Where This Noise Come From?

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goodoboy

Senior Member
Location
Houston
Hello,

I have a 4-20mA HART valve positioner in the field that is controlled via the DCS (analog output card) connected with a #16 2 wire cable.

I have intermittently Not Communicating With Device alarms at the DCS. Which means the HART signal goes away.

  • I put an oscilloscope on the wire and saw voltage spikes up to 9V.
  • We check the cable and it is good.
  • Valve specialist came and inspected the valve and checked out good. The specialist
  • The shield is terminated to references ground back that DCS cabinet. and the shield is not terminated back at the device.
  • I check for noise on similar valve in a different area and I saw small noise of about 1V spike and this valve works perfectly good with no communication issues.
  • The problem valve is located next to 2 VFD medium voltage pumps, so I suspect this AC voltage could be causing noise.

When I check resistance between earth ground and the negative terminal at the device, I notice resistance up to 80Kiloohms, so maybe floating ground. :?

Any suggestions or recommendations.

Thank you
 
Have you checked for AC ripple on the DC supply to the instrument? HART does not like much AC ripple and I have run into that issue a number of times.
 
don_resqcapt19 said:
Have you checked for AC ripple on the DC supply to the instrument? HART does not like much AC ripple and I have run into that issue a number of times.
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Thank you don_resqcapt19,

What did you do to resolve the issues you had? Please share.

Yes, I measured (positive to power supply common and positive to instrument reference ground) the AC ripple with an oscilloscope .

positive to power supply common : 240 to 260mV peek to peek
positive to instrument reference ground : 240 to 260mV peek to peek
 
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