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Isolated potentiometer design (Part 1)

Posted: 17 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:potentiometers? digipots? anapots? isopot? power supply?

Digitally-actuated potentiometers, or digipots, are commercially available from multiple suppliers but not analogue-actuated potentiometers, or anapots. The following isolated anapot, or isopot, design is intended as a design "template" for general application wherever a manually-actuated pot is to be replaced and actuated by an analogue voltage.

A manual potentiometer has three terminals, labelled VP+, the highest-voltage end, VP? the lowest-voltage end, and VPW, the wiper. The pot is emulated by the following simplified circuit.

The goal is to duplicate the pot wiper voltage, VPW as a fraction, a, of the voltage across the pot,

as

where a ranges from 0 to 1. The above circuit outputs VPW as the voltage,

At full-scale (a = 1), iO x RO = VP and a is varied by varying iO using a multiplier. The differential V/I converter converts VP to a current input to the multiplier, MULT. This is the full-scale iO that is scaled by the other MULT input, from a V/I converter that converts the control voltage (controlling a) to a multiplier input current.

The following design is limited to non-negative terminal voltages, though by using a negative power supply (instead of ground for −VEE), pot voltages can be readily extended to negative voltages. The scheme is shown below.


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