Engineer's Classroom: Common Problems and Application Experience Summary of Digital Potentiometers

Digital potentiometers are becoming more and more important to designers. They have many advantages, but also many limitations. The following compares the similarities and differences between mechanical potentiometers and digital potentiometers to help readers understand how to use digital potentiometers.

Potentiometers have a long history and are used in a variety of ways in a wide range of applications such as constant adjustment and measurement. The most common use is to set and fine-tune resistance values ​​to fine-tune circuits, set levels and adjust gain . Potentiometers are also used to design position feedback in robots and industrial equipment. Potentiometers have various aspects to consider and need to be set according to the various requirements of a specific application. Such as the maximum voltage across the potentiometer, the maximum current that each arm can provide, the maximum power that can be consumed and the most important resistor issues. Everything from power to noise. The error of a single resistor is usually +/-20% to +/-5%, and temperature will also cause the resistance value to drift, so it is necessary to consider the accuracy of the potentiometer, linearity, monotonicity or not, and whether to consider other factors in the design. For example, the frequency response of the human ear to sound will be more important. The change in resistance when power is off and on, cost and size, and reliability such as assembly, moisture, etc.

Among Edison's more than 1,000 inventions, the potentiometer is often forgotten. It was invented in the 1870s and used in switches. See Figure 1.

Over the past 100 years, as materials and shapes have changed, mechanical potentiometers have received great attention in some primary applications. There are undoubtedly many differences between mechanical potentiometers and digital potentiometers, but their commonalities are surprising. The biggest similarity is that they are both adjustable and can provide a wide range of end-to-end resistance.

Mechanical potentiometers can withstand high voltages of thousands of volts, while digital potentiometers are limited by their small size and usually have voltages of less than 30 volts. Mechanical potentiometers also have larger resistance capacity than digital potentiometers. However, we can solve the above problems with a little consideration. Mechanical potentiometers will cause problems in the design when resistance drifts due to vibration. The contact points of mechanical potentiometers will increase resistance or fail due to wear and aging, making the performance of mechanical potentiometers unpredictable. Digital potentiometers do not have the above problems caused by mechanical structure, and can remain consistent after tens of thousands of switching operations.

Digital potentiometers usually use polysilicon or thin film resistor materials, which have low noise, high accuracy and excellent temperature coefficient.

The size comparison between mechanical potentiometer and digital potentiometer is shown in Figure 2.

Another significant advantage of digital potentiometers is programmability. They can be programmed to adjust resistance like EEPROM, can replace voltage followers, and can control or set voltage and current like digital-to-analog converters. The main parameter characteristics of digital potentiometers are shown in the figure below.

When using a digital potentiometer to set the voltage, if you need to limit the voltage output range, just add a resistor in series to the power supply loop of the digital potentiometer . The figure below shows how to change the output voltage range from 0 to 15V to 6+/-1V, where only resistors R1 and R3 need to be added.

 Circuits that use potentiometers to adjust amplifier gain are widely used, such as contrast adjustment in liquid crystal displays (LCDs), sensor calibration, and digital multimedia playback. Due to process reasons, the industry standard for end-to-end resistance error of mechanical potentiometers is +/-20%. When the resistance value is too large, the circuit resolution is reduced. When the resistance value is too small, the circuit adjustment range is reduced. As shown in the figure below. The gain fluctuation caused by this 20% error will cause serious consequences in open-loop applications due to the lack of compensation control. Digital potentiometers can achieve a matching accuracy of 1% for channel resistance, thereby effectively solving the gain fluctuation problem caused by resistance error.

In digital audio applications, digital potentiometers have largely replaced mechanical potentiometers because of their high reliability, digital control, easy conversion between linear and logarithmic, and better stability. The engineering challenges are:

The audio signal voltage range should be within the circuit power supply range, that is, it should not be higher than Vdd nor lower than Vss.

The power-on sequence is: first the power ground and positive and negative power supplies, then the digital signal, and finally the A, W, and B ports of the built-in ESD of the digital potentiometer.

Potentiometer end-to-end resistance error problem. The potentiometer can be connected in the circuit as a resistor divider, so that the output of the potentiometer depends on the position of the cursor and has nothing to do with the resistance error. As shown in the figure below.

Potentiometer cursor value problem when power is turned on . For mechanical potentiometers, as long as the cursor position is not changed, the cursor value remains unchanged after power is turned off and then back on. The situation is different for digital potentiometers. Some digital potentiometers have built-in EEPROM to record the cursor value, and the cursor value remains unchanged after power is restored. Some digital potentiometers automatically set the cursor to the middle value after power is restored. Some digital potentiometers set the cursor to a random value after power is restored. This requires users to carefully read the relevant specifications and cannot be generalized.

Power-on noise problem. When the audio circuit is powered on or the circuit is switched, it is easy to make a "pop" sound in the speaker due to the sudden change of voltage, which is a noise in terms of sound quality. Some digital potentiometers have a built-in zero-crossing circuit, so that the audio circuit is powered on or the circuit is switched at the voltage zero point, thereby avoiding the sudden change of voltage and eliminating the "pop" sound.

Volume adjustment uniformity problem. The human ear actually responds to audio in a logarithmic rather than linear manner. Most mechanical potentiometers are designed to be linear, so when adjusting the volume, the sound level does not increase or decrease uniformly. Digital potentiometers can be designed logarithmically, that is, in dB, so that the volume adjustment uniformity problem can be solved without additional circuit design.

Digital potentiometers can also be used in digital filter circuits. The following figure shows the circuit diagram and calculation formula provided by Analog Devices. It should be noted that the bandwidth limitation of the digital potentiometer itself is related to the setting of the cursor value. Please refer to the manufacturer's application manual for details.

Digital potentiometers cannot completely replace mechanical potentiometers. The reasons include the input voltage of digital potentiometers must be limited between Vdd and Vss, current limitation (such as @1K=5.5mA@10K=0.55mA, please refer to the datasheet for details), power-on sequence requirements, power-on initialization, EEPROM considerations, digital interface considerations, resistance value cannot be too large, response time must be considered when tracking input signals to adjust gain, etc.

Conclusion: The biggest disadvantage of digital potentiometers compared to mechanical potentiometers is that they cannot currently handle high voltage and high current, but there are many other advantages that allow electronic engineers to develop more new functions and reduce costs.