[sdiy] Pots read by microcontroller and sourcing question

[Richie Burnett]

http://fa.utfs.org/diy/jx3p/schemo/JX3P_schematics_06_pg-200.jpg

...shows a bunch of 100k linear pots fed through CMOS multiplexers, a 
unity-gain follower, and a "discrete" successive approximation ADC in 
Roland's PG-200 programmer.

I've not had any problems using 10k linear pots feeding PIC ADC inputs 
directly, provided that you put the 100nF capacitors between the pot wipers 
and ground.  This 100nF capacitor lowers the AC impedance at the input of 
the ADC and it is this that really matters.  It provides some local stored 
charge to "stiffen" the analogue voltage so that it doesn't sag dramatically 
when the ADC's sample and hold circuit takes its sample.

The total current consumption starts to add up though if you have lots of 
10k pots across a 5V or 3V3 supply!

-Richie,

-----Original Message----- 
From: David G Dixon
Sent: Monday, February 02, 2015 7:27 PM
To: 'Jack Jackson' ; 'Sdiy'
Subject: Re: [sdiy] Pots read by microcontroller and sourcing question

> Hi thanks for the replies.
>
> I'm using an Arduino. Looks like ideal input impedance of 10kb or less

If you use an opamp buffer in front of every pot, then the output impedance
to the uC is effectively 0 ohms.  This will cost you about 10 cents per pot,
so not really a bank-breaker.  I personally would never take a voltage
directly from a pot for this sort of control, since you really can't
guarantee linearity unless you feed the pot to a near-infinite impedance
such as the + input of an opamp.

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