Digital Potentiometers Replace

文章摘要：Figure 1. Effect of Switching at the 0V Level on Audible Clicks and Pops.In addition to the analog-domain considerations mentioned above, each digital pot has a digital interface. Most are programmable through a conventional serial interface like I²C or SPI™, and some offer the useful up/

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Figure 1. Effect of Switching at the 0V Level on Audible Clicks and Pops.

In addition to the analog-domain considerations mentioned above, each digital pot has a digital interface. Most are programmable through a conventional serial interface like I²C or SPI™, and some offer the useful up/down interface.

Better Performance

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.

Conventional digital potentiometers do not require extensive explanation, because they substitute directly for conventional mechanical pots and operate in a similar fashion. Nonetheless, an application-specific device, such as one designed for low-cost stereo volume control, warrants some comment. Application-specific devices, for audio, often operate over a wide voltage range to accommodate the wide range of audio signals. They are often log-taper devices, whereby the number of decibels of attenuation per step increases as the steps increase, which better reproduces the response of the human ear. Sometimes the device includes a mute function, which further attenuates the signal by a significant amount (e.g., 30dB).

Temperature Issues

One of the typical characteristics of digital potentiometers is the temperature coefficient (TC), specified over the rated temperature range. Two different TCs must be specified for most potentiometers. First, the absolute end-to-end TC is a large value indicating the absolute variation of resistance with temperature, and is calculated as:

ΔR = R_UNCOMP × TC × ΔT/10⁶

where:
R_UNCOMP is the uncompensated resistance value,
TC is the temperature coefficient, and
ΔT is the temperature variation.

Thus, for example, a digital potentiometer with 20kΩ resistance and absolute TC of 35ppm would exhibit a variation of 35Ω (0.2%) over the 50°C temperature range. Also, the initial value of the 20kΩ end-to-end resistance could vary significantly; a range from 15kΩ to 25kΩ is possible. In that case, the resistance value of the 32 increments (steps) would range from 470Ω to 780Ω. This variation is, of course, much higher than the absolute TC deviation.

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.

High-Resolution Applications

The digital potentiometers found in programmable gain amplifiers (PGAs) and instrumentation amplifiers (IAs) require much higher accuracies than do those used in standard adjustable circuits (Figure 2

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