comparator problem

Hey I'm actually designing a comparator circuit

|--------feedback resistor------| | | | |---------------------| | RC CIRCUIT ----|------|+ | | | COMPARATOR CIRCUIT | | INPUT----------|- |--|---------OUT |---------------------| The task is to design this cirucit in such a way that when an input pulse (0.5,-2.5) with a pulse width of 8micro seconds is given it should produce me an output which remains high for atleast like

64micro seconds..i tried using R=2248.5k and C=8500pF and the feedback resistor i used was 90k..i was able to get an o/p which remains high for 64microseconds but when looked at it carefuly i was able to figure out that the outout remained high just beacuse of the large difference between the resistors(the storage capacitance is not much observed in the o/p graph) should basically work like a monostable multivibrator (without using ic 555 timer though) would be of great help to me if u could give me some suggestions as how could i do this..

thanks sk

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Since you're posting to the FPGA newsgroup....

STOP IT !! Don't try to add analog components - however simple they may seem in concept - when you have the FPGA resources to give you what you need with very little overhead. You need a clock resolute enough to discern 8ms from undesired shorter values. Use this clock to count when 8ms is valid then generate your 64ms pulse with the same clock. No fuss no muss.

If you're trying to use the differential inputs to an FPGA as a comparator, keep in mind the voltage tolerances on the input to your circuit as well as the voltage driving the feedback. The tolerances on your resulting circuit will be sloppy and not necessarily repeatable.

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I'm not sure I understand what you are doing, but if I get it right, your input pulse goes from 0.5 volts to -2.5 volts and 8 uS later returns to 0.5 volts. You want an output pulse starting when the input pulse goes low and lasts for 64 uS. If this is right, I would say you need to swap the feedback resistor with the RC. Actually, you don't need an RC, just a C will do.

But this circuit will have all the problems with noise that the 555 timer has and may even be worse since noise on the comparator output can affect your circuit.

BTW, where did you get a 2248.5k ohm resistor? What is that, .001% tolerance?

Rick "rickman" Collins
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