Zero Crossing Detector

[josrodmar]

Hi, I've been trying to build a simple zero crossing detector with an LM741 opamp with no much success. This zero crossing must work with a period sine wave of 60hz and provide an output of 5V when the wave is positive and 0V when the wave negative. The problem is the signal that I'm going to measure has an amplitude as little as 1mV. I figured out that if I connect this signal to the positive pin of the comparator and a ground to the negative pin, it won't work in single power supply configuration. I'm planning on raising this signal to an offset and compare the signal input with the offset. do you guys have any suggestion, or design available for signals that are as low as 1mV for 60hz periods.

 

[keith1200rs]

You really need to look for something far better than the 741. The 741 isn't a comparator and its offset it larger than your signal! Can you AC couple your signal? What is the maximum size of the signal? Is it a clean signal? What power supplies can you use? I would probably AC couple and add some gain first, if that was an option. Then look for a decent comparator.

Keith.

 

[josrodmar]

The thing is I'm using a true rms to dc converter in a single supply mode, which adds an offset of 5v to the input signal (in this mode). the signal range goes from 0mV to 200mV. I recently bough a ald2321 which is a precision comparator that can read signals of 1mV+ o so I read from the data sheet. I haven't been able to test it yet but in theory it should work I think. btw it is a clean signal, maybe with a little distorsion. I can use 9V and 5V power supply. Thank you.

 

[keith1200rs]

Sounds like you should be able to AC couple it. So, I would do that and put some gain in there. If you only have a 1mV signal then the offsets will have a huge effect on the zero crossing point. With only 200mV of signal you have a lot of room for a gain of 10 or more.

Keith.