Replace mechanical potentiometers with digital potentiometers

introduction

The reliability of digital potentiometers is much higher than that of mechanical potentiometers, and they can easily guarantee more than 50,000 reliable read and write times, while the number of repeated adjustments of mechanical potentiometers can only reach thousands or even hundreds of times. The resolution of digital potentiometers ranges from 32 levels (5 bits) to 256 levels (8 bits) or higher. For applications such as LCD contrast adjustment that do not require high dynamic range, choosing a lower resolution device can meet the requirements of actual applications. At present, some high-resolution digital potentiometers have become an ideal choice for high-fidelity applications such as audio, and can provide a dynamic adjustment range of up to 90dB.

Non-volatile

Some applications require nonvolatile storage in digital potentiometers, and both types of devices (volatile and nonvolatile memory) are popular in the market. Nonvolatile digital potentiometers are closer to mechanical potentiometers in that they can maintain resistance values ​​under different external conditions (external power supply or not).

Audio equipment needs to store volume settings internally, and the potentiometer must maintain the same resistance value when the device is powered on again, even when the power is completely turned off.

The MAX5427/MAX5428/MAX5429 family of digital potentiometers offers a unique programming feature. These devices feature one-time programming (OTP) memory that sets the power-on reset (POR) position of the potentiometer wiper to a user-defined value (the wiper position remains adjustable, but always returns to the fixed setting position after power is cycled). In addition, the OTP can also disable interface communication, latching the wiper to the desired fixed position and preventing further adjustment. In this case, the device becomes a fixed-ratio resistor divider rather than a potentiometer.

Audio Design Considerations

Potentiometers have logarithmic and linear taps. Logarithmic potentiometers are generally used for volume adjustment in high-fidelity audio equipment because logarithmic tapers can achieve linear volume adjustment considering the nonlinear filtering characteristics of the human ear. Currently, highly integrated digital potentiometers can integrate six independent potentiometers in a single chip to support multi-channel audio systems, such as stereo and Dolby surround sound systems.

In audio applications, especially when the digital potentiometer has a low adjustment resolution (32 levels), special attention should be paid to the change process between tap levels. If the tap does not change at 0V, the audio system will produce clicks and pops (Figure 1). Fortunately, the new generation of digital potentiometers has a so-called zero-crossing detection function that can reduce audio noise when the tap jumps. The internal zero-crossing and timeout detection circuit ensures that the tap jumps after detecting the zero-crossing (0V) signal or after a 50ms delay (depending on which condition occurs first).


Figure 1. The impact of audio clicks and pops when switching at 0V level

In addition to the analog circuits in the digital potentiometers mentioned above, each digital potentiometer also contains a digital interface. Most potentiometers are programmable via conventional I²C or SPI™, and some offer a convenient up/down adjustment interface.

Performance Improvements

Digital potentiometers offer another advantage over mechanical potentiometers. The adjustment taps of digital potentiometers are mounted directly in the signal path of the circuit board, and the use of electronic adjustment avoids the complex and expensive mechanical adjustment device. Digital potentiometers have improved noise suppression indicators and eliminate the noise pickup of the interface cable of mechanical potentiometers.

Traditional digital potentiometers can directly replace mechanical potentiometers and have the same working method without much explanation. However, in some special applications, such as low-cost stereo volume control, some additional explanation is required. For this special application of audio, it is generally required to operate over a wide voltage range to support a wide range of audio signals. Logarithmic taps are generally selected. As the number of tap levels increases, the attenuation decibel number increases accordingly, which is very suitable for the frequency response characteristics of the human ear. Some devices have a mute function to provide greater attenuation (for example: 30dB).

Temperature considerations

One of the typical parameters of digital potentiometers is the temperature coefficient (TC), which is defined over the rated temperature range. Most potentiometers require two different TCs to be defined. One is the absolute end-to-end TC, which represents the absolute value of the resistance change with temperature and is calculated as follows:

ΔR = RUNCOMP × TC × ΔT/106

where:
RUNCOMP is the uncompensated resistance value,
TC is the temperature coefficient, and
ΔT is the temperature change.

For example, a digital potentiometer with a resistance of 20kΩ and an absolute TC of 35ppm will produce a resistance change of 35Ω (0.2%) over a temperature change of 50°C. In addition, the initial value of the 20kΩ end-to-end resistance may vary significantly, ranging from 15kΩ to 25kΩ. In this case, for a 32-tap potentiometer, the corresponding resistance value (increment) may be 470Ω to 780Ω. This change is much larger than the deviation of the absolute TC.

Another typical TC is the resistance ratio TC. Potentiometers are often used as voltage dividers, especially in ratiometric designs where the requirements for absolute resistance change (absolute temperature coefficient) are less stringent than for ratio change. For example, a 5ppm ratiometric TC can achieve very stable gain over the entire temperature range.

High resolution applications

When digital potentiometers are used in programmable gain amplifiers (PGAs) and instrumentation amplifiers (IAs), the accuracy requirements are usually higher than those of standard adjustment circuits (Figure 2). These applications typically require a voltage divider ratio error (accuracy) of less than 0.025% over the -40°C to +85°C range.


Figure 2. A precision programmable gain amplifier is constructed using an op amp and a digital potentiometer (IC below).

in conclusion

Digital potentiometers have many advantages over mechanical potentiometers. In addition to improved reliability, they also take up less space; due to reduced parasitic effects, digital potentiometers can provide better electrical characteristics and are less susceptible to noise. Digital potentiometers can replace mechanical potentiometers in a variety of applications, benefiting designers and end users.