[sdiy] Outputs grounded! Re: SSI2164 datasheet updated

David G Dixon dixon at mail.ubc.ca
Sat Aug 11 23:46:08 CEST 2018


> > In fact you are *required* to force the output to the same(ish) 
> > voltage as the GND pin this usually done with the virtual 
> earth node 
> > of an inverting op-amp, but there is no reason for 
> connecting it directly to GND.
> Okelidokkeli , i dont really understand ,so if no reason to 
> directly connect unused output to GND why is it a requirement 
> then? If output loaded with say 1Mohm to GND would that make 
> that section go berserk?
> 
> > See  the datasheet where it talks about "Output Compliance" 
- that is 
> > how  far the outputs are allowed to stray from GND before 
> bad things 
> > happen.
> Yes its low but how bad, latch-up? Or severely affecting 
> other sections used?

The old datasheet says no more than 100 mV on the output pin.  I can say
from experience that it is actually less than that.  If the 2164 doesn't
terminate to virtual ground, it just goes berserk.  The circuit simply will
not work.  I've made this mistake twice, so I speak from experience.  This
is the single most inconvenient thing about the 2164, because it precludes
using it for certain filter topologies (like Sallen Key).

> Ok ,so now DIXON says they left thousands outputs unconnected 
> with no problem!
> I have to take him for his words really.

Yes, I have, and it's not a problem.  I have also not compensated the unused
VCAs, and this also has not been problematic.  I don't like to leave 2164
VCAs unused, but sometimes you just don't need one of them.  It doesn't
happen often, but it does happen.

It would be interesting to know what other builders have done with unused
VCAs.  I suspect that most people just leave them unconnected.



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