more than 800mV noise in digital measurement (SPEED)

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kumecepent

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I am using a proximity sensor to measure the speed of a motor. THat proximity switches 24VDC. I am sourcing it from my control panel and measuring it back at the same panel. It has the 30 mtr length of wire to connect it with proximity sensor. I am getting some noise more than 800mV in that digital measurement line.Because of this, my digital measurement system detects it as HIGH signal and counts it as well for the speed measurement. how to avoid that noise?.....please help me...
 

A few things you need to clarify:
Are you using some sort of bridge to drive the motor? Where is it located?
Do the sensor and motor share any common signal or power paths?
Any ground loops?

A diagram of the system would be of great help.
 

Noise immunity requires specifying the signal, and the noise characteristics in full as well as the impedance of the sensor and the source of the noise.

Solutions may include,; Isolation, source filtering, sensor filtering, cable shielding or twisted pair, Ferrite baluns or CM chokes, RF caps, impedance matching, input hysteresis, etc.
 

Can it be the resonance due the wire parasitic inductance. Add a damping resistor is signal path and check.
 

24V signal with 800mV noise should not be difficult to detect with 20% hysteresis. Can you scale down to 5V and use a Schmitt trigger CMOS gate? A small ceramic cap will assist in some noise reduction.
 

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