Potentiometer Input Impedance For AtoD Inputs

This is based on the discussion found here.

AtoD inputs will be specified with a maximum source resistance for them to function correctly to all of their specifications.  The source resistance of a pot is highest at its midpoint setting, but even at that position it is only 1/4 of the pot total resistance.  This means a 10K pot has an effective source resistance at its mid position of only 2K5 ohms.

If you don't need very high speed readings you can often allow for higher source resistances (and lower noise) by adding capacitor from the AtoD input to ground which has a much larger capacitance than an AtoD's internal sample and hold capacitance (e.g. 100nF).  This capacitor is charged to the pot voltage continuously (and also acts as a RC low pass filter).  The capacitor transfers its charge to the AtoD internal capacitor very quickly when the ADC is switched to that signal, with little voltage change.  

The impedance on the pin is as seen by the pin. 8V going through a potential divider made of two 5K resistors will make an impedance of 10K as far as the 8V source is concerned, but only 2K5 as far as the pin is concerned. 

A consideration though:

The impedance limitation due to AtoD channel switching can also be countered simply by waiting longer to take a reading, if your AtoD allows it. However, the DC leakage current at the AtoD input pin itself can present a problem and if so no amount of waiting or adding capacitance will fix that. Pin leakage is not reduced when you play tricks with AtoD inputs and can be temperature and voltage dependent so always bear this in mind.

Potentiometer Types

Good resources:

sound.westhost.com/pots.htm

Carbon

Good general use pot type.

Cermet

Ceramic/metal composite.
Good temperature stability, so often a good choice for trim pots.
Can have low operations life – check datasheet

Conductive Plastic

Usually high quality pots with good life, low noise and good mechanical feel

Wire Wound

High power and long fife, but resistance changes are in small steps as wiper moves from 1 wire to the next.

 

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