AN795 datasheet(3/15 Pages) MICROCHIP

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DS00795A-page 3

AN795

TEMPERATURE METER

A temperature meter using the voltage output of a probe, such as one of the three shown, can be economically and straightforwardly

implemented with the TC9400 V/F converter. The V/F output is simply counted to display the temperature. For long-distance data

transmission, the TC9400 can be used to modulate an RF transmitter.

TC9400

V/F

Temperature Display

Reset

Latch

Gate

Temp

Probe

50/60Hz

Gate

Latch

Reset

Temperature

Probes

A. Thermocouple

B. Thermistor

C. Transistor

Junction

One

Shot

One

Shot

Preamp

FIGURE 5: Temperature meter.

A/D CONVERSION WITH A MICROCONTROLLER

There are two schemes that can be utilized to accomplish A/D conversion with a microcontroller:

Depending on the number of digits of resolution required, VIN is measured by counting the FOUT frequency for 1ms, 10ms,

100ms, or 1 second. The final count is then directly proportional to VIN. (The microcontroller provides the time base.)

VIN is measured by determining the time between two pulses (negative edges). FOUT is used as a gate for counting the

microcontroller's clock. The final count will then be inversely proportional to VIN.

By taking the one's complement (changing 1's to 0's and 0's to 1's) of the final binary count, a value directly proportional to VIN will result.

This technique will give a faster conversion time when resolution is very important, but dynamic range is limited.

Digital

Output

VIN

TC9400

V/F

PIC

Microcontroller

FOUT

FIGURE 6: A/D conversion with a microcontroller.

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