AN795 MICROCHIP | Alldatasheet
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© 2002 Microchip Technology, Inc.DS00795A-page 12 FREQUENCY DIFFERENCE MEASUREMENT Frequency-difference measurement is accomplished by using two TC9400's in the F/V mode to convert both frequencies into two proportional analog voltages (V1 and V2). V2 is inverted by a unity gain inverter. V1 and –V2 are then added by the summing op-amp to give a voltage proportional to the frequency difference between F2 and F1. Since the TC9400 V/F input is actually the summing junction to an op-amp, V1 and –V2 can be summed at the TC9400 input to generate a frequency output proportional to the difference between F1 and F2. CONVERTERS SIMPLIFY DESIGN OF FREQUENCY MULTIPLIER* By using a programmable digital-to-analog converter in combination with frequency-to-voltage and voltage-to-frequency converters, this circuit can multiply an input frequency by any number. Because it needs neither combinational logic nor a high-speed counter, it is more flexible than competing designs, uses fewer parts, and is simpler to build. As shown in the figure on the next page, the V/F converter, a TC9400, transforms the input frequency into a corresponding voltage. An inexpensive device, the converter, requires only a few external components for setting its upper operating frequency as high as 100kHz. Next the signal is applied to the reference port of the DAC-03 D/A converter, where it is amplified by the frequency-multiplying factor programmed into the converter by thumbwheel switches or a microcontroller. The D/A converter's output is the product of the analog input voltage and the digital gain factor. R 3 sets the gain of the op-amp to any value, providing trim adjustment or a convenient way to scale the D/A converter's output to a much higher or lower voltage for the final stage, a TC9400 converter that operates in the voltage-to-frequency mode. The op-amp and R 3 can also be used to set circuit gain to non-integer values. The V/F device then converts the input voltage into a proportionally higher or lower frequency. FIGURE 25: Frequency difference measurement. TC9400 F/V TC9400 F/V TC9400 V/F Op Amp –V2 FOUT = K1 (F1 – F2) VOUT = K2 (F2 – F1)
DS00795A-page 13© 2002 Microchip Technology, Inc. AN795 FIGURE 26: Circuit uses frequency-to-voltage-to-frequency conversion, with intermediate stage of gain between conversions, for multiplying input frequency by any number. Digital-to-analog converter is programmed digitally, by thumbwheel switches or microcontroller, for coarse selection of frequency-multiplying factor; op-amp provides fine gain, enables choice of non-integer multiplication values. *Reprinted with permission from Electronics, October 12, 1978; Copyright © Mc-Graw-Hill, Inc., 1978. All rights reserved. TC9400 Frequency- to-Voltage Converter Comp In Offset Gnd Amp Out DAC-03 Digital-to-Analog Converter Offset Gnd Out Amp Out Comp In Gnd Analog Gnd V MSB LSB +5V Digital Gain Factor (Programmable) –5V 100kΩ 10kΩ 50kΩ –15V +15V 180pF 180pF +5V +5V 20kΩ 47pF 0.1 µF 1MΩ 15 1MΩ 100kΩ TC9400 Frequency- to-Voltage Converter +15V –15V MSB = Most Significant Bit LSB = Least Significant Bit 100kΩ 2.2kΩ REFOUT IBIAS VREF VSS VDD fIN IIN RGAIN 10kΩ VDD OUTGND IBIAS REFOUT IIN VSS VREF 510kΩ fOUT REFOUT REFIN VOUT Op Amp
2002 Microchip Technology Inc. DS00795A - page 14 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, PIC- START, PRO MATE, SEEVAL and The Embedded Control Solu- tions 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 T otal 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.
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