Analog multiplexer issue

[Murugesh_89]

Hi,

The input to the 4 channels are connected to a analog sensor monitoring a rotating wheel. The input is of 20% on duty and 80% off duty cycle. The frequency is 500Hz. The amplitude is 3.3V during the 20%
and 0V during the 80% off duty.



I am scanning each channel one by one for 100 milli seconds.

The problem is sometimes when a wheel is not rotating i am getting some signal( similar to noise ) as running. but other wheels are running.

Please give me any solution on this...

Thanks,
Murugesh

 

[erikl]

1. I think you should add some hysteresis to the U15A amplifier: some positive feedback resistor between output and non-inverting input.
Depending on your sensor output impedance, try 100kΩ or less.

2. Don't leave unused inputs open - they might increase noise. Connect CH4..7 to GND.

 

[Murugesh_89]

I tied the unused pin to gnd. Now i have removed the U15 part and sees the waveform at the output of Analog mux.

Again i am getting the same issue?

My doubt is..
1. Is there any analog and digital ground separation to be made
2. Is the pull down 10k to the each channel of analog mux creating the issue? Is it good to put or bad?

 

[D.A.(Tony)Stewart]

- phase noise on the edge of a static sensor is normal.
- the quadrature noise should not exceed +/-45deg , so only 1 line should have noise.

A state machine should ignore the commutation noise on one channel if you execute this properly in a PLL state machine. Start with a State machine filter that ignores dither noise and only processes valid progress in CW or CCW direction.

Start with a proper FInite State Machine design for a PLL rotational detector with noise rejection.