I've a zero-cross circuit for low Hz (<200Hz) but potentially higher voltage (80v) which is working well in terms of reading a signal but less so in terms of phase.
Originally the circuit was modelled in LTSpice using a LM2901 with a little additional hysteresis. Since lower value zeners tend to have a bit of capacitance the resistor R5 would bleed this off. My first question is whether when it comes to the AVR comparator this is needed as the hysteresis isn't modelled in current but appears to be a voltage injection (10mV/25mv/50mv). Obviously it has some impedance but it's not specified. I can't see it being a traditional resistor from the output when specified in mV because the output voltage is unknown (Vdd).
The second question is to do with phase. I'm attempting to get a measurement but I can already tell by the way it controls another circuit that it's not cutting in or that close to the zero cross point (absolute accuracy isn't required). As I see it the D31 and Z2 have some capacitance which I can't do much about. I can take R7 down quite a bit without violating the Z2 rating but that leaves R5 again.
I'm wondering if I can take R5 completely. The hysteresis shouldn't be required as anything remotely <0.55v dac value/ref is fine to be considered as a zero-cross.