Digital Potentiometers

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What are digital potentiometers, and how are they used?
Digital potentiometers are integrated circuits that implement a resistive ladder and a digital means of addressing a particular tap on the ladder that corresponds to the wiper position of a mechanical potentiometer. They’re used to calibrate system tolerances or dynamically control system parameters. Some of them have no on-chip memory. Others incorporate nonvolatile memory for saving the wiper position. Still other digital potentiometers are one-time programmable (OTP).

Some digital pots make all three terminals (top, bottom, and wiper) available, providing a voltage divider function, as in a conventional mechanical pot. Others connect the wiper to the low-side or high-side terminal, providing a rheostat function. (Of course, a three-terminal digital pot can also be configured as a rheostat.) And, digital potentiometers are available in singlechannel or multichannel (up to six) configurations.

What kinds of applications are appropriate?
Any kind of application that would normally use a trim pot or require precise resistor matching is a candidate. As an example, consider a state-variable filter (Fig. 1). All parameters are independent and tunable, so designers could implement electronic control of frequency, Q, and cutoff frequency via digital potentiometers for R1 through R4. Digital pots are finding wide use in LCD panels for VCOM adjustment and panel contrast/brightness control, programmable power supplies, RF amplifier biasing, and automotive electronics.

What advantages do digital potentiometers have over mechanical pots?
Obviously, digital pots can be operated in a closed control loop, and they don’t require physical access for adjustment. In addition, they offer higher resolution than mechanical pots, along with better reliability and stability, faster adjustment, better dynamic control, and a smaller footprint. Being digital, they potentially provide additional functionality as well.

What are some typical operating characteristics?
Resolution is specified in terms of number of positions, from 32 to 1024. The most common arrangement is 256 positions (8 input bits). Programming is generally serial, via I2C or serial peripheral interface (SPI) bus, with pushbutton and up/down interfaces also available. End-to-end resistance values start at 1 kΩ and run to 1 MΩ. Resistor tolerance is typically ±20%.

The single-ended dc voltage range is most often 5.5 V, though some devices are spec’d for 30 V. Some digital pots are intended to handle bipolar or ac signals. Most of these are spec’d for ±3 V. A few are rated for ±15 V. Operating current can be as low as 0.01 µA.

What are some advantages of on-chip memory?
Products built using digital pots with memory can be factory-programmed or calibrated and shipped. This removes the requirement for an onboard microcontroller. Or if there is a microcontroller in the design, freeing it of pot-control functions reduces the coding and memory required and leads to faster power-up times.

What’s new in digital pots?
Recent advances have made it possible to economically fabricate digital pots with 1% end-to-end resistor tolerances. Some recently introduced parts can retain a factory-measured resistor tolerance value in memory. External software can access and use the stored value for compensating for resistance errors when computing wiper settings.

Both are significant because, until recently, end-to-end tolerances have been limited to ±20% to ±30%, which is a major drawback when the digital pot is used in ratiometric circuits with a fixed external resistor or in circuits with multiple digital pots. Positive absolute resistance errors will result in a loss of resolution, while a negative absolute resistance error will reduce range in most applications.

Until now, there have been only two ways of dealing with these limitations: multichannel digital pots, which typically have 0.1% channel-to-channel matching (Fig. 2), or factory calibration, with the errors measured during board assembly and stored in memory for readback and customer calibration. New devices with ±1% end-to-end resistor tolerance now offer greater flexibility for system designers and avoid the suboptimal solutions previously implemented.

Recent digital potentiometers include arrays of OTP memory words that allow the wiper position to be programmed in memory multiple times. This feature is ideal for factory programming applications where calibration may require more than one adjustment step.

What does a designer need to watch out for in designing with digital potentiometers?
The voltage that can be applied across the resistor network is limited to the range of the supplies. The system designer must restrict the digital code range to ensure that the maximum voltage input signal divided by the minimum resistance does not exceed the maximum current allowed.

Product Q&A

1% Resistor Tolerance DigiPots Simplify Designs
Analog Devices offers a broad portfolio of digital potentiometers offering different interfaces, resolutions, end-to-end resistances, and memory options. These integrated-circuit digital potentiometers can be used to adjust and trim electronic circuits similar to variable resistors, rheostats, and mechanical potentiometers.