I need to compare a AC voltage against a reference voltage. I'm only interested in the positive part of the cycle so I need to clamp the negative part of the cycle to protect the comparator input. I need some degree of precision so I'd prefer not putting a diode in series with the AC signal. The first thing that comes to mind is a simple diode clamp:
This will clamp the input to approximately -0.65V. However, the LM393 datasheet says -0.3V is lowest voltage allowed on a input, stating in the notes that going any lower may cause the output to behave erratically due to parasitic action:
1) Is there a simple, common way to solve this problem?
2) I've seen the above solution with a simple clamp diode used in commercial devices. Does this mean the problem is not as bad in reality and the datasheet is just overly protective or the commercial product was just poor design ?
Best Answer
You could use a Schottky diode, at the currents you're running it should stay in the 0.3V range. Alternatively (or additionally) you could put a resistor from the positive input to ground, creating a divider that will keep the input in the recommended range. This will also reduce the voltage seen on the positive half of the wave, so you may want to modify your REF accordingly.