terkio wrote: ↑
Fri Apr 05, 2019 11:48 am
mediatechnology wrote: ↑
Thu Apr 04, 2019 6:50 am
@terkio We had a servo discussion that began with the OPA2188. I did a test check with slow op amps as simple integrators to see when/where they stopped being integrators due to bandwidth and load-driving. This exercise reinforced the point JR has made regarding the importance of passive input poles when slow op amps are used. https://proaudiodesignforum.com/forum/p ... ?f=6&t=419
They stop being integrators at DC, where they become high gain amplifiers instead of infinite gain as would do a perfect integrator.
This DC high gain comes from the cap(s) resistor leakage ( stray PCB resistance included ) or the open loop gain of the op-amp(s) witchever comes first.
This high gain, one whishes to be infinite, will decide of the residual error of the servo.
I'll save you the trouble of reading my old comments
Typical op amps have DC voltage gain on the order of 100dB (100,000 : 1) , some even more. Not infinity but more than adequate for typical applications. All this gain has to be attenuated down below unity to assure stability with 100% capacitor feedback, before reaching the high frequency where internal lag flips negative feedback to positive.
From this POW, I do not see how a RC cell at the input or the output of the intergrator can give an improvement ( apart from filering noise ). May be I am missing something, I must look harder in the informations from the servo discussion with the OPA2188.
The front end passive pole scrubs high edge rate signals from the servo loop, making life easier for modest speed op amps. Even with 100% capacitor feedback the output may not have to swing much voltage in response to HF transients, it still has to deliver the HF current, so internally is slewing like crazy.
Contrary to popular opinion servos remain in the audio path, so transient perturbations could be audible and are therefore undesirable. Of course the value/benefit of this additional real pole depends on the servo topology, if the signal feeding it is already adequately LPF it could be redundant.
_The Deboo's resistance tolerances turns it into an equivallent resistor leakage at the cap. This is the reason, I have abandonned the Deboo.
_The input voltage offset of the op-amp(s) participates in the residual error, in a direct way.