Signal-off detection circuit and optical receiving device using the same

A signal-off detection circuit counting input data signals per a fixed time for detecting a disconnection state of said data signal according to whether the count value reaches a predetermined value (a positive integer greater than 1), the circuit comprising:

• an amplifier for amplifying said input data signal;
• a direct current feedback circuit for adding an offset voltage to the input of the amplifier;
• a counter for counting said input data signals every fixed time and causing transition of its output level when reaching to said predetermined value;
• a first flip flop taking a predetermined level of said counter at the timing of the output level transition thereof and holding it for said fixed time;
• a second flip flop taking the holding level of the first flip flop every said fixed time and holding it;
• means for generating a reset pulse for resetting said counter and said first flip flop every said fixed time;
• and
• delay means for delaying said data-taking-in timing pulse by a predetermined time to output as a data-taking-in timing pulse,
• wherein the output of said amplifier is adapted to be counted,
• wherein the holding output of the second flip flop is used for an alarm output, and
• wherein said data-taking-in timing pulse is also used as the data-taking-in timing pulse for said second flip flop.