dB input.
> All the classic filter designs (including ones carried
> over to LM13x00 datasheets) assume max 10V p-p for
> 1% distortion, but actually 220 ohms should be 100 ohms
> for full +/- 10 volts channel headroom at a maximum of 1% distortion.
> (assuming the Bernie rule of max 10mv p-p into a CA3080
> with a worst case of +/-20v p-p and no more than 1%THD).

Hi Grant,

that's one way to look at it.

Here's another one:
Who wants 10V headroom @ 1% distortion, when a couple of volts
higher your next opamp clips (or does worse things like phase reversal),
producing the most ugly kind of distortion ?

There was one classical design at least (if the elektor Formant counts
as classic) that used a remarkably *smaller* resistor ratio than the

If you have a nominal level of +/-5V in a system with highly resonant
filters, you cannot expect 6dB headroom (+/-10V) being enough. So
IMO it's no use optimizing the THD for +/-10V.
If your system does weird things @ +/-13V, it's totally unimportant
if your THD @ +/-10V is 1%, 0.1%, or 5%.


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