DS00795A-page 5

© 2002 Microchip Technology, Inc.

AN795

LONG-TERM INTEGRATOR FOR BIPOLAR (

±) SIGNALS

When the input signal is negative as well as positive, there has to be a way of generating "negative" frequencies. An absolute value

circuit accomplishes this by giving the V/F converter a positive voltage only; and also telling the counter to count up for a positive

voltage and to count down for a negative voltage.

ANALOG SIGNAL TRANSMISSION OVER TELEPHONE LINES

The TC9400's square-wave output is ideal for transmitting analog data over telephone lines. A square wave is actually preferred over

a pulse waveform for data transmission, since the square wave takes up less frequency spectrum.

The square wave's spectrum can be further reduced by use of low-pass filters.

At the other end of the telephone line, the TC9400 converts the frequency signal back into a voltage output linearly proportional to the

original input voltage.

Reset

Up/Down

Counter

Up/Down

TC9400

V/F

+

–

+

–

Op

Amp

Absolute Value Circuit

Op

Amp

1M

Ω

47k

Ω

47k

Ω

500k

Ω

TC9400

F/V

9400

V/F

Telephone

System Linearity

0.03%

~

Telephone

FIGURE 10: Long-term integrator for bipolar (

±) signals.

FIGURE 11: Analog signal transmission over telephone lines.