[sdiy] simple s&h problem

[simon.oo.o at xs4all.nl]

Hi Karl,

Karl Ekdahl zei:
> hi list, i'm trying to do a simple s&h with two opamps
> (tl0x1) a mosfet (bs107), a 1uF cap and misc
> resistors. the problem is that it drops inbetween
> 10-20mV *every time* i stop sampling, it's reallly
> annoying....
>
> so, just generally, should this setup work with a
> +-1mV error tolerance? it might be my psu maybe....

I could think of two effects,
1) the capacitor memory effect which has been mentioned already.
Polystyrene  would be best, you could also try polycarbonate.

2) gate to channel capacitance of the MOSFET. When you switch the MOSFET,
the voltage transition on the gate causes a small amount of charge to be
transferred into the hold capacitor.
However, for the BS107, the gate capacitance (at Vds=0V) is on the order
of 100 pF. If your gate control voltage is (for example) 0-10V then this
would result in an offset of 10V*100pF/1uF=1 mV. So this alone does not
seem t explain the observed 10-20mV drop.

Vriendelijke groet,

Simon Brouwer
--> nl.openoffice.org <--