 |
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
| Datasheet File OCR Text: |
| AN219 Comparing Digital Potentiometers to Mechanical Potentiometers Author: Bonnie C. Baker, Microchip Technology Inc. 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. 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. PA PW PB PA PW PB contact resistance wiper resistance MCP41010 Digital Potentiometer Model An example of PCB mountable Mechanical potentiometers Mechanical Potentiometer Model MCP42010 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. 2000 Microchip Technology Inc. Preliminary DS00219A-page 1 AN219 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. 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. Changes of Resistive Element Due to Environmental Cycling Environmental changes such as temperature or humidity 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 potentiometer to these types of environmental changes is shown in Table 1. Environmental Event (per Mil-R-94 standard) Temperature Cycling High Temperature Exposure Humidity excursions Maximum Allowable Resistance Change of Mechanical Potentiometer 1% to 10% 2% @ 125 C for 250 hours 15% 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. PA0 PW0 PB0 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. PA1 PW1 PB1 RDAC1 Data Register 0 D7 D0 RDAC2 Data Register 1 D7 D0 RS Decode Logic CS D7 D0 16 Bit Shift Register SCK SI SO SHDN Figure 2: This is an example of a dual digital potentiometer. The digital potentiometer is programmed via a serial interface. DS00219A-page 2 Preliminary 2000 Microchip Technology Inc. AN219 Since digital potentiometers are manufactured using a standard CMOS process with no moving parts, the reaction to these environmental changes are significantly reduced. 1k through 1M. This range of potentiometers are available in both the digital and mechanical potentiometers. Total Resistance Tolerance The total resistance tolerance of the element between terminal PA and terminal PB varies from part to part. With digital potentiometers that variance is dependent on processing variance of the resistive material and switches. Typical digital potentiometer total resistance tolerances are between 20% to 30%. On the other hand, variance of the cermet material in mechanical potentiometers range from 10 to 25%. Although there seems to be a degree of difference between the digital potentiometer and mechanical potentiometer, the variability of the nominal resistance of both devices is considerably larger than standard 1% discrete resistors. In some applications, these tolerance values can cause errors that are too large. For additional design help, refer to the numerous circuit ideas in Microchip's application note, AN-691,"Optimizing Digital Potentiometer Circuits to Reduce Absolute and Temperature Variations". Vibration or Shock Vibration or shock can also have an effect on an application circuit by causing physical movement. All devices that are soldered on a PCB can have failures due to vibration or shock, but the moving mechanism of mechanical potentiometers may also move. A typical specification for a mechanical potentiometer would be a 2% change due to vibrations that span from 10Hz to 2kHz. Another way of describing the effects of movement on the mechanical potentiometer is force. Typically 20Gs of force on a higher quality mechanical potentiometer would cause a maximum of 1% resistive change. Since there are no moving parts in digital potentiometers, the element will remain unchanged with vibration or shock tests unless discontinuities occur in the PCB construction. Mean Time to Failure Life One type of failure that is quantified with mechanical potentiometers is the mean time to failure life of the wiper adjustment capability. A typical specification for this type of failure would be that the device could survive several hundred cycles without discontinuity. A cycle is defined as changing the wiper position across full scale once. With thin film mechanical potentiometers, such as those constructed of cermet, a failure resulting from repeated cycles manifests itself as reduced performance. Since the wiper of the digital potentiometer is controlled by electrical switches, the resistive elements are not effected by repeated cycles. Consequently, the digital potentiometer is a more robust solution. Temperature Coefficient Mechanical potentiometers and digital potentiometers drift with temperature. The range of typical drift specifications for the total resistance of the mechanical potentiometer is from 100ppm/C to 300ppm/C. Typical drift versus temperature specification for the digital potentiometer is around 800ppm/C. With both types of potentiometers, the temperature coefficient difference between the A element (resistance between PA and PW minus the wiper resistance) and B element (resistance between PB and PW minus the wiper resistance) is very low. The magnitude of these specifications may or may not affect the performance of the circuit. If it is found that they do, numerous circuit ideas are available in Microchip's application note, AN-691,"Optimizing Digital Potentiometer Circuits to Reduce Absolute and Temperature Variations". Nominal Total Resistance The nominal total resistance of a potentiometer is the typical specified resistance (in ohms) that can be measured between terminal PA and terminal PB per Figure 1. Typical values for digital potentiometers are 10k, 50k, and 100k. Nominal resistance values below 10k become difficult to implement in silicon because of the switch resistances. Values higher than 100k are possible but require more silicon, which increases the cost of the device. The range of the selection of the mechanical potentiometer is considerably wider with values such as 10, 20, 50, 100, 200, 500, 1k, 2k, 5k, 10k, 20k, 25k, 50k, 100k, 250k, 500k, 1M, and 2M. The mechanical potentiometer might be considered attractive because of the wide range of nominal resistance offerings. However, the most common nominal resistance ranges used in adjustment type circuits are Power Rating Mechanical potentiometers can sustain more power dissipation than the digital potentiometers. It is not unusual to have a mechanical potentiometer that is capable of dissipating 0.5W @ 70C (usually specified for 1000 hours). However, the wiper of the mechanical potentiometer usually can only conduct up to 1mA of current. This becomes a limitation if the potentiometer is configured so that the wiper is directly connected to terminal A or terminal B. The digital potentiometer is capable of conducting power up to 0.0055W @ 70C. It also has a 1mA maximum wiper current restriction. 2000 Microchip Technology Inc. Preliminary DS00219A-page 3 AN219 Temperature Range Both the mechanical potentiometer and digital potentiometer are specified to be able to operate over industrial temperature range of -40C to 85C. Most typically, the mechanical potentiometer is specified to operate over the military range of -55C to 125C. REFERENCES: Baker, Bonnie C., "Optimizing Digital Potentiometer Circuits to Reduce Absolute and Temperature Variations", AN-691, Microchip Technology Inc. Todd, Carl David, "The Potentiometer Handbook: Users' Guide to Cost-effective Applications", McGrawHill, 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. 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 overriding factors are related to environmental and reliability 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 potentiometer is considerably more robust that its predecessor, the mechanical potentiometer, in terms of environmental 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. DS00219A-page 4 Preliminary 2000 Microchip Technology Inc. Note the following details of the code protection feature on PICmicro(R) 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 knowledge, 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. 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 components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, 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 Technology 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. (c) 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(R) 8-bit MCUs, KEELOQ(R) 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. 2002 Microchip Technology Inc. M WORLDWIDE SALES AND SERVICE AMERICAS Corporate Office 2355 West Chandler Blvd. Chandler, AZ 85224-6199 Tel: 480-792-7200 Fax: 480-792-7277 Technical Support: 480-792-7627 Web Address: http://www.microchip.com ASIA/PACIFIC Australia Microchip Technology Australia Pty Ltd Suite 22, 41 Rawson Street Epping 2121, NSW Australia Tel: 61-2-9868-6733 Fax: 61-2-9868-6755 Japan Microchip Technology Japan K.K. Benex S-1 6F 3-18-20, Shinyokohama Kohoku-Ku, Yokohama-shi Kanagawa, 222-0033, Japan Tel: 81-45-471- 6166 Fax: 81-45-471-6122 Rocky Mountain 2355 West Chandler Blvd. Chandler, AZ 85224-6199 Tel: 480-792-7966 Fax: 480-792-7456 China - Beijing Microchip Technology Consulting (Shanghai) Co., Ltd., Beijing Liaison Office Unit 915 Bei Hai Wan Tai Bldg. No. 6 Chaoyangmen Beidajie Beijing, 100027, No. China Tel: 86-10-85282100 Fax: 86-10-85282104 Korea Microchip Technology Korea 168-1, Youngbo Bldg. 3 Floor Samsung-Dong, Kangnam-Ku Seoul, Korea 135-882 Tel: 82-2-554-7200 Fax: 82-2-558-5934 Atlanta 500 Sugar Mill Road, Suite 200B Atlanta, GA 30350 Tel: 770-640-0034 Fax: 770-640-0307 Singapore Microchip Technology Singapore Pte Ltd. 200 Middle Road #07-02 Prime Centre Singapore, 188980 Tel: 65-6334-8870 Fax: 65-6334-8850 Boston 2 Lan Drive, Suite 120 Westford, MA 01886 Tel: 978-692-3848 Fax: 978-692-3821 China - Chengdu Microchip Technology Consulting (Shanghai) Co., Ltd., Chengdu Liaison Office Rm. 2401, 24th Floor, Ming Xing Financial Tower No. 88 TIDU Street Chengdu 610016, China Tel: 86-28-6766200 Fax: 86-28-6766599 Taiwan Microchip Technology Taiwan 11F-3, No. 207 Tung Hua North Road Taipei, 105, Taiwan Tel: 886-2-2717-7175 Fax: 886-2-2545-0139 Chicago 333 Pierce Road, Suite 180 Itasca, IL 60143 Tel: 630-285-0071 Fax: 630-285-0075 Dallas 4570 Westgrove Drive, Suite 160 Addison, TX 75001 Tel: 972-818-7423 Fax: 972-818-2924 China - Fuzhou Microchip Technology Consulting (Shanghai) Co., Ltd., Fuzhou Liaison Office Unit 28F, World Trade Plaza No. 71 Wusi Road Fuzhou 350001, China Tel: 86-591-7503506 Fax: 86-591-7503521 EUROPE Denmark Microchip Technology Nordic ApS Regus Business Centre Lautrup hoj 1-3 Ballerup DK-2750 Denmark Tel: 45 4420 9895 Fax: 45 4420 9910 Detroit Tri-Atria Office Building 32255 Northwestern Highway, Suite 190 Farmington Hills, MI 48334 Tel: 248-538-2250 Fax: 248-538-2260 China - Shanghai Microchip Technology Consulting (Shanghai) Co., Ltd. Room 701, Bldg. B Far East International Plaza No. 317 Xian Xia Road Shanghai, 200051 Tel: 86-21-6275-5700 Fax: 86-21-6275-5060 Kokomo 2767 S. Albright Road Kokomo, Indiana 46902 Tel: 765-864-8360 Fax: 765-864-8387 France Microchip Technology SARL Parc d'Activite du Moulin de Massy 43 Rue du Saule Trapu Batiment A - ler Etage 91300 Massy, France Tel: 33-1-69-53-63-20 Fax: 33-1-69-30-90-79 Los Angeles 18201 Von Karman, Suite 1090 Irvine, CA 92612 Tel: 949-263-1888 Fax: 949-263-1338 China - Shenzhen Microchip Technology Consulting (Shanghai) Co., Ltd., Shenzhen Liaison Office Rm. 1315, 13/F, Shenzhen Kerry Centre, Renminnan Lu Shenzhen 518001, China Tel: 86-755-2350361 Fax: 86-755-2366086 New York 150 Motor Parkway, Suite 202 Hauppauge, NY 11788 Tel: 631-273-5305 Fax: 631-273-5335 Germany Microchip Technology GmbH Gustav-Heinemann Ring 125 D-81739 Munich, Germany Tel: 49-89-627-144 0 Fax: 49-89-627-144-44 San Jose Microchip Technology Inc. 2107 North First Street, Suite 590 San Jose, CA 95131 Tel: 408-436-7950 Fax: 408-436-7955 Hong Kong Microchip Technology Hongkong Ltd. Unit 901-6, Tower 2, Metroplaza 223 Hing Fong Road Kwai Fong, N.T., Hong Kong Tel: 852-2401-1200 Fax: 852-2401-3431 Italy Microchip Technology SRL Centro Direzionale Colleoni Palazzo Taurus 1 V. Le Colleoni 1 20041 Agrate Brianza Milan, Italy Tel: 39-039-65791-1 Fax: 39-039-6899883 Toronto 6285 Northam Drive, Suite 108 Mississauga, Ontario L4V 1X5, Canada Tel: 905-673-0699 Fax: 905-673-6509 India Microchip Technology Inc. India Liaison Office Divyasree Chambers 1 Floor, Wing A (A3/A4) No. 11, O'Shaugnessey Road Bangalore, 560 025, India Tel: 91-80-2290061 Fax: 91-80-2290062 United Kingdom Arizona Microchip Technology Ltd. 505 Eskdale Road Winnersh Triangle Wokingham Berkshire, England RG41 5TU Tel: 44 118 921 5869 Fax: 44-118 921-5820 03/01/02 2002 Microchip Technology Inc. |
Price & Availability of AN219
 |
|
|
|
|
All Rights Reserved © IC-ON-LINE 2003 - 2022 |
| [Add Bookmark] [Contact Us] [Link exchange] [Privacy policy] |
|
Mirror Sites : [www.datasheet.hk]
[www.maxim4u.com] [www.ic-on-line.cn]
[www.ic-on-line.com] [www.ic-on-line.net]
[www.alldatasheet.com.cn]
[www.gdcy.com]
[www.gdcy.net] |