You didn't mention what the 6.67 volt signal really is or what granularity of measurement is required .. For pulsating dc counting the pulses would be a possibility. The easiest method for measuring DC I can think of is to scale the voltage by a factor of four and using an op-amp for dc level shifting to move the +/- 6V signal to a +/- 1.5V signal where 0 volts is centered in the scaled measurement window. This way 0 RPM is centered in the detection window and variations speed and rotation affect the polarity and magnitude of the measured signal from the motor tachometer. The only real issue I see is the possible requirement for a different AVref to calibrate the device since it is measuring a small piece of the tach signal.Bob
Yes, that is exactly what I have. I think I will try the precision rectifier route with the voltage divider . Thank you all for the help.