Operational amplifiers and feedback loop : low pass decoupling capacitor : which frequency ?

[diyiggy]

Thanks Rayma

 

[diyiggy]

The best way to select the cap is to get the DAC to output a square wave. Any capacitance from the op-amp summing junction to ground is going to cause overshoot on a square wave. The cap across the feedback resistor can then be adjusted for max square wave fidelity. Unfortunately, even if you know the DAC output capacitance, you probably won’t know the layout parasitic capacitance. Therefore you have to do this on the actual final PCB layout. Additionally, keep any ground plane well away from the summing junction as this will cause problems as well.

I tried to do so. As one can see, no ground on the 4 mm circa between the output of the dac and the inverted pin... it is perhaps even too much (but the ground pin of that dac chip is just the next sided one, so signal can find a short return path away from the PS ground btw.

Edit : the name Extinction DR(for dead rooster) is a pun story to give a stupid name to the pcb as some do

 

[MarcelvdG]

If I were to design a circuit like this, I would choose some odd order for the reconstruction filter and then use this capacitor to make the first-order section (realize the filter's negative real pole).

I was assuming an ordinary unity-gain-stable op-amp here.

 

[MarcelvdG]

Regarding small resistors, for voltage followers, some op-amp datasheets recommend using small resistors either in series with the input or in the feedback or both. Those are then either meant as base stoppers or to limit the current flowing through the input protection diodes when there is a sudden voltage step at the input. That second reason doesn't apply here because the DAC output current is limited anyway, but a base stopper may be needed, depending on the op-amp design.