Dividing voltage on comparator input or something else?

[paradajz]

Hello. I am using salvaged HDD motor (motor only, no PCB or anything else) and I wanted to feed its inputs to LM339 comparator. Motor generates two phase-shifted sine waves (+-500mV max), and purpose of comparator is then to convert those signals into digital ones (0/5V) so that I can use motor as rotary encoder. However, datasheet for LM339 says I can feed -0.3V into inputs, and I can generate -500mV, so that's an issue. Obvious solution is to use voltage divider on input, but am I going to lose some resolution from disk then? Any suggestions what to do?

 

[jiripolivka]

Any comparator needs to compare two voltage inputs: one is your input signal, the other the reference voltage. Use an adjustable reference from a potentiometer. If your input signal is not exactly rectangular, you can generate a nice 0/+5 V output and avoid any response to noise or unwanted ripple.

 

[paradajz]

Yeah I'll go with voltage divider after doing some maths. One more question:

What is the difference between input voltage and common-mode voltage range? I'm not sure what is the minimum voltage I can apply to LM339 inputs (considering that I can generate -500mV).

 

[Audioguru]

Didn't I answer your questions a few minutes ago on another website forum, or from another kid in your class?

The maximum allowed negative input voltage is -0.3V, more might cause damage. One circuit on the other website shows a series resistor and a Schottky diode clamping the input voltage so the -0.3 is not exceeded.
The input common-mode voltage range is the voltages where the inputs work normally. It is from 0V to 1.5V less than the positive supply voltage.