Welcome to this module on Digital Potentiometers. In this article we will discuss the advantages of Digital Potentiometers comparing with the Conventional Mechanical Potentiometers.

Mechanical Potentiometers

The mechanical potentiometers are also called resistor potentiometers or timmer potentiometers, which are still available in a wide range applications. The adjustment of wiper of a mechanical potentiometer is manually by a knob or a screwdriver. Typically, they are used in a voltage divider configuration for different types of task, such as gain adjust.

Digital Potentiometers Digpots

Digital potentiometers, or digpots, exhibit the same fundamental operation as the mechanical parts with only one exception. The wiper position is digitally adjusted. Digpots produce an analog output in the form of a variable resistance. They use digital input signals to set an analog output. A digpot usually has three analog connections: the high connection, the wiper (or analog output), and a low connection. However, it can also be configured as a two-terminal variable resistor by connecting the wiper to either the high or low end, or by floating either the high or low end.

For certain applications, the digital pot requires a nonvolatile memory function. A nonvolatile digital pot is most similar to its mechanical counterpart because it retains the resistance value, regardless of external conditions such as whether or not the supply voltage is present. Digital pots with nonvolatile memory are widely used in audio equipment because the internal stored value guarantees the same resistor settings always after power up, even if the power supply was completely switched off.

Logarithmic potentiometers are often used to adjust volume in hi-fi audio, because each step of a logarithmic pot creates a linear change of volume for the nonlinear human ear. Today’s highly integrated digital pots offer as many as six independent potentiometers in a single chip, which can support the many audio applications that require multiple channels, such as stereo and Dolby-surround systems. If a wiper does not switch exactly at 0V, the audio can include annoying clicks and pops. Fortunately, certain recent-generation digital pots include a feature called zero-crossing detection, which reduces the audible noise generated during wiper transitions. Sometimes the device includes a mute function, which further attenuates the signal by a significant amount.

Digital pots have yet another performance advantage over mechanical potentiometers. Digitally adjustable pots can now be mounted on the circuit board directly in the signal path, thereby eliminating the need for complex and expensive mechanical integration with the electronic controls. Digital pots improve the rejection of electrical noise, and they eliminate the disturbances picked up by the cables required to interface a mechanical potentiometer.

One of the typical characteristics of digital potentiometers is the temperature coefficient (TC), specified over the rated temperature range. First, the absolute end-to-end TC is a large value indicating the absolute variation of resistance with temperature, and is calculated as the equation listed here, where RUNCOMP is the uncompensated resistance value, TC is the temperature coefficient, and ∆T is the temperature variation. The second type of TC is a ratio-metric TC. Potentiometers are typically used as voltage dividers, especially in ratio-metric applications for which the absolute resistance value is far less critical than the absolute TC and the variation between steps. A ratio-metric TC of only 5ppm, for example, would allow a very stable configuration for adjustable gain over temperature.

The digital potentiometer found in programmable gain amplifiers and instrumentation amplifiers require much higher accuracies than do those used in standard adjustable circuits. Divider-ratio tolerances (accuracies) as low as 0.025% over the -40 ℃ to +85 ℃ range are often needed for these applications.

Advantages of Digital Potentiometers

Digital potentiometers have many significant advantages over mechanical potentiometers. Digital potentiometers are far more reliable than mechanical potentiometers. Digital pots can easily guarantee 50,000 writing cycles, while the mechanical parts rate only a few thousand, and sometimes only a few hundred. Digital pots are insensitive to dust, dirt, and moisture, which can kill a mechanical part if they intrude. They use less space and have better electrical performance because of the reduced parasitics and lower susceptibility to noise. Digital pots can replace mechanical types in virtually every application, thus allowing the designer and the eventual user to enjoy the associated benefits.

MAX5427/MAX5428/MAX5429 linear-taper digital potentiometers
The MAX5427/MAX5428/MAX5429 (datasheet) linear-taper digital potentiometers function as mechanical potentiometers, but replace the mechanics with a simple 2-wire digital interface. These digital potentiometers are unique in that they have an optional one-time programmable feature that either sets the wiper's power-on reset (POR) position to a user-defined value, or the wiper can be set and the interface disabled to prevent further adjustment.

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