Design Ideas: November 10, 1994
The circuit in Fig 1 looks simple but usually comes as a revelation to designers. Under control of the system's µP, the digital potentiometer, IC1, becomes a variable set-point reference that can adapt to the long-term drift of the variable measured. The digital potentiometer replaces schemes involving A/D and D/A converters (see "Autocalibrator nulls dc offsets," EDN, June 9, 1994, pg 139, and "Infinite-hold circuit zeros out long-term drift," EDN, March 3, 1994, pg 90).
Depending on how you configure IC1, its resolution will be 0.79, 0.53, or 0.40%. Cascading two devices yields 0.024% resolution. Internal EEPROM registers store the digital potentiometer's "wiper" position, allowing the system µP to determine the current set point, even after power outages. (DI #1615)