Fine Tune Your Signal Chain with Integrated Digital Potentiometers

Digital potentiometers are components used to adjust resistance within a mixed signal design. This allows use in mixed-signal circuits where dynamic tuning is desirable. Dynamic tuning of circuits may be used for one-time calibration of hardware in the factory or for real-time adjustment of volume, sensor tuning, op-amp feedback compensation, or contrast/brightness adjustments in LCD screens.

Microcontrollers and memory are used to dynamically tune circuits with the use of digital potentiometers. Measurements from sensors or other circuit blocks are read, converted, and stored into memory. Information stored into memory is used by the microcontroller to calibrate circuit blocks for intended use. In this way, digital data is converted into analog voltages or currents to accurately drive volume or brightness, or whatever your circuit is wanting to refine on the fly.

Digital-to-analog conversions occur when using digital potentiometers. Bit streams from the microcontroller represent step adjustments in voltage or current along the signal chain. Consider tolerances when choosing a digital potentiometer for your application as variation causes nonlinearity in the transfer function. Nonlinearity can reduce tuning accuracy causing your circuit to fail. Most vendors include modes to help compensate for irregularities.

Digital Potentiometers Effect Tuning in the Signal Chain

Potentiometers originated as mechanical devices able to manually tune circuits by varying resistance. Think of the volume dial in your car radio. They operate employing Ohm’s Law which states that current is directly proportional to the voltage across two points in a circuit. The proportionality constant is resistance. If you change the resistance across two points, you change the value of the current with respect to the voltage present on the node.

Accurate tuning happens with digital potentiometers

When mixed-signal circuits came about, microcontrollers used digital bit streams to automatically vary resistance across two points. Able to read and measure nodes for storage, and further analysis led to the microcontroller’s ability to analyze and fine-tune circuits without the need for human intervention. Able to move information quickly at high transmission speeds further enables computers to dynamically tune circuits while operating.

Digital potentiometers are used by computers to adjust circuits dynamically within changing environments. Think about cars moving down the road, its sensors acquiring positional information. Circuits are required to respond to new information every split second. Computers are able to perform in these environments and use digital potentiometers within the signal chain to effect tuning by varying proportional resistance between two points.

Consider Memory and Linearity When Choosing Digital Pots

If your circuit has need of automatic tuning, you may want to consider adding a digital potentiometer within your design’s signal chain. Things to consider include whether you’ll need to make the adjustment once or whether you’ll want to continuously tune your circuit during operation.

Tuning a guitar with a digital potentiometer in the signal chain

For one-time calibration of hardware within the factory, you can choose a digital potentiometer able to program a fixed value for lifetime operation, and we look at one below. For dynamic and continuous tuning during lifetime operation, you’ll want to consider a digital potentiometer able to adjust its relational resistance continuously. For this type of operation, we consider two possible parts below.

Calibration requires digital-to-analog conversions that follow predictable behavior. Predictable behavior in proportional relationships best occurs when the response is linear. Variation in tolerance effects linearity and vendors offer varying tolerance specifications along with biasing schemes to get the linearity needed for your design needs. Knowing you circuit accuracy needs will help you find a good solution at a competitive price.

Let’s take a look at a few digital potentiometers below.

Analog Devices, AD5144

This part varies up to 100kΩ with 8% tolerance variation. It incorporates embedded EEPROM and a dedicated register for setting the wiper of the potentiometer. The register may both be written to and read from by the controller in your signal chain. Analog Devices provides several modes for linear gain setting to enable accuracy for conversions.

The AD5144 digital programmable potentiometers are designed to operate as true variable resistors for analog signals within the terminal voltage range of VSS < VTERM < VDD. The resistor wiper position is determined by the RDAC register contents. The RDAC register acts as a scratchpad register that allows unlimited changes of resistance settings. A secondary register (the input register) can be used to preload the RDAC register data.

Shift Register and Timing Diagrams for AD5144 Found on page 11 of AD5124/AD5144/AD5144A datasheet

This part has several package options including 4mm x 4mm Lead Frame Chip Scale Package or alternatively the rectangular Thin Shrink Small Outline Package. To investigate performance response and variation, Analog Devices offers an evaluation board, the EVAL-AD5144. The evaluation board allows you development space to drive the 256-position, non-volatile memory while investigating the best linear mode for your application.

Maxim Integrated, MAX5527

This is a one-time programmable part suitable for factory calibration of hardware without need for dynamic tuning. Two pins are dedicated for one-time programming by your factory test equipment. After programming, a fuse is activated, and the pins may be disabled to prevent tampering after the hardware leaves the factory. The MAX5527 offers resistance adjustments up to 100kΩ using 64 tap positions.

The MAX5527 linear-taper digital potentiometer performs the same function as mechanical potentiometers, replacing the mechanics with a simple 2-wire up/down digital interface. These digital potentiometers provide an optional one-time programmable feature that sets the power-on reset position of the wiper. Once the wiper position is programmed, the 2-wire interface can be disabled to prevent unwanted adjustment.

AD5527 Functional Block Diagram Found on page 1 of Maxim One-Time Programmable, Linear-Taper Digital Potentiometers datasheet

The part comes in 8-pin, 3mm x 3mm, Thin Dual Flat No Lead, or a proprietary, 5mm x 3mm, µMAX^® packaging. Both are small form factor, using very little real estate in your layout. In addition, Maxim Integrated offers a development board to investigate programming functions and ensure you get precision in the factory before activating the one-time fuse. The evaluation board, MAX5527EVKIT, is listed on our website.

Microchip MCP4661

This part is fully programmable with read and write functionality to its registers across a full 8-bit array, similar to the Analog Devices part listed above. It, too, offers single or dual resistor network options with several choices to effect accurate linearity. Its serial protocol allows high-speed read or write options to memory using SPI or I2C interface interoperability. The part also allows further control with separate enable/disable pins.

The MCP4661 uses an I2C interface. This part supports 8-bit resistor networks, nonvolatile memory configurations, and Potentiometer and Rheostat pinouts. WiperLock Technology allows application-specific calibration settings to be secured in the EEPROM.

I2C Increment Command Sequence Found on page 63 of MCP454X/456X/464X/466X datasheet

The 4661 comes with several packaging options which is good for customizing to your particular circuit block PCB real estate considerations. In addition, Microchip offers the MCP46XXEV Evaluation Board so algorithms may be fully tested at the bench before integration into final system blocks.

Whatever your need for tunable functionality within your mixed design signal chain, our website has the part. Our database contains over 50 million parts, along with tools to help find similar parts and to find best sourcing and costing.

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