Attila wrote:
I think the proper solution here would be to use a high speed comparator instead (with hysteresis).
See below for three possibilities, in addition to the Wenzel-style squarer I posted previously.
Circuit A is the simplest of these (and, in my view, best, because it minimizes noise). It is DC-coupled (although, if the input may have a DC offset, you should add a 10n coupling cap before R2). The LT1719 uses separate supplies for the front end and the output, so you can run the input on split +/- 5v supplies and still have whatever logic level you want. The down side is that you need +5v and -5v supplies, which you may not already have. Still, I think it is better to add these supplies than to try to do the whole conversion with only a 3v supply, because (i) these comparators work better on 5v than on 3v, and (ii) with 3v supplies you have to be very careful with the input level so as not to exceed the allowable input voltage.
Circuit B uses the simple-to-apply LT1720 in an AC-coupled circuit. Again, it uses a 5v supply (but no -5v supply) for the reasons given above. Use your favorite 5v to 3v logic converter, or just put a 220 ohm (series) and 330 ohm (shunt) resistive divider on the output to drive 3v logic.
Circuit C allows using the LT1720 with the existing 3v supply, at the cost of adding an input transformer. This connection keeps the input voltage within the comparator's allowable input voltage range.
Any of these should work better than a logic gate. Best regards, Charles
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