Isolated potentiometer design (Part 1)

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鈭? 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, V_PW as a fraction, a, of the voltage across the pot,

as

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

At full-scale (a = 1), i_O x R_O = V_P and a is varied by varying i_O using a multiplier. The differential V/I converter converts V_P to a current input to the multiplier, MULT. This is the full-scale i_O 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 鈭扸_EE), pot voltages can be readily extended to negative voltages. The scheme is shown below.