Op Amp comparator problem

[dfox416]

Hi,
I want to use an op amp comparator to convert a 1V p-p sine wave to a 0-5V square wave. I tried using a standard comparator circuit and I got no output. Then, I put a 10pF capacitor between the negative op amp supply rail and ground and it worked. I am using an LF353. I attached schematics of the two circuits. Does anybody know what is happening?

[w2aew]

Hi,
I want to use an op amp comparator to convert a 1V p-p sine wave to a 0-5V square wave. I tried using a standard comparator circuit and I got no output. Then, I put a 10pF capacitor between the negative op amp supply rail and ground and it worked. I am using an LF353. I attached schematics of the two circuits. Does anybody know what is happening?

There are a couple of problems here.
1) Op amps aren't really optimized for use as a comparator (poor dynamic saturation characteristics, etc.)
2) The LF353 is designed primarily for supplies greater than +/-6V (split), which means at least 12V from VCC to VEE.  Operation at lower supplies will be sub-par.  Proper operation with only 5V is probably not going to work well at all.
3) The input common mode range and output swing are not compatible with what you are doing.
4) If you're judging this circuit based on simulation - then the simulator is NOT predicting reality.  Models used in simulation make the assumption that the device is being biased and used correctly.  If you don't  use the part correctly, the simulator won't tell you that - it will just lie to you.  After all, it is nothing more than a set of equations designed to mimic *proper* operation of the circuit.  If the circuit is not setup right, the equations, and results, will be meaningless.

So, I'd say to start off using a comparator, bias one input with a DC voltage that is at the mid-point of your sine wave.  You may need to add a DC component to your sinewave to balance it around your decision point.  Or, bias both inputs, and AC couple your sinewave in...