Electronic Design

Digital Pots Leverage Reliability And Hands-Off Programmability

For decades, mechanical (or trimmer) potentiometers within a voltage-divider configuration have tackled such tasks as gain or offset adjustment. Digital pots, in which the wiper position is digitally programmed with a microcontroller, offer a type of "hands-off" programmability that raises flexibility in a number of applications.

In addition to programmability, digital potentiometers provide a number of distinct advantages over their mechanical predecessors. While mechanical pots are susceptible to movement caused by vibration or shock, digital pots (with no moving parts) are resistant to shock and vibration. Another major consideration is cycle life, or the pot's mean time to failure. Generally, mechanical pots can withstand several hundred cycles without reduced performance. Since the wiper of a digital pot is controlled by electrical switches, the resistive elements aren't affected by repeated cycles. Mechanical pots do, indeed, hold certain advantages in terms of available resistance values and power dissipation. Yet, digital potentiometers often win out in the design choice due to their flexibility, enhanced cycle life, and remote-control capabilities.

Within the last year or so, the leading manufacturers of digital pots have introduced a range of devices for use in optical networking, telecommunications, instrumentation, automatic test equipment, industrial process controllers, and communication systems. Most notable are the recent releases of six major vendors in the digital pot market. Brief descriptions of the new releases can be found on the bottom of the previous two pages. The table below lists a number of the pots' key specifications, including resolution, configuration, supply voltage, resistance options, and interface type.

Catalyst's CAT5111 and CAT5113 devices target conventional digital pot applications and are ideal for automated adjustments on high-volume production lines. The wide voltage ranges of Analog Devices' AN5263 and Maxim's MAX5436-MAX5439 series devices make them especially useful for LCD screen contrast adjustment. The WMS7202 device from Winbond targets LCD contrast adjustment as well as laser-diode adjustment in optical networks. Xicor's X9460 digital pot, which can address up to four devices within the same system by using two address pins, targets audio control in set-top boxes, DVD players, and home-entertainment systems utilizing surroundsound. The MCP42100 from Microchip works for audio applications and LCD contrast control, as well as for medical equipment and instrumentation.

While the resolution (number of steps) and the applications of these devices vary, many share similar specs. All have an operating temperature range of −40°C to 85°C, with the exception of the AD5263, which has an extended range of −40°C to 125°C. While programming is conducted via a digital up/down increment interface in the CAT5111 and CAT5113 digital pots, all of the other devices feature an SPI-compatible interface. Two of the product offerings provide notably versatile interfacing capabilities: The MAX5436-MAX5439 pots have a three-wire SPI/QSPI/MICROWIRE-compatible serial interface, and the AD5263 has a pin-selectable three-wire serial peripheral interface (SPI) or two-wire I2C-compatible interface. While resistance options vary across the new digital pots, from the X9460's 33-kÙ resistance to the AD5263's 200-kÙ resistance option, the most common resistance values across the board are 10, 50, or 100 kÙ.

Supply voltages also vary widely across the products, with the MAX5436-MAX5439 and AD5263 offering the widest voltage ranges. The MAX5436-MAX5439 devices can be used with single-supply voltages from 0 to 30 V or dual-supply voltages of ±5 to ±15 V. In addition to its single-supply voltage range of 5 to 15 V, the AD5263 can be used with a ±5-V dual supply.

A distinguishing feature among some of the digital pots, such as the CAT511, is the inclusion of a buffer-configured op amp to minimize errors if there is potential wiper loading. Nonvolatile wiper storage is another main feature of the CAT5111, as well as the CAT5113 and the WMS7202.

WMS7202
The WMS7202 is a 256-tap, dual-channel digital potentiometer available from Winbond. It can be used as a three-terminal potentiometer or as a two-terminal variable resistor. Wiper position varies linearly according to the content stored in the volatile Tap Register (TR), the settings of which can be provided either directly by the user through the serial peripheral interface, or by the flash-based nonvolatile memory where the previous settings are stored. When changes are made to the TR to establish a new wiper position, the value of the setting can be saved into any of the nonvolatile memory locations. Each channel has its own selectable output buffer plus four nonvolatile memory locations, which allow direct read or write. With the output buffer inactive, the WMS7202 features a low standby current of 1µA maximum.
Winbond Electronics Corporation America
www.winbond-usa.com; (800) 677-0769

See associated figure

 

CAT5111, CAT5113
Catalyst's 100-tap CAT5111 and CAT5113 single digitally programmable potentiometers (DPPs) boast nonvolatile NVRAM wiper storage for reliability. The wiper position can be adjusted to test new system values without affecting the stored wiper position. The CAT5111 wiper is buffered by an op amp that operates rail to rail, suiting it for use as a three-terminal voltage divider. The non-buffered CAT5113 functions as a two-terminal variable resistor or as a three-terminal resistive divider. Both pots feature 1% resolution and are specified with a tight tolerance of ±15%. Within the range of 2.5 to 6.0 V, supply current measures under 100 µA. The pots target optical networking, telecommunications, and instrumentation applications.
Catalyst Semiconductor
www.catalystsemiconductor.com;
(408) 542-1000

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MAX5436-MAX5439
The MAX5436-MAX5439 high-voltage digital potentiometers pack 128-step performance into a miniature package. Two of the Maxim devices, the MAX5437/9, integrate a general-purpose operational amplifier. All feature low temperature coefficients of 35 ppm/°C end-to-end and 5 ppm/°C ratiometric. Digital logic is powered by a separate single 2.7- to 5.25-V supply. The resistor string uses its own analog supplies, allowing for greater flexibility in voltage ranges that can be used with these devices. All of the parts provide an amplifier shutdown mode. At power-on, the wiper is reset to midscale. The MAX5436-MAX5439 suit liquid-crystal-display (LCD) contrast adjustment applications. They also work for audio volume control and gain adjustment applications.
Maxim Integrated Products
www.maxim-ic.com; (888) 998-8800

See associated figure

 

AD5263
With the AD5263 quad-channel, 256-position potentiometer, control comes via a pin-selectable three-wire serial peripheral interface (SPI) or a two-wire I2C-compatible data interface. At power-on, the wiper is reset to the midscale position. The AD5263 has a low temperature coefficient of 50 ppm/°C. Its hardware override shutdown function shorts the wiper to the A-terminal and open-circuits the B-terminal without disturbing the contents of the wiper register. The AD5263 suits applications like LCD contrast control, gain and offset adjustment in instrumentation, and automotive electronics adjustment.
Analog Devices Inc.
www.analog.com; (800) 262-5643

See associated figure

 

MCP42100
Microchip's MCP42100 dual digital potentiometer boasts 8-bit resolution (256 wiper steps) and consumes less than 1 µA during static operation. The unit's serial peripheral interface includes both the SI and SO pins, allowing daisy-chaining of multiple devices. Channel-to-channel resistance matching varies by less than 1%. The MCP42100 features ±1 LSB maximum integral nonlinearity (INL) and differential nonlinearity (DNL). The wiper resets to midscale position upon power-up. A software shutdown feature maximizes power savings, as does a hardware shutdown pin.
Microchip Technology Inc.
www.microchip.com; (480) 792-7200

See associated figure

 

X9460
Targeted at audio applications, Xicor's X9460 32-tap dual digitally programmable potentiometer IC enables full digital control of audio volume via front-panel pushbuttons or remote control. Integrating two digitally controlled potentiometers (XDCPs) on a monolithic IC, the unit provides 6-bit resolution over the 0- to −62-dB audio level control range. A −90-dB mute function suits the X9460 for adjusting audio signals in preamplifier stages of audio equipment. The device features low, −102-dB channel crosstalk (at 1 kHz). A zero-crossing detection circuit eliminates audible clicks that occur in other digital potentiometers during wiper changes. On power-up, the wiper is reset to the mute position.
Xicor Inc.
www.xicor.com; (408) 432-8888

See associated figure

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