Re: gEDA-user: opamp slew rate limiting

[gene glick <carzrgr8@xxxxxxxxxxxxx>]

Martin Maney wrote:
> 
> Nah.  You should use a good comparator with controlled hysteresis.  An
> opamp, any opamp, makes at best a mediocre comparator.  

Actually, it worked!  Not sure why I didn't think of it earlier, but I 
threw the circuit into simulation to see.  By cranking the gain up from 
-10, to -200, the amp saturates much earlier in the cycle. 
Interestingly, with gain of -10, and input signal of about 2 Vrms, the 
output looked exactly like it does on the bench with edge rate of 20 uS. 
  In simulation, changed the gain to -200 and the edges go to about 2 
uS.  Worked precisely the same on the bench.  This I can live with, and 
solved my problem.

I'm pretty sure this supports the notion that the output was really just 
following the input, saturating later in the cycle.  Regarding John's 
point, I did in fact, change to FET input by going from opa2227 to 
opa2132.  While the slower bipolar input device would probably work, I 
think I prefer the faster device for various reasons, including yours.

Yeah, I totally agree that a comparator is the way to go - but it's too 
late for that.  It's not my design, I'm just the flunky who gets to make 
it work right - or at least as good as possible.  The digital logic used 
is old CD4000 stuff which is pretty slow.  There's a D flip-flop in the 
circuit that calls for edges no slower than 10 uS - pretty slow by 
modern standards, but this circuit failed to even meet that.  Oh, btw, 
this opamp used drives a schmitt trigger - sort of a poor mans 
comparator.  Same problem though, with the slow edge rate fooling it.


This turned out to be a fun thread, thanks for the help :)

gene


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