[sdiy] How droopy is too droopy in a S & H?

[George Hearn]

Indeed I would also say it is the application that really decides how droopy
is too droopy.  For example if you are refreshing the sample and hold every
1ms or so like you might when demultiplexing a single DAC to give multiple
channels, 2mV/s would give you a ripple of 2uVpp, a respectable figure.  If
the sample and hold is for lo frequency effects like modulation of a VCO
pitch then 2mV/s would give you a droop of 1/4 semitone every 10s if the VCO
is 1V/Oct.  G

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of rdrake
Sent: 26 January 2009 18:44
To: mverbos at earthlink.net; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] How droopy is too droopy in a S & H?

here's a page (http://mysite.du.edu/~etuttle/electron/elect53.htm) with
measurements for a lf398 and .01 polypropylene cap giving  droop of -3.0
mV/s... which seems w/in range of what's promised on th smp04.  those
components sound like typical sdiy S&H values, but i don't have my docs here
with me. 

th other question to consider is the application... if you're sampling a
keyboard in order to hold that note, you'd need much more stringent specs
than if you were doing bleep-bloop scifi sounds, or wogglebugginess... 

bbob

>----- ------- Original Message ------- -----
>From: mverbos at earthlink.net
>To: synth-diy at dropmix.xs4all.nl
>Sent: Mon, 26 Jan 2009 13:16:45
>
>yeah, probably.
>
>
>On Jan 26, 2009, at 1:14 PM, Tom Bugs wrote:
>
>> Just a thought, but I'd imagine that engineer
>desires for sample and  
>> hold would be over much shorter times than those
>for synthy people.
>> So, I'd have thought that 2mV per second would be
>a decent figure  
>> for 'general' use though much less so for 'our'
>use?!
>>
>> mark verbos wrote:
>>> I have been seduced by the Analog Devices SMP04
>quad sample and  
>>> hold chip. It has 4 independent S&H circuits
>with built in	
>>> capacitors, in a 16 pin chip. I claims a "low
>droop rate" of 2 mV  
>>> per Second. Doesn't that seem like a lot? How
>much does a normal  
>>> sample & hold droop? How much does an LF398 with
>polystyrene caps  
>>> droop? Is this a waste of time and money? The
>AD684 seems to be  
>>> about 10 time better, but also $50! It's hard to
>tell because they  
>>> don't use the same units for measuring the specs
>on all the chips...
>>>
>>>
>>> Mark
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