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Issue #213 April 2008
Digital Touch
A Potentiometer with No Moving Parts
by Jeff Bachiochi
Start | Touch Slider IC | Digital Potentiometers | Control and Feedback | Value to Digits | Linear Segmentation | Worth It | Sources & PDF
DIGITAL POTENTIOMETERS
Mechanical potentiometers wear out over time, rotary potentiometers are fairly well sealed, and slide potentiometers have an inherent “open” slot where environmental contaminants can enter. Worn potentiometers add noise that will make an audio buff cringe, as if listening to AM radio during a thunderstorm. Digital potentiometers have no moving parts. Therefore, they can provide years of service without the breakdown of the mechanical wiper/element connection. They use a digital value to connect the wiper to a corresponding position on a chain of discreet resistors. Figure 3 is a block diagram of a Microchip Technology MCP41010 digital potentiometer.
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| Figure 3 — This digital potentiometer has a 256-tap resistor array totaling 10 kW. The wiper (tap position) is set via a SPI. The digital potentiometer is totally isolated from the rest of the circuitry. |
A SPI command to the MPC41010 consists of 16 bits. The first 8 bits contain a command and the last 8 bits contain the wiper’s data value. While the shutdown (0x20) command does not require any data, 16 bits must be transmitted for each command, so the data in this case is “don’t care.” Shutdown will disconnect the “A” terminal from the top of the resistor chain and connect the wiper to the “B” terminal (bottom of the resistor chain). The WR command will set the wiper state during the rise of the *CS line. The wiper value can be set during a shutdown condition, but it will activate the device during the rise of the *CS line. A data value of 0x00 is the minimum position (B) and a data value of 0xFF is the maximum position (A). This device defaults to mid-position during powerup.
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