[sdiy] Singin' the S&H blues...

[Czech Martin]

A simple VMOST Source follower (BS170 e.g.) can have leakage in the
1 pA range. Errrr, as long as I did not damage it with ESD...

For short and medium hold times the capacitor soakage will
be more harmfull than this leakage droop, even with poly styrol types.
At least this was my result when I tested some s&h a few weeks
ago.

The follower as such is unprecise (wandering offset), but
this can be fixed by putting it in a op amp control loop.

So only short time temperature effects remain, but this should
be not so bad if no draft is present. Also the source resistor could be changed
to another VMOST, giving some hope for error cancelation.

The CA3140 has much higher leakage due to zener input protection network.

I think that VMOST transistors are a good alternative to very expensive
very low input bias current op amps. These have often low  and mono rail
supply. With +- 15V supply the VMOST should be able to do +-10V.

Of course, this works only for sense.
For drive one would choose perhaps two jfets in series with
bootstrapped mid-point. This gives a pretty low leakage 
switch which works for current in both directions.


Should be better than the CA3080/CA3140 usual s&h.


m.c.

-----Original Message-----
From: Tim Ressel [mailto:madhun2001 at yahoo.com]
Sent: Freitag, 14. März 2003 10:06
To: Synth-Diy
Subject: [sdiy] Singin' the S&H blues...


Yo,

Play me a durge, matie ... I be tellin' a tale of
sample and holds.

Yes, the drama is required. Silly me, I thought an
LM324 would be okay for a sample and hold buffer. It
looked good on the scope. Sadly, ears are very
sensitive to tiny little changes in pitch. So when my
okay-looking s&h output was tied to my VCO, well, it
was not good.

A quick look-see at the datasheets tells the tale: The
LM324 has worse-case 100nA of input bias current. The
TL074 is about 500 times better at 200pA input bias
current. Luckily, the pinout is the same.

So tomorrow I will try it and see if it fixes the
problem.

Sigh...

--Tim



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