[volt-nuts] 731A output impedance

ed breya eb at telight.com
Tue Nov 27 16:24:23 UTC 2012

The simplest way to drop the output impedance without adding much 
circuitry is to just change the series R to 100 ohms or so - that 
would still give pretty good isolation from capacitive loading.

If the R is dropped to zero, the DC performance will be best, but 
you'll have to worry about the amount of capacitive loading. If the 
lines are short - say a couple of meters or less of open wire, it 
would probably be OK, but that much coaxial cable may make it oscillate.

The suggestion to get the feedback right from the output terminal,or 
even with external sensing at the load would be best for DC accuracy, 
but would have the same problems as above.

You can also take the DC feedback from the output directly, and the 
AC feedback from the amplifier output, while the series resistor 
isolates the two. This would give good DC accuracy and AC stability, 
but would alter the dynamic response and LF noise shape somewhat.

If you add an amplifier, you'll of course have to consider its offset 
and noise contribution, and it will have the same stability issues to resolve.


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