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.
I was hoping more Information about Pot Resistance Tolerance 20% for CAT5113V -01, what's mean that? and some Information about INL, DNL and Ratiometric TC.
Best regards kelmi
kelmi -July 06, 2007
If you could do a follow-up article that shows the difference between digital and mechanical pots, this would be interesting. I hear that the digital pots are more rugged and the mechanical pots have more resolution. Is this true, and to what degree? Thanks for the article!
Mark pearson -January 03, 2005
I think the article has lots of useful information. I was hoping to fine some information about the 20 to 30% resistance tolerance associated with digital pots and the maximum voltage limitation across the pots. In our application we need a much tighter resistance tolerance from device to device or unit to unit. The pots are under software control so the op amp gains set by the pots much be consistent between units. For the user like ourselves this is a problem consistant with digital pots but not the cousin mechanical pot.
John Kajander -March 21, 2004
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