Digital potentiometer

A digital potentiometer (informally a digipot) is a digitally controlled electronic component that mimics the analog functions of a potentiometer. It is often used for trimming and scaling analog signals by microcontrollers. It is either built using an R-2R integrated circuit or a Digital-to-analog converter, with resistor ladder construction being the most common. Every step on the resistor ladder has its own switch which can connect this step to the output terminal of the potentiometer. The selected step on the ladder determines the resistance ratio of the digital potentiometer. The amount of steps is normally indicated with a bit value e.g. 8 bits equals 256 steps. A digital potentiometer is often controlled by digital protocols like I²C and SPI, as well as more basic Up/Down protocols. Some typical uses of digital potentiometers are in circuits requiring gain control of amplifiers (frequently instrumentation amplifiers), small-signal audio-balancing, and offset adjustment.

Sometimes this device is also referred to as an RDAC, Resistive Digital-to-Analog Converter.

Some digital potentiometers come with non-volatile memory, so that they retain their last programmed position after they have been power cycled. Most, though, are volatile, i.e. after they are power cycled they will default to a standard value, which is usually the midpoint.

The former can be useful, but when they are controlled by a microprocessor, or even via a Field Programmable Gate Array (FPGA), these devices can retain, in other non-volatile memory, the value to initialize the digital potentiometer with.

Limitations

Despite their usefulness, digital potentiometers have some limitations. While quite similar to a normal potentiometer, digital potentiometer is constrained by current limit in the range of tens of milliamperes. Also, most of digital potentiometers limit the input voltage range to the digital supply range (0–5 VDC), so additional circuitry is required to replace conventional potentiometer. Further, instead of the seemingly continuous control that can be obtained from a multiturn resistive potentiometer, digital potentiometers have discrete steps in resistance. Eight-bit resolution (giving 2⁸ = 256 steps) is the most common, but resolutions between 5 and 10 bits (32 to 1024 steps) are available. A fourth constraint is that special logic is often required to check for zero crossing of an analog AC signal to allow the resistance value to be changed without causing an audible click in the output for audio amplifiers.

The volatile digital potentiometers also differ from electro-mechanical ones in that on power up, the resistance will default to (possibly) a different value after a power cycle. Similarly, their resistance is only valid when the correct DC supply voltage is present. When voltage is removed, the resistance between the two end points and the (nominal) wiper are undefined. In an operational amplifier circuit, the off-state impedance of a real potentiometer can help stabilize the DC operating point of the circuit during the power-up stage. This may not be the case when a digital potentiometer is used.

Also, both electro-mechanical and digital potentiometers generally have poor tolerances (typically +/- 20%), poor temperature coefficients (many hundreds of ppm per degree C), and a stop resistance that is typically about 0.5-1% of the full scale resistance. Note that stop resistance is the residual resistance when the terminal to wiper resistance is set to the minimum value.

See also

• Potentiometer
• Console automation

External links

• Digital potentiometer - the Resistor Guide
• Designing with Intersil Digitally Controlled Potentiometers (XDCPs)
• FAQ