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Low Frequency Noise Shielding Wires

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veeru612

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Hi,
I hope everyone is staying safe and well in these tough times.

I have a PCB that is about 1" x 1" in size. It is a 4 layer board with a dedicated ground and power plane. I have 3 LED drivers that get pulsed at 5KHz and need to support up to 3A load on each channel at a time using a STM32 micro. I have a motion sensor whose pins are very close to the LED pins. It looks like the motion sensor wires are picking up EMI whenever the LEDs are pulsing at high loads. I need to shield my motion sensor wires from this EMI.
I tried to shield these wires and found that Cu foil works the best when I wrap these wires with it and connect the foil to the closest ground. Al foil helped but not to the same extent.
I also tried Cat5 shielded wire but it didn't help at all, guess it doesn't shield low frequency noise.

I am wondering if anyone could recommend a low cost shielded wire that I can use in a production environment.

Thank you so much in advance.

Regards,
Veeru
 

Hi,

What about using a meaningful LPF at your motion sensor signals. They usually don´t need to operate up to 5kHz, do they?
Maybe on the analog side, maybe on the digital side...

Klaus
 

Hi KlausST,
Great suggestion but I cannot modify the PCB as it is locked and already scaled up. We kind of overlooked higher amperage loads and the noise is more pronounced at higher loads. We did put a DSP filter in the firmware to clean up the noise but then the response of the motion sensor is a bit slow and undesirable but functional. The LEDs are always pulsed at 5Khz and one channel is always ON (steady state). Depending on the motion detection, other channels get turned ON and OFF after certain time.
So I am looking for a low cost and easy to implement shielding solution for this generation of boards. Any leads would be great.

Thank you.
 

Cat5 cable has good shielding working from DC to GHz. If it doesn't work, you didn't connect it right.

It's not clear what kind of "motion sensor" you are talking about. I agree with the component that its output signal can be probably separated from LED driver noise if filtered appropriately.
 

Hi,

Please confirm:
* PWM is generated by STM?
* ADConversion is done by STM? Which sampling rate?

If so, my idea:
Just synchronize ADSampling with PWM frequency. No need to be the same frequency, but any integer multiple will do.
Then a simple averaging filter (in simplest case) will do. --> no ripple at the digital side. Any PWM_caused AC noise at motion signals will be canceled out.
Simple and fast firmware. No hardware modifications nor shieldings.

Klaus
 

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