[sdiy] Servo / Hold?

[ASSI]

On Wednesday 12 February 2003 21:02, patchell wrote:
>     In order to have a "feedback" circuit, you need a reference.  A
> Sample and hold circuit is usually used with a signal that is not
> going to be around for very long, so the only reference you would
> have concerning the voltage you are trying to hold would be on the
> capacitor itself....

If you know what the leakage current is and where it flows you can (at 
least partly) compensate for it. Input current compensation for 
instance has been done for OpAmp input stages for ages. Since there's 
always leakage that you don't know about beforehand, you can improve 
hold time, but of course not infinitely.

>     Your best bet is to try and minimize leakage currents going to
> the cap, primarily from the switch and buffer amp...but doing it
> digitally with an A-D/D-A will give you that infinate hold time at
> the expense of quantizing errors (TANSTAAFL)...

This is a commercial product already:

AD5532 and AD5516 Infinite Sample&Holds (14bit resolution in S&H mode).

If I find out how to handle the MapBGA they're housed in I'll 
definitely get a bunch of those. I'm currently thinking of 
doing/getting a MapBGA->PGA adapter. Some variants can also be used as 
12bit (with slewing to 14bit) resolution DACs, which makes them 
extremely interesting for MIDI->CV conversion.


Achim.
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