Telephone line is designed to carry audio signal, which is an alternating current in nature. To transmit measurement signal, which has very low frequency or DC signal, we have to convert the signal into AC with acceptable frequency range for telephone line transmission.
For transmitting analog data over telephone lines, its ideal to use the TC9400’s square-wave output. Since the square wave takes up less frequency spectrum a square wave is actually preferred over a pulse waveform for data transmission. By use of low pass filters, the square wave’s spectrum can be further reduced. The TC9400 converts the frequency signal back into a voltage output linearly proportional to the original input voltage at the end of the telephone line. [System’s block diagram source: Microchip Application Note]