I'd have to ask PCW, my gut tells ne that the resolver input gets slapped with clipping diodes to a square wave it then looks at the zero crossings, and converts that into a count cycle and calculates angles against a tpu in the asic.
but that's how automotive stuff handles those kinds of signals.
The zero crossings don't tell you much, it is the relative amplitudes of the two outputs that indicate the angle, and there is often only one electrical cycle per shaft rotation. It is necessary to measure analogue voltages.
For a 5V excitation voltage the output at 0 degrees of shaft rotation might be a 5V p-p 10kHz signal on the S pair and a 0V p-p signal on C.
Then at 45 degrees it would be a 3.53 V 10kHz signal on the S pair and also 3,53V p-p signal on the C pair.
At 225 degrees rotation there would be a signal that looks identical to 45 degrees, but out of phase with the excitation voltage. If the voltages are sampled at the peak of the excitation signal then the measured amplitude would be -3.53 on both.
The 7i49 manual says "The 7I49 uses a oversampling A-D followed by a tracking filter."
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