Digital pots shrink volume controls

It is now common for portable equipment to include some sort of stereo audio playback circuitry, perhaps for MP3 or Web radio applications. Many specialist ICs are available to use as building blocks for these circuits, but widely available, low-power components can also be used effectively. An often-overlooked component is the stereo volume control.

Historically, audio volume controls used a special kind of potentiometer with a "log" (or sometimes "audio") taper or law. This derives from the roughly logarithmic response of the ear to changes in sound pressure levels.

In general, the result is that the midpoint of the rotation usually gives around 20-dB attenuation of the audio signal, giving rapidly increasing attenuation from the midpoint counterclockwise and finer control over the "louder" settings from the midpoint clockwise.

While this works well, there are a number of reasons not to use a a bulky mechanical pot in a small portable device, space constraints and reliability issues being just two. Use of up/down buttons in conjunction with some form of host processor is a common volume control interface for modern devices, providing a cheap, usable solution.

Also, stereo (or ganged) rotary pots have a mechanical-tracking issue, in that mechanical tolerances mean left-right tracking suffers as the volume is adjusted. Moreover, the desired transfer function should be considered: Is full attenuation desired, or rather a gain trim control offering (say) 30 dB of adjustment range, but no "fully off" position?

Over the past few years, digital potentiometers have become available, using resistive ladders and FET switching under digital control to effectively replace mechanical potentiometers in a number of areas. On the surface, using a pair of these would seem a logical solution for stereo volume control. However, a number of issues must first be addressed.

The most commonly available types are effectively "linear" pots, meaning that their resistive increments are equally weighted. A reasonably constant "decibel-per-step" law is desirable in an audio volume control, so the design may have to emulate this log behavior in some way-we are no longer constrained by the mechanical potentiometer audio taper. A second issue is that while the digital pot steps are usually designed to give resistive increments of equal value, a by-product of the process variation is that the total end-to-end resistance varies widely from part to part-as much as 30 percent in some types. Developers must take this into account when designing a circuit requiring close matching between two channels using separate digital pots. A further requirement is that the transitions should be as glitch-free as possible, so a make-before-break wiper arrangement should be considered mandatory.

One example of both a gain trim design (where the control is applied over a set attenuation range, without reaching full attenuation) and more traditional "full clockwise, off counterclockwise" volume controls assumes a supply voltage (Vcc) between 2.7 and 5 V, and a low impedance VREF = Vcc/2. Assume too, the IN input signals are from a low-impedance voltage source.

Using two MAX5160L digital pots (assuming their digital inputs are suitably controlled) around a MAX4252, the circuit should give evenly tracking gain or attenuation over a nominal 6-dB range. The circuit should work over a Vcc range from 2.7 to 5 V and have 32 usable gain settings; even the power-on-reset state of the MAX5160L gives approximately a unity-gain setting. The drawback to this implementation is the 25 percent overall resistance variation of the digital pot. This could lead to a wide gain tolerance, especially on the settings at the extremities of resistance, channel-to-channel as well as unit-to-unit.

This amount of left-right mismatch is easily audible. This circuit can be made to work by selecting or trimming the support resistors, or selecting the digital pot, to suit, but this approach is not feasible in mass production. A design approach must be found that minimizes or eliminates this gain error.

Resistively loading the wiper of a linear pot to "bend" the characteristic is an old trick, usually with the load resistor being about 1/20th the value of the linear pot resistance. The two disadvantages of applying this with a linear digital pot is that the input impedance of the pot is now dependent on gain setting (lowest at maximum volume) and, again, the wide tolerance on end-to-end resistance means left-right tracking suffers at any setting apart from the extremes.

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