Digital Potentiometer learning notes

AN219 Comparing Digital Potentiometers to Mechanical Potentiometers 

http://ww1.microchip.com/downloads/en/AppNotes/00219.pdf

Author: Bonnie C. Baker, Microchip Technology Inc

INTRODUCTION

Resistor potentiometers can be found in electronic circuits across a wide spectrum of applications. Most typically, they function in a voltage divider configuration in order to execute various types of tasks, such as offset or gain adjust. The two types of potentiometers compared in this application note are the mechanical potentiometer (also called a trimmer potentiometer) and the digital potentiometer. The physical descriptions and circuit models of these two devices are shown in Figure 1.

Basics of Mechanical Potentiometers

The first type of potentiometer on the market was mechanical in nature. This type of potentiometer is still available and adjustments of the wiper are implemented by twisting a knob, moving a slider, or using a screw driver. Although this method seems awkward, given the advent of the digital potentiometer, mechanical potentiometers still find their way into various electronic circuits.

Earlier mechanical potentiometers were built by wrapping a resistive wire around a cylinder. With this construction, the wiper moves from one winding to the next. As the wiper is moved across the element, there are discrete steps in resistance. Following this style of fabrication, the mechanical potentiometer was built using a resistive thick film that was screened onto a ceramic substrate. With this construction, the change in resistance across the element is continuous.

There are a variety of resistive materials that are used by mechanical potentiometer manufacturers. They include molded conductive plastic, conductive plastic film, screened conductive plastic, and cermet. Each resistive material has its own set of performance characteristics. In this application note the digital potentiometer will only be compared to the more popular cermet potentiometer. Cermet is a thick film resistive material that is a mixture of fine particles of ceramic or glass and precision metals such as silver, platinum, rhodium, or gold.

The wiper of the mechanical potentiometer slides along the distance on the resistive material providing an analog resistive output that has an infinite number of positions across the span of the element.

The metal contacts of the mechanical potentiometer can affect the performance and reliability of the device.

Higher cost potentiometers use multi-fingers made from precious metals in order to promote longer life as well as improve electrical performance in all environments. These higher quality potentiometers are not included in the discussions in this application note.

Basics of Digital Potentiometers

Digital potentiometers (Figure 2) were introduced in the market after the mechanical potentiometer. The digital potentiometer is fabricated using the same silicon technology used in active analog and digital integrated circuits use. This device comprises a combination of segmented resistive elements and on-chip switches.

The resistive elements are manufactured using standard p-type silicon diffusions. Each resistive element can be switched from one side to the other side of the wiper using a serial digital command.

The digital potentiometer exhibits the same fundamental operation as the mechanical potentiometer with one primary exception. The wiper position is digitally programmed with a microcontroller. This style of adjustment allows the designer to adjust circuit performance dynamically using a digital controller. The additional programmability provides a solution where human intervention is not required. With this “hands-off” programmability, the digital potentiometer offers significant flexibility for a variety of applications.

Because this system is digital, the number of wiper positions is no longer infinite. For example, Microchip’s MCP41XXX and MCP42XXX family of potentiometers are all 8-bit and have 256 unique linear positions along the total resistive element.

Beyond the basic differences in fabrication and functionality of these two styles of potentiometers, there are several specifications that describe the difference and similarities of these devices further.

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