[sdiy] Sample and Hold

Ben Bradley ben.pi.bradley at gmail.com
Fri Jun 8 23:46:47 CEST 2018


I'm comparing the data sheets, and I wonder that the  AD781 may be "too
good" in some respects for synth use, and not good enough in others. It has
an internal capacitor, unlike the  LF398 which uses an external one (that
you can replace with different values/types to fit the app), Its sample
rate is more than fast enough for audio, but the droop rate spec of 1uV/uS
looks bad for sub-audio sampling (as in generating slow stepped control
voltages) - I read that as one volt per second! Looking at online prices,
it's also a lot more expensive than the Natio ... TI part, even considering
that it's from Analog Devices.

Reading the LF198 datasheet reminds me of the "capacitor soakage"
article(s) by Bob Pease. He even developed circuits to (somewhat)
counteract the effect. If high precision and long hold time is needed, I
think a two-stage circuit (small capacitor in the first, larger capacitor
in the second) might be workable.

For a synth module, I'd expect a S/H device to work well from as low as 50
uS sampling (of a 20kHz waveform) and as long as a minute hold time (a long
sustained CV for a VCO), to cover all possible applications.


On Thu, Jun 7, 2018 at 5:49 PM, Tom Bugs <admin at bugbrand.co.uk> wrote:

> If anyone would be interested in taking a stash of the similar Analog
> Devices AD781 chip..
> I probably have 20+ unused DIP + some working pulls.
> Happy to work something out if anyone is interested in making use of them.
>
> Tom
>
> On 07/06/2018 22:44, Tim Ressel wrote:
>
> Make sure to buffer the signal going in to the LF398. It will not work
> well if driven with a soft source.
>
> --timmers
>
> On 6/7/2018 9:21 AM, Elain Klopke wrote:
>
> Hey all,
>
> In my poking around the internets for sample and hold circuits, I ran
> across the Texas Instruments LF398 monolithic sample and hold. It looks
> promising, +/-5 to +/-18V supply range, up to +/-18V input range, TTL and
> CMOS compatable logic input, all in a DIP8 package.
>
> Does anyone have any experience with this chip?
>
>
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> Circuit Abbeytimr at circuitabbey.com
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