AN795 datasheet(12/15 Pages) MICROCHIP

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AN795

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.

R3 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 R3 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

Amp

–1

–V2

FOUT = K1 (F1 – F2)

VOUT = K2 (F2 – F1)

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