[sdiy] Paralleling lots of ADC inputs

Scott Gravenhorst music.maker at gte.net
Tue May 1 00:45:55 CEST 2018


Tom Wiltshire <tom at electricdruid.net> wrote:

>Thanks all.
>
>Perhaps Brian is right, and I should just use a single ADC input 
>at the master uP and then send the data via the comms link. After 
>all, vibrato isnt that fast, so it wouldnt imply a super-fast 
>sampling frequency. It seems like software might be a better 
>answer than hardware for this one. 
>
>Thanks,
>Tom

I agree, this seems like the best way from several points of view.  Digitally, each CPU gets
an exact copy of the data and avoids a bunch of extra messy analog circuitry (flame suit now
donned).

>==================
>       Electric Druid
>Synth & Stompbox DIY
>==================
>
>> On 30 Apr 2018, at 20:49, ASSI <Stromeko at nexgo.de> wrote:
>> 
>> On Monday, April 30, 2018 5:14:51 PM CEST Tom Wiltshire wrote:
>>> So - I need to tie together several ADC inputs. Can I just tie them
>>> together? Should I drive the tied-together-inputs with an op-amp follower
>>> or something to make sure Ive got a nice low impedance output? Or do the
>>> inputs themselves need buffers in front of them (e.g. tied-together-signal
>>> -> opamp buffer -> ADC input). Has anyone tried anything like this?
>> 
>> The datasheet should have some details what kind of signal conditioning the 
>> particular ADC is expecting, but it's usually a bad idea to just tie together 
>> multiple ADC inputs unless you can make sure they don't sample at the same 
>> time.  Most ADC inputs present a time-dependent impedance that can be quite 
>> low at the sampling point.  Also if capacitive sampling is used (very likely 
>> these days), the paralleled input capacitances will influence each other and 
>> degrade the ADC performance significantly.  At the minimum and only if your 
>> actual source is already reasonably low-impedance I'd put an isolation network 
>> in front of each input (RC lowpass is the most simple one).  An actual buffer 
>> amplifier is better, but you might need an auto-zero buffer for your 
>> application to get rid of offset voltages.
>> 
>> 
>> Regards,
>> Achim.
>> -- 
>> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
>> 
>> Wavetables for the Terratec KOMPLEXER:
>> http://Synth.Stromeko.net/Downloads.html#KomplexerWaves
>> 
>> 
>> 
>> 
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>> Synth-diy mailing list
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>
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>class="">Thanks all.<div class=""><br class=""></div><div 
>class="">Perhaps Brian is right, and I should just use a single 
>ADC input at the master uP and then send the data via the comms 
>link. After all, vibrato isnt that fast, so it wouldnt imply a 
>super-fast sampling frequency. It seems like software might be a 
>better answer than hardware for this one.</div><div class=""><br 
>class=""></div><div class="">Thanks,</div><div 
>class="">Tom</div><div class=""><br class=""></div><div 
>class=""><div class=""> <div style="word-wrap: break-word; 
>-webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" 
>class=""><div style="color: rgb(0, 0, 0); font-family: Helvetica; 
>font-size: 12px; font-style: normal; font-variant-caps: normal; 
>font-weight: normal; letter-spacing: normal; orphans: auto; 
>text-align: start; text-indent: 0px; text-transform: none; 
>white-space: normal; widows: auto; word-spacing: 0px; 
>-webkit-text-stroke-width: 0px;">==================<br 
>class="">       Electric Druid<br 
>class="">Synth & Stompbox DIY<br 
>class="">==================</div></div> 
>
></div>
><div><br class=""><blockquote type="cite" class=""><div 
>class="">On 30 Apr 2018, at 20:49, ASSI <<a 
>href="mailto:Stromeko at nexgo.de" 
>class="">Stromeko at nexgo.de</a>> wrote:</div><br 
>class="Apple-interchange-newline"><div class=""><div class="">On 
>Monday, April 30, 2018 5:14:51 PM CEST Tom Wiltshire wrote:<br 
>class=""><blockquote type="cite" class="">So - I need to tie 
>together several ADC inputs. Can I just tie them<br 
>class="">together? Should I drive the tied-together-inputs with 
>an op-amp follower<br class="">or something to make sure Ive got 
>a nice low impedance output? Or do the<br class="">inputs 
>themselves need buffers in front of them (e.g. 
>tied-together-signal<br class="">-> opamp buffer -> ADC 
>input). Has anyone tried anything like this?<br 
>class=""></blockquote><br class="">The datasheet should have some 
>details what kind of signal conditioning the <br 
>class="">particular ADC is expecting, but it's usually a bad idea 
>to just tie together <br class="">multiple ADC inputs unless you 
>can make sure they don't sample at the same <br class="">time. 
> Most ADC inputs present a time-dependent impedance that can 
>be quite <br class="">low at the sampling point.  Also if 
>capacitive sampling is used (very likely <br class="">these 
>days), the paralleled input capacitances will influence each 
>other and <br class="">degrade the ADC performance significantly. 
> At the minimum and only if your <br class="">actual source 
>is already reasonably low-impedance I'd put an isolation network 
><br class="">in front of each input (RC lowpass is the most 
>simple one).  An actual buffer <br class="">amplifier is 
>better, but you might need an auto-zero buffer for your <br 
>class="">application to get rid of offset voltages.<br 
>class=""><br class=""><br class="">Regards,<br class="">Achim.<br 
>class="">-- <br class="">+<[Q+ Matrix-12 WAVE#46+305 Neuron 
>microQkb Andromeda XTk Blofeld]>+<br class=""><br 
>class="">Wavetables for the Terratec KOMPLEXER:<br class=""><a 
>href="http://Synth.Stromeko.net/Downloads.html#KomplexerWaves" 
>class="">http://Synth.Stromeko.net/Downloads.html#KomplexerWaves</a><br 
>class=""><br class=""><br class=""><br class=""><br 
>class="">_______________________________________________<br 
>class="">Synth-diy mailing list<br 
>class="">Synth-diy at synth-diy.org<br 
>class="">http://synth-diy.org/mailman/listinfo/synth-diy<br 
>class=""></div></div></blockquote></div><br 
>class=""></div></body></html> 
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- http://scott.joviansynth.com/
-- When the going gets tough, the tough use the command line.
-- Matt 21:22



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