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2000 Microchip Technology Inc. Preliminary DS00219A-page 1 AN219 INTRODUCTION Resistor potentiometers can be found in electronic cir- cuits across a wide spectrum of applications. Most typ- ically, 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 com- pared in this application note are the mechanical poten- tiometer (also called a trimmer potentiometer) and the digital potentiometer. The physical descriptions and cir- cuit 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 imple- mented by twisting a knob, moving a slider, or using a screw driver. Although this method seems awkward, given the advent of the digital potentiometer, mechani- cal potentiometers still find their way into various elec- tronic circuits. Earlier mechanical potentiometers were built by wrap- ping a resistive wire around a cylinder. With this con- struction, 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 char- acteristics. In this application note the digital potentiom- eter 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. Figure 1: The mechanical potentiometer is constructed so that the user can easily adjust the position of the wiper (PW) by hand or with a screw driver. The digital potentiometer is manufactured so that the position of the wiper is adjusted by means of a serial digital code. The circuit representation of the digital potentiometer and the mechanical potentiometer is fundamentally the same. Author: Bonnie C. Baker, Microchip Technology Inc. Mechanical Potentiometer Model Digital PA PB PW Potentiometer Model An example of PCB mountable Mechanical potentiometers wiper PA PB PW MCP41010 MCP42010 contact resistanceresistance Comparing Digital Potentiometers to Mechanical Potentiometers
DS00219A-page 2 Preliminary 2000 Microchip Technology Inc. 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 environ- ments. 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 tech- nology used in active analog and digital integrated cir- cuits use. This device comprises a combination of segmented resistive elements and on-chip switches. The resistive elements are manufactured using stan- dard 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 fundamen- tal operation as the mechanical potentiometer with one primary exception. The wiper position is digitally pro- grammed with a microcontroller. This style of adjust- ment 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” pro- grammability, the digital potentiometer offers signifi- cant 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 func- tionality of these two styles of potentiometers, there are several specifications that describe the difference and similarities of these devices further. Changes of Resistive Element Due to Environmental Cycling Environmental changes such as temperature or humid- ity can have an adverse effect on an application circuit where a mechanical potentiometer is used. Since mechanical potentiometers have moving parts, they can be more sensitive to these types of environmental changes. The reaction of a typical mechanical potenti- ometer to these types of environmental changes is shown in Table 1. Figure 2: This is an example of a dual digital potentiometer. The digital potentiometer is programmed via a serial interface. Environmental Event (per Mil-R-94 standard) Maximum Allowable Resistance Change of Mechanical Potentiometer Temperature Cycling ±1% to ±10% High Temperature Exposure ±2% @ 125 °C for 250 hours Humidity excursions ±15% Table 1: The environment can have an adverse effect on the reliability of the mechanical potentiometer. The specifications in this table were taken from data sheets of higher quality mechanical potentiometers. RDAC1 SCK SOSI Decode Logic
16 Bit Shift Register
mechanical potentiometers may also move. potentiometer is a more robust solution. Figure 1. Typical values for digital potentiometers are increases the cost of the device. terminal PA and terminal PB varies from part to part. potentiometers range from ±10 to ±25%. and Temperature Variations”. imum wiper current restriction.
DS00219A-page 4 Preliminary 2000 Microchip Technology Inc. Temperature Range Both the mechanical potentiometer and digital potenti- ometer are specified to be able to operate over indus- trial temperature range of -40°C to 85°C. Most typically, the mechanical potentiometer is specified to operate over the military range of -55°C to 125°C. CONCLUSION Mechanical potentiometers have advantages in terms of having a wide variety of values available and tighter specifications such as nominal resistance, tolerance, temperature coefficient, power rating and temperature range specifications. But in many applications the over- riding factors are related to environmental and reliabil- ity issues. These characteristics are not necessarily specified by the mechanical potentiometer vendor. Digital potentiometers go hand in hand with the drive towards digital system control. This type of potentiom- eter is considerably more robust that its predecessor, the mechanical potentiometer, in terms of environmen- tal exposure issues and longevity with repeated use of the wiper. But beyond the reliability issues, the digital potentiometer offers hands-off programmability. This programmability also allows the user to repeatedly and reliably return to the same wiper position. REFERENCES: Baker, Bonnie C., “Optimizing Digital Potentiometer Circuits to Reduce Absolute and Temperature Varia- tions”, AN-691, Microchip Technology Inc. Todd, Carl David, “The Potentiometer Handbook: Users’ Guide to Cost-effective Applications”, McGraw- Hill, 1975. Baker, Bonnie C., “Using a Digital Potentiometer to Optimize a Precision Single Supply Photo Detection Circuit”, AN-692, Microchip Technology Inc. Baker, Bonnie C., “Using Digital Potentiometers to Design Low Pass Adjustable Filters”, AN-737, Microchip Technology Inc.
2002 Microchip Technology Inc. Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical com- ponents in life support systems is not authorized except with express written approval by Microchip. No licenses are con- veyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, FilterLab, KEELOQ, microID, MPLAB, PIC, PICmicro, PICMASTER, PICSTART, PRO MATE, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Tech- nology Incorporated in the U.S.A. and other countries. dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2002, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999. The Company’s quality system processes and procedures are QS-9000 compliant for its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001 certified. Note the following details of the code protection feature on PICmicro ® MCUs. The PICmicro family meets the specifications contained in the Microchip Data Sheet. Microchip believes that its family of PICmicro microcontrollers is one of the most secure products of its kind on the market today, when used in the intended manner and under normal conditions. There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowl - edge, require using the PICmicro microcontroller in a manner outside the operating specifications contained in the data sheet. The person doing so may be engaged in theft of intellectual property. Microchip is willing to work with the customer who is concerned about the integrity of their code. Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable”. Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our product. If you have any further questions about this matter, please contact the local sales office nearest to you.
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