00538c

00538c - AN538 Using PWM to Generate Analog Output Author:...

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Unformatted text preview: AN538 Using PWM to Generate Analog Output Author: Amar Palacherla Microchip Technology Inc. FIGURE 2: FREQUENCY SPECTRUM OF A PWM SIGNAL Fundamental component Harmonics Pulse Width Modulation (PWM) modules, which produce basically digital waveforms, can be used as cheap Digital-to-Analog (D/A) converters only a few external components. A wide variety of microcontroller applications exist that need analog output but do not require high resolution D/A converters. Some speech applications (talk back units, speech synthesis systems in toys, etc.) also do not require high resolution D/A converters. For these applications, Pulse Width Modulated outputs may be converted to analog outputs. Conversion of PWM waveforms to analog signals involves the use of analog low-pass filters. This application note describes the design criteria of the analog filters necessary and the requirements of the PWM frequency. Later in this application note, a simple RC low-pass filter is designed to convert PWM speech signals of 4 kHz bandwidth. In a typical PWM signal, the base frequency is fixed, but the pulse width is a variable. The pulse width is directly proportional to the amplitude of the original unmodulated signal. In other words, in a PWM signal, the frequency of the waveform is a constant while the duty cycle varies (from 0% to 100%) according to the amplitude of the original signal. A typical PWM signal is shown in Figure 1. Frequency spectrum of baseband signal FPWM = 1/T 3/T Frequency 5/T The band-width of the desired signal should be Fbw <= (FPWM = 1/T) If FBW is selected such that FBW = FPWM, then the external low-pass filter should be a brick-wall type filter. Brick-wall type analog filters are very difficult and expensive to build. So, for practical purpose, the external low-pass filter should be as shown in Figure 3. FIGURE 3: Bandwidth of desired signal EXTERNAL LOW-PASS FILTER Unwanted spectra due to PWM pulses FBW FPWM = 1/T Frequency 3/T 5/T This means, Fbw << FPWM FIGURE 1: Vo A TYPICAL PWM WAVEFORM or FPWM >> FBW => FPWM = K FBW (1) where, K is a constant such that K >> 1 The value of K should be chosen dependant upon the number dB the inherent fundamental noise component of PWM will be rejected. An example follows: Fixed period W1 W2 T Time 2T W3 3T A Fourier analysis of a typical PWM signal (such as the one depicted in Figure 1) shows that there is a strong peak at frequency Fn = 1/T. Other strong harmonics also exist at F = K/T, where K is an integer. These peaks are unwanted noise and should be eliminated. This requires that the PWM signal be low-pass filtered, thus eliminating these inherent noise components as shown in Figure 2. Example: It is required to design a simple RC low-pass filter to obtain an analog output from a pulse width modulated speech signal of bandwidth 4 kHz. From eqn (1), choosing arbitrarily K = 5, FPWM = K fBW = 54 kHz = 20 kHz. 1997 Microchip Technology Inc. DS00538C-page 1 AN538 FIGURE 4: RC FILTER CONNECTED TO PWM1 OF PIC17C42 For many applications, this rejection of -14 dB will not suffice. Therefore instead of a simple RC low-pass filter, a higher order active low-pass filter may be necessary. Or, if the microcontroller is capable of modulating at higher PWM frequencies, the rejection of noise will be greater. For example, using 8-bit resolution, the PIC17C42 can generate PWM frequency of 62.5 kHz. At this frequency the attenuation of the PWM frequency is: (dB)62.5 kHz = -10log[1+ (2 f.RC)2] = -24 dB. The higher frequency of the PIC17C42 PWM outputs makes it easier to generate analog output. PWM1 R C +10V OPAMP Analog out -10V PIC17C42 Choosing, the -3 dB point at 4 kHz, and using the relation RC = 1/(2 f), we get R = 4 k, if C is chosen as 0.01 F: R = 4.0 k C = 0.01 F Since the PWM frequency is selected as 20 kHz, the fundamental noise peak to be filtered is at 20 kHz. Now, lets calculate by how many dB the main peak of PWM signal is cut-off at 20 kHz: (dB) 20 kHz = -10log[1+ (2 f.RC)2] = -14 dB. DS00538C-page 2 1997 Microchip Technology Inc. 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 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. 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. 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. 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This note was uploaded on 07/23/2011 for the course EEL 4920 taught by Professor Arellano during the Spring '10 term at FIU.

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