Super S&H

[tomg]

I kept at it and the results are almost unbelievable!! I sampled 
+5.01V. It's now about 4 1/2 HOURS! later and it's sitting at +5.01V.

What is it?
It's a "low drift sample and hold". It's made of several circuits
built around a lm3900 quad Norton amp.  A1 is a ramp and
hold amp. A2 is used to bias the input current of A1. A3 is
used to compare Vo - Vi.

How does it work?
If sample is high, S2 closed and a 0-12vdc voltage is applied to A3. 
S1 closes turning Q1 and Q2 off. A1 ramps up or down to match the 
voltage output of A3. 
If sample is low, S2 opens and the voltage is removed from A3,  S1 
opens turning Q1 and Q2 on putting A1 in hold mode. The stored
voltage appears at Vo of A1. And apparently just sits there!!

I'm using the divider in the note box right now for the test. I can't
swear you'll get the same results as I did because the stability
is dependant on the cap used. I have several 1uf types the ceramic
and plastic show some minor drift. I have these large old
things with the color code painted on them, I have no Idea what
kind they are but that's whats in the circuit right now. I'm not kidding
guys it hasn't even tried to drift... you know....the last digit on my
meter flipping back and forth...It hasn't moved! I'm gona leave it on
all night, just to see if it drifts at all.

btw - it's just sitting on the proto-board...wires going everywhere.

-tg

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