[sdiy] Sample and Hold

[Tim Ressel]

Maybe its time to breadboard the beastie?


On 6/8/2018 2:46 PM, Ben Bradley wrote:
> 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 
> <mailto: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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>>
>>     -- 
>>     --Tim Ressel
>>     Circuit Abbey
>>     timr at circuitabbey.com <mailto:timr at circuitabbey.com>
>>
>>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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