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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a 311 comparator. I connected Vc to 10v, gnd to gnd. I connected Vee to gnd.
I tried the setup with and without a pull-up resistor of 40k on pin 7, yet I could not get the comparator to work. I tried it with V- connected to ground, and then I tried it with V+ connected to ground. I tried toggling the other input to the 10v or the ground to see what would happen.
My ultimate goal is to take a floating ac signal and feed it into a comparator or a non-inverting opamp with a large gain to convert a sinusoidal wave into a clean square-wave for a PLL.
EDIT:
I seemed to make some progress. Am I suppose to have a resistor going into the V+ or V- input? I got it to work with one, and found I must have fried the other chips. I also noticed that at high frequencies the chip can not reproduce a clean square wave, and instead starts to make a ramp.
Edit:
Well, it seems I had some bad connection within the breadboard. The 311 chips were not blown. I also found that I could get a cleaner square-wave at high frequencies by dropping the value of the pull-up resistor on pin 7 to 1k. I does seem that I need some amount of resistance going to pin 2 (V+) in order to get a good square-wave; otherwise, my 50% duty wave becomes a 95% duty wave. Why is this?
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
It's possible that the 311 doesn't like it's inputs at Vss. You're coupling the signal in with a capacitor, right?
Try this: Connect the - input to a voltage divider to put it halfway between Vdd and gnd, and connect the + input to a high impedance divider. Couple you signal in with a capacitor at the midpoint of the second divider (same node as + input).
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
You need a pullup resistor on the output. Fast comparators like the 311 can go into oscillation so have a .1uf decoupling capacitor across the supply as physically close to the IC as possible.
I have worked a lot with the 311. You may wish to use a high-value feedback resistor to put some hysteresis into your circuit. Also, a 100 to 1000pF cap across the inputs can help with response/minimize output oscillation.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
MinorityCarrier wrote ...
I have worked a lot with the 311. You may wish to use a high-value feedback resistor to put some hysteresis into your circuit. Also, a 100 to 1000pF cap across the inputs can help with response/minimize output oscillation.
You mean put it across V- and V+ ? How does this reduce output oscillations?
What is the quad equivalent for the 311 for a fast comparator? Is it the 339?
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Help with comparator
