[sdiy] S&H problem -- Help??

[Tim Ressel]

Hey all,

I have this nasty little problem with the S&H on my
midi-2-cv convertor. It's kinda hard to describe,
especially without a schematic, but I'll try: I use a
DAC with a opamp buffer on the output which drives a
CD4051 1-to-8 mux. The 4051 attaches the DAC to a cap
to ground followed by a TL071 unity gain buffer. The
sequence is this: first the uP sets the DAC. Then the
mux is set to the channel to update. Then the 4051 is
enabled for a short time, then disabled. 

On the outputs of channels I get a 5-15mV bump which
lines up, more or less, with the addressing of the
mux. But here is the wierd part. The first channel has
a bump for 1/8 the total scan time for all 8 channels.
The second channel has a bump for 1/4 the time, the
third for 3/8 the time, etc. The 8th channel is
different: it has a ramp for the total scan period
that is about 10mV.

There is a resistor between the DAC opamp and the
input of the 4051. Changing the resistor affects the
amplitude of the bumps. Changing to a MAX4051 had no
effect. 

I'll post a schematic as soon as I can transcribe my
scribbles into Orcad.

--TR
 

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