Digital potentiometer

Friday, 17 November 2006

Potentiometers are resistors in which the resistance is variable by adjusting a control device. Potentiometers typically provide a variable resistance to the flow of electrical current in a circuit. The resistance provided by a potentiometer ranges from a very small resistance to a maximum resistance which is arbitrarily chosen by the user for the particular application involved. A typical potentiometer comprises a resistive element, a conductive element, and a wiper that has two or more metal contacts which provide movable electrical contact between points along the elements. As the wiper moves along the elements, it continuously varies the resistance between the conductive element and either end of the resistive element. Traditional analog potentiometers are mechanical devices whose resistance varies according to a selected physical position of a wiper, which can produce selected electrical potentials. Such potentiometers may take the form of a movable wiper which can traverse a wire wound resistor and short out a variable portion of the resistor. An electric potential of constant magnitude is applied to the opposite ends of the resistor, and the wiper and one of the ends are connected in an electric circuit also containing one or more elements to be controlled or otherwise acted upon. Wiper location is controlled by rotating or sliding the wiper to the desired position. The magnitude of the resistance interposed in the circuit by the conductor depends upon the axial position of the wiper relative to the ends of the conductor. Analog potentiometers have long been used to provide variable resistances in applications such as audio volume control and speaker balancing light dimmer control, and CRT brightness control. An analog potentiometer is often employed to provide such high resolution. A typical analog potentiometer contains a resistor which can be tapped at different points to attain a corresponding resistance. As the tap points can potentially be contiguous, a correspondingly high resolution can be attained. However, such mechanical devices are bulky and awkward to combine with integrated circuits on circuit boards.

Digital potentiometers, sometimes referred to as voltage-scaling digital-to-analog converters (DAC), are electronic circuits that can provide a variable impedance as a result of processing a digital sequence. Digital potentiometers are comprised of a resistor array with a plurality of selectable wiper points. Resistor arrays used in digital potentiometers may be generated from discrete resistors or from commercially available integrated circuits including a plurality of resistors. The resistors are typically coupled with electrical relays or switches, such as dual in-line package (DIP) switches, to provide a selectable range of resistance values. For each selectable wiper point, there is a semiconductor gate device for switching a signal path through the gate device and into the resistor array at the specified wiper point. A digital potentiometer has a fixed-value impedance connected between two reference terminals. This impedance is provided by a string of impedance devices that can be selectively connected to a third terminal through electronic switches controlled by digital signals. Digital potentiometers often contain digital switching elements which are operated to connect only some of the available resistors to provide a desired resistance value. Voltage-scaling digital potentiometers produce an analog output voltage by selectively tapping a voltage-divider resistor string connected between high and low reference voltages, with the low reference generally being set at ground. These types of converters are used most commonly as building blocks in metal oxide semiconductor (MOS) analog-to-digital conversion systems which function as the DAC subsection of a successive-approximation-type analog-to-digital converter. Semiconductor digital potentiometers provide a variable resistor with on the order of 100 different resistance levels by the use of a resistor string and selection of a particular tap on the resistor string controlled by a digital input. Potentiometers are often characterized by resolution. Resolution refers to the minimal change in resistance that can be offered by (or attained by using) a potentiometer. The fineness of adjustment or resolution of a digital potentiometer is typically determined by the number of digital bits used for the selection of the desired wiper position.

Digital potentiometers are replacing analog potentiometers because they are smaller, more easily and accurately set, are controllable remotely, and cost effective. Digital potentiometers are suitable for integration into electrical/electronic environments since the switching elements can be implemented at least substantially using only electronic/electrical technologies. Digital potentiometers can be used in digital to analog converters, and as replacements for mechanical potentiometers and rheostats. Nonvolatile programmable digital potentiometers have been used in LCD (liquid crystal display) screen adjustment, volume control, automated product calibration, remote adjustment of equipment, signal processing, and other applications requiring the replacement of mechanical potentiometers.

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