[sdiy] Sequencer good (and bad) news update

[Tony Clark]

> We still haven't heard back from Tony as to how he got a 1.2 V saturation.  :)

   Well I was going by experience from what was used in an older product 
(that has now been discontinued).  In that circuit, it had a two op-amp 
feedback loop to get a really high stable gain.  The first op-amp was an 
OP-77 for low-noise, and the second op-amp was the LM324 running off of 
+-5V.
   The circuit routinely clipped at about +-3.5 or so, giving you the 
1.2V saturation that I was talking about.
   However, after looking at the datasheet, (which I hadn't looked at in 
years), I can see that it _is_ supposed to go close to the negative rail, 
but I still imagine that the actual results aren't quite as nice as a 
true rail-to-rail chip would be.
   I also recall now that on that particular obsoleted product (which, 
BTW, I didn't have anything to do with design-wise), the OP-77 was 
incorrectly filtered with a 3.3K and 10uF cap, meaning that the voltage 
at the chip wasn't no where NEAR +-5V, but more like about +-3V.  So the 
saturation that I was thinking of is probably just the OP-77 hitting the 
rails before the LM324 got there.

   Cheers,

   Tony

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