| 摘要 |
A circuit for inputting an ac signal to a digital processor includes a capacitor connected to the input and ground. A rectifier, preferably a transient voltage suppression device, permits the capacitor and also the stray capacitance which couples false signals to the input lead, to charge to one polarity only. A shunt resistor discharges the capacitor on half cycles of opposite polarity, but there is no discharge path for the stray capacitance which therefore charges up to block steady state false ac signals. Series resistors between the rectifier and the input determine, together with the shunt resistor, the minimum input signal level and increase the over surface breakdown voltage to protect the input from transient voltage conditions. A resistor connected between the series resistors and the rectifier, and to ground determines the charging rate of the stray capacitance. The TVS device provides high transient voltage withstand and breaks down without damage in the event of large transients. The signal applied to the digital processor, which is clipped to logic level by clamping diodes on the input, is checked by the digital processor for magnitude and frequency compatible with a true input signal.
|
