Digital Potentiometer Circuit Design

A digital potentiometer is an array of switchable resistors with a tap that a serial command can move. It replaces a trimming screw with something a processor can set, and it brings a set of limitations that a mechanical part does not have: voltage range, current handling, bandwidth and a wiper resistance that changes with the tap position.

What Is Inside the Package

The element is a string of resistors with a switch at every node. Only one switch is closed at a time, so the wiper connects to a single tap and the resistance between the wiper and each end follows from the number of steps passed.

That structure defines the accuracy. A part with two hundred and fifty six steps has a resolution of about four tenths of a percent of the end to end resistance, and the integral non linearity over the range is quoted separately. A part advertised as ten kiloohms may be anywhere in a band around that value, so matching between two channels is not guaranteed.

The switches are field effect transistors and they have an on resistance, which appears in series with the wiper. That wiper resistance is a few tens of ohms and it varies with the tap position, with the supply voltage and with temperature.

Ratiometric Use Versus Rheostat Use

Used as a three terminal divider, the digital potentiometer is ratiometric: the output is a fraction of the voltage across the element, and the absolute resistance and its tolerance cancel. This is the mode in which these parts behave most like an ideal divider.

Used in rheostat mode, with the wiper and one end connected, the absolute resistance matters and so does the wiper resistance. The tolerance of the end to end resistance, often twenty percent, then appears directly in the result, and two channels of the same part will not match.

Choose the mode from the requirement. A feedback network that sets a gain wants a rheostat and a calibration step, while a reference divider or a level setting wants the ratiometric connection where the tolerances cancel.

Voltage and Current Limits

The signal across the element must stay within the supply rails of the part, and it must not go below ground. A digital potentiometer used in a circuit with a negative swing, such as the feedback path of an inverting amplifier, will conduct through the substrate and may be destroyed.

Current through the wiper is limited to a few milliamps in most parts, and the limit applies to the switch rather than to the resistor string. A design that drives a load directly from the wiper exceeds the rating, and a buffer amplifier is the correct answer.

The end to end current also has a limit, set by the power the thin film element can dissipate. A small package with a low resistance value carries a surprising current at five volts, and the resulting temperature rise shifts the resistance and the wiper.

Bandwidth and Capacitance

The element has capacitance to the substrate and the switches add capacitance at the wiper, so the bandwidth of a divider falls as the resistance rises. A hundred kiloohm part with a wiper capacitance of a few tens of picofarads gives a corner in the kilohertz region.

That makes a digital potentiometer unsuitable for signals above a few tens of kilohertz unless the resistance is low. An audio volume control works, a wideband filter does not, and the limitation is a property of the structure rather than of the particular part.

Where a wideband signal has to be attenuated, a multiplying digital to analog converter is the right component. It has a similar interface, a much higher bandwidth, and a cost that reflects the difference.

Settings, Storage and Start Up

A volatile part forgets its setting when the supply drops, so the firmware must write the value again at every power up. That is a problem when the setting is a calibration value that has been trimmed in production: the value has to be stored somewhere else.

A part with nonvolatile memory keeps the setting across a power cycle and loads it automatically. The write endurance of the memory is finite, so the firmware should only commit changes when the value has settled, not on every step of a ramp.

A third option is a part with a defined power on value, usually the midpoint. That is enough for a safe default, and the firmware can then adjust from a known starting point rather than from an unknown one. Whatever the choice, the start up state should be defined in the design description so that the behaviour after a brownout is predictable.

Interface and Control

The interface is a two wire serial bus or a three wire up and down scheme. The two wire version shares the bus with other devices and needs an address, while the up and down version is simple and needs a chip select and two pins from the controller.

The command latency matters in a control loop. A write over a slow bus takes tens of microseconds, and a loop that adjusts the tap on every cycle will be limited by the bus rather than by the process. Update the tap at a rate the loop can tolerate and filter the value.

Some parts include a shutdown pin or a mute function that opens the wiper. That is useful for a pop free start up in an audio path, and the behaviour of the wiper during the transition should be checked rather than assumed.

Layout and Decoupling

Decouple the part at its supply pin and keep the digital lines away from the analogue terminals. The element is a resistor string with a capacitance to the substrate, so a digital line running underneath couples switching noise into the signal path.

Keep the ground return of the divider separate from the return of the digital section and join them at one point. The current in the element is small, but the divider is often used in a feedback path where any error appears directly in the output.

Where the part sets a gain, place it close to the amplifier it controls. The trace between the wiper and the amplifier input adds capacitance and picks up noise, and the same reasoning applies as for any feedback network. The wider layout discipline for analogue control nodes is covered in our guide to mixed signal board design.

Calibration and Verification

Verify the part by stepping through its taps and measuring the output. A plot of measured fraction against commanded tap shows the integral non linearity directly, and an unexpected jump at one code points to a damaged switch.

Check the temperature behaviour if the setting is critical. The wiper resistance rises with temperature and the end to end resistance has its own coefficient, so the ratio in a divider drifts less than the absolute value in a rheostat.

Store the production trimming values with the unit serial number. When a board is repaired, the values can be restored without repeating the whole calibration, and a drift in one unit can be spotted against the record of the others. The release checks that capture these details are collected in our PCB design release checklist, and the assembly points we inspect are listed in judging PCB quality.

FAQ

Can a digital potentiometer replace any mechanical trimmer? Only within its voltage and current limits. The signal must stay inside the supply rails and the wiper current is limited to a few milliamps.

Why does my digital potentiometer read a different value from the setting? The end to end resistance has a wide tolerance and the wiper resistance is in series. Use it as a ratiometric divider where the tolerances cancel.

Does the setting survive a power cycle? Only on a part with nonvolatile memory. Otherwise the firmware has to rewrite the value at start up from stored calibration data.