Digital Pot
A digital potentiometer (digipot) emulates a 3-terminal mechanical pot using an integrated resistor array and CMOS switches. The wiper position is set by a digital interface (I²C or SPI) and is preserved across power cycles in EEPROM-backed parts.
The IC contains a series string of N matched resistors with N+1 tap nodes. A wiper-decoder selects one tap node to connect to the wiper pin via a CMOS analog switch; the user writes the wiper code (typically 7–10 bits) over a digital bus. EEPROM-backed parts (e.g. Microchip MCP4661) store the last position so the pot returns to its setting at next power-up.
In plain terms
A staircase of resistors with a CMOS multiplexer for the wiper; tell it 'step 64' and the multiplexer connects tap 64 to the wiper terminal.
Why designers use it
- Microcontroller-controllable gain and offset trim in instrumentation.
- Volume control in audio products without mechanical wear.
- Calibration trims in production lines that need to be set once and locked.
Best for
- Programmable gain
- Audio volume
- Production trim
Key specifications
- Resolution: 32 – 1024 steps
- End-to-end resistance: 1 kΩ – 1 MΩ
- Interface: I²C, SPI, or up/down
- Wiper current: ≤ 1 mA typ.
- Voltage: 2.7 – 5.5 V
When not to use it
- When the required analog voltage, current or resistance behavior falls outside the selected device's operating limits.
- When the application's nonvolatile save frequency would exceed the chosen EEPROM endurance rating.
- When the startup setting, resolution, interface or bandwidth cannot meet the application requirements.
Common mistakes
- Applying analog terminal voltages outside the selected part's permitted range.
- Treating typical wiper resistance as a guaranteed minimum or ignoring its dependence on voltage and temperature.
- Exceeding terminal current or resistor-network power limits.
- Writing every control-loop adjustment to EEPROM instead of checking the difference between volatile adjustment and nonvolatile storage.
Where you will find it
- Hi-fi DAC designs use 256-step digital pots in the analog gain stage so a single rotary encoder on the front panel can command both channels at once with no scratchy carbon track.
- Lab analytical instruments (multimeters, spectrum analysers) use digital pots in the calibration network: a production tester writes the trim value once, and the EEPROM keeps the setting for life.
- An automatic gain control (AGC) loop in a software-defined radio uses a digital pot at the IF amp input — the firmware adjusts the wiper to keep the ADC's headroom regardless of antenna conditions.
A short history
Microchip's MCP454X/456X/464X/466X documentation distinguishes volatile wiper registers from nonvolatile EEPROM storage. This lets a circuit make temporary adjustments and separately save a startup setting. The digital interface does not turn the resistor terminals into unrestricted analog connections: their voltage and current limits still apply.
Good to know
- Changing a volatile wiper register and saving a setting to EEPROM are different operations. Do not save every adjustment to nonvolatile memory without checking its write-endurance limit.
- Microchip lists 75 Ω as a typical wiper resistance for the cited family, not a universal minimum for all digital potentiometers. Its value depends on operating conditions.
- No moving wiper does not mean no failure modes. Terminal voltage, current, temperature and memory endurance still have limits.