XICOR, Inc. announce a digital potentiometer that replaces four mechanical knob or slide potentiometers. The chip has important benefits over mechanical potentiometers including low noise, and immunity to wear, shock, vibration, dust, oil, moisture and temperature extremes. The device also has the benefit that it can be adjusted remotely via a telephone or wireless connection.

The X9408 contains four EEPOTs (i.e., nonvolatile digital potentiometers) each with 64 different settings. The X9418 version of the device contains two EEPOTs also each with 64 settings. The devices set a new standard for low noise with a noise figure of -140 dB[root]Hz. This is important because it improves the fidelity of the signal being controlled. A standby current of 1uA and the ability to operate down to 2.7V makes the chip ideal for battery applications.

According to Gary Craig, Director of Product Marketing, "In terms of reliability, the weakest link in any system is its mechanical components. Our digital potentiometers eliminate this weak link by allowing engineers to replace mechanical dials and sliders with our solid-state EEPOTs. Our devices give engineers a tool that improves system reliability, and provides features such as automatic calibration and remote adjustment."

Applications for the new potentiometers include communications, consumer, computer, office automation, industrial, automotive and military products. In addition to improved reliability, these systems need the ability to adjust parameters for a variety of reasons including volume control, screen brightness, as well as calibration and filter settings that automatically maintain system performance during changing conditions.

EEPOT Features
Xicor's new devices use an embedded EEPROM to store up to four wiper positions for each potentiometer. The sixteen, 8-bit registers that hold the wiper positions in EEPROM can also be used for general purpose nonvolatile storage.

The wiper settings, which can range through 64 different taps, are set using a 400KHz two-wire interface. After power up, the devices can return to their previous positions or can be programmed to go to a fixed starting point.

Internally, each potentiometer consists of an array of 63 resistors connected in series. The voltage to be divided is applied across the ends of the array. By sending a command to the chip, the wiper pin can be switched to any one of the 64 resistor taps. The devices have an end-to-end resistance of 10K ohms and each resistor in the array has a value of 158.5 ohms.

Lower Noise
Unlike the previous generation of EEPOTs, these new devices use CMOS transistor gates to switch the wiper to the desired tap. This approach provides a lower noise figure because it eliminates the need for an on-chip oscillator that can couple noise to the wiper output. Oscillators were used to boost the voltage driving the wiper transistors. The lower noise figure of the new devices makes them ideally suited for applications such as multimedia audio systems.

EEPOT Applications

• 2-Way Radio
  EEPOTs replaced several potentiometers in the base station of a 2-way radio. This allows automated resistor adjustment using digital signals from an electronic tester instead of time consuming manual adjustments.
• Power Supplies
  EEPOTs replaced mechanical potentiometers in power supplies to provide protection from shock, vibration, dust and moisture.
• Automobile Engines
  EEPOTs improved the maintainability of an automobile engine controller by allowing it to easily achieve and maintain precise ignition timing.
• Displays
  EEPOTs reduce warranty returns for manufacturers by eliminating mechanical potentiometer failures in the control of display contrast (from keyboard) in notebooks and PDAs.
• Remote Cameras
  EEPOTs in the Mars Pathfinder lander allowed remote camera adjustments that helped to bring us exciting view of Mars from 150 million miles away.

The great jump with these new versions of digital pots, to my mind, is the change of technology in the wiper switch; instead of having an on-chip oscillator that is used to drive up the supply voltage to the wiper transistors, these new EEPOTs use CMOS gates to switch to the wanted taps. The improved noise performance opens up a number of applications that were closed before including, as the company observes, multimedia audio. The same applies in any situation where the signal of interest is actually passing through the pot. The low-voltage operation and the low standby current are also both attractive.

One of the great advantages of using EEPOTs, I have been told by designers, is the flexibility of deciding what the initial conditions are going to be at power-up: they can be left where they were when the power went away, or they can go to a preset point which can be at any value of the resistance.

Although there are several manufacturers of digital pots these Xicor ones seem to be the most accepted, so far, and the improvements can only increase that acceptance; the uses for the parts are still hidden in way-open fields of green: the idea of having an automatic tester set up the correct resistance value without human intervention is just a wondrous notion for all sorts of high-volume production lines. A negative is that not every potentiometer situation can be solved with a 10 k linear pot and I am sure that Xicor must be under some OEM pressure to offer other values and styles to "complete the range" which would probably mean lost focus and profitability. And although it is the modern habit to think in binary numbers there must surely be a lot of projects wher multiples of 3 pots are needed?

Samples of the 24-pin SOIC-packaged version of the parts will be available this month and the parts will be priced at $2.95 for the X9408 in 10,000 piece lots, and $1.95 for the X9418; there will also be a 24-pin DIP and 24-pin TSSOP versions.