[volt-nuts] 731A output impedance

ed breya eb at telight.com
Wed Nov 28 01:37:44 UTC 2012

I'm not sure how much elaboration is needed, but here's some:

If you take all of the feedback from the output terminal, that's 
better for DC accuracy by eliminating the voltage drop of the series 
resistor, while still providing some overload protection to the 
opamp. But, it also decreases phase margin so that it will be more 
prone to oscillate with capacitive load. If the series R becomes 
zero, the voltage drop and the extra loss of phase margin are 
eliminated, but the inability to drive large capacitive loads remains 
- it is a limitation of the amplifier.

Usually a small amount of series R can help a lot with capacitive 
loading stability, but even when small it can drop enough DCV to be a 
problem. A common way to solve both problems is to sense the DC right 
at the output to eliminate the drop in the series R as above, but to 
increase stability by taking some AC ahead of the resistor - usually 
at the output of the amplifier.

If the amplifier has an integrating feedback capacitor, it's usually 
already connected that way, so only the resistive part of the 
feedback needs to go to the terminal. If there is no feedback 
capacitance, then a small amount can be added from the amplifier 
output to the effective inverting input.

I don't know what the output stage of the 731A looks like, but it 
must be an inverting (integrator) amplifier or a buffer, if using an 
opamp. In either case there should be a way to modify the feedback 
network. However, whatever is changed or added may affect the overall 
frequency response and noise.


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