Digital Potentiometers Replace

文章摘要：Digital Potentiometers Replace Mechanical PotentiometersAbstract: Digitally adjustable potentiometers, also known as digital potentiometers, or digpots, are replacing the old-fashioned mechanical versions in all electronic systems, not just in the more well-known audio applications. A simpler and le

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Digital Potentiometers Replace Mechanical Potentiometers

Abstract: Digitally adjustable potentiometers, also known as digital potentiometers, or digpots, are replacing the old-fashioned mechanical versions in all electronic systems, not just in the more well-known audio applications. A simpler and less expensive version of the digital-to-analog converter (DAC), digital pots produce an analog output in the form of a variable resistance. Available in volatile and nonvolatile types, the digital approach has many advantages. Digital pots are insensitive to dust, dirt, and moisture, which can kill a mechanical part if they intrude.

Introduction

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 pot resolution can range from 32 steps (5-bit) to 256 steps (8-bit), and more. Low resolution is acceptable for LCD-display contrast and other applications in which dynamic range is not critical. Recent high-resolution digital pots are ideal for achieving the typical dynamic range up to 90dB for some audio and hi-fi applications.

Nonvolatility

For certain applications, the digital pot requires a nonvolatile memory function. Both part types (volatile and nonvolatile memory) are well established in the market. 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.

The MAX5427/MAX5428/MAX5429 family of digital pots offers a unique programming feature. Each device has a one-time programmable (OTP) function that sets the wiper's power-on reset (POR) position to a user-defined value. (The wiper position remains adjustable, but always returns to the programmed position on power-up.) As an alternative, the OTP capability lets you disable the interface and permanently lock the wiper in a desired position, thereby preventing further adjustment. As a result, the device becomes a fixed resistive divider instead of a potentiometer.

Audio Considerations

Electronic pots offer logarithmic and linear functions. 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. Consequently, these pots also support the many audio applications that require multiple channels, such as stereo and Dolby-surround systems.

For audio, and especially for low-attenuation (32-step) digital pots, one must pay special attention to the step-by-step wiper transitions. If a wiper does not switch exactly at 0V, the audio can include annoying clicks and pops (Figure 1). Fortunately, certain recent-generation digital pots include a feature called zero-crossing detection, which reduces the audible noise generated during wiper transitions. An internal zero-crossing and timeout circuit ensures that the wiper changes position only after detection of a zero crossing (0V) or a 50ms delay, whichever comes first.

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