AN795

© 2002 Microchip Technology, Inc.

DS00795A-page 10

ANALOG DATA TRANSMISSION ON DC SUPPLY LINES (TWO-WAY TRANSMITTER)

By converting an analog voltage to a linearly-proportional pulse train of short duration, it is possible to transmit this data on the same

wires used to energize the V/F converter.

The TC9400 V/F converter shorts out the DC supply for 3

µsec out of each period. At 100kHz, the supply line is down 30% of the

10

µsec period. As the frequency is lowered, the down-time decreases, so that at 1kHz the line is down only 0.3% of the time.

Two precautions are necessary to assure that the system does not stop functioning during the shorting period. At the power supply

end, a 1.2k resistor limits the current to 10mA on a 15V supply line. This prevents the TC9400 from being operated beyond its output

rating and at the same time prevents the supply from being shorted out. At the V/F end, a capacitor is used to keep the TC9400

energized, while the diode keeps the capacitor from being discharged.

Since the TC9400 requires only 2mA of current, a 1

µF capacitor ensures a stable voltage (the ripple is only 6mV). Since the 3µsec

pulses appear at the left side of the 1.2k

Ω resistor, it is easy to sense the signal here and convert the data back into a recognizable

format. A frequency counter connected at this point will directly display the input voltage by counting the frequency.

If an analog output is required, a TC9400 in the F/V mode can be used to convert the frequency back into a voltage. The overall

linearity is on the order of 0.03%, when both V/F and F/V are used. If only the V/F is used, 0.01% linearity can easily be achieved.

DIGITALLY CONTROLLED FREQUENCY SOURCE

This system generates frequencies controlled by a microcontroller counter, register, or thumb-wheel switches. Applications for such a

system include computer-controlled test equipment and numerically-controlled machine tools.

FIGURE 21: Analog data transmission on DC supply lines (two-way transmitter).

FIGURE 22: Digitally controlled frequency source.

Frequency

Counter

+

Remote Sensor

Analog Display

Analog

Input

TC9400

V/F

Digital Display

+

–

49

8

14

+

1

µF

1.2k

Ω

8-15V

DC

Power

Supply

3

–

TC9400

F/V

8

10

D/A

TC9400

V/F

Digital

Signal

Source