Odp: [sdiy] DAC for MIDI2CV

[Seb Francis]

>
> I've really no experience of trying to make a precision S&H, so (as always) any tips would be greatly appreciated.  In particular I'm unsure what the cause of Linearity Error is in an S&H - this seems significant in some S&H IC datasheets.  Obviously I need to use precision opamps (especially for an open-loop S&H), but I have no idea what the best caps are.  I'm guessing low leakage is of some importance, although I will be refreshing the S&H very often, so maybe there's some other attribute of the caps which is more important?
>
> [I've seen in Tony Allgood's MIDI DAC that he uses 10n polyester caps with precision opamps (at least for the pitch), although I've read that polycarbonate and polypropylene caps have lower leakage]

And now I've also found this on the internet which suggests polystyrene caps ..

http://www.electrocube.com/bulletins/APPLICATIONS%20GUIDE.htm
<quote>
TIMING, SAMPLE & HOLD

In this type of application, the capacitor is used as a temporary storage cell until the time constant is reached or, in a sample and hold circuit, until the next sample is taken. To change the voltage across a capacitor, it is necessary to change the stored charge which takes a finite time. This phenomena is put to use in timing circuits, such as oscillators, signal generators, and latch timers. Capacitors selected for this application must have extreme capacitance stability, high insulation resistance, relatively low ESR, and a low dielectric absorption. The polystyrene capacitor would be the proper device to select for this critical application.
</quote>

My guess is that capacitance stability is not so important for S&H, therefore I'm not sure if polystyrene are really necessary.

Seb