I made an amplifier of PT100 bridge which operates on 24V DC supply and gives 0 - 10V DC for 0 - 400°C when tuned. The circuit is as follows : enter image description here

Here P2 + and P4 S+ are shorted and P4 S- is left open. PT100 is connected between P2 + and -.

The device works fine and gives proper output in my lab but it is to be used in an electrical control panel that has VFDs (Variable Frequency Drives). When the load is connected to the drives, the output of my amplifier turns 0. The weird part is if I connect a multimeter to measure the output, it gets to a value near the desired output but fluctuates. I put a 1M resistor across the output but it makes no difference.

The input power supply remains same as well as the input to the amplifier across P4 S- and P4 S+ remains very stable.

This might be the noise from the VFD harmonics causing this. I tried using shielded wire but no difference.

Is there a way to prevent the effect of noise on my amplifier. Some sort of a filter. Please suggest. Thanks for all the help in advance.

Also, I don't have access to the VFD. All I can do is modify the transmitter (Amplifier) and replace the existing one.

  • \$\begingroup\$ What does an oscilloscope show, used as a magnetic field probe? Simply clip the alligator clip ground to the center pin of the (10 MegOhm) probe, and you now have a 3" by 3" magnetic-field probe. \$\endgroup\$ – analogsystemsrf Feb 5 '19 at 10:05
  • \$\begingroup\$ @analogsystemsrf I've never tried that, neither do I have an oscilloscope laying around. There is no 10 MegOhm probe. Anyway, I'll see about that for sure. \$\endgroup\$ – Ohbhatt Feb 5 '19 at 10:09
  • \$\begingroup\$ @analogsystemsrf I ordered a portable oscilloscope as the one I had was a bulky CRO. I'll check the magnetic field probe as you said very soon. But meanwhile, I would want your opinion connecting a ceramic capacitor across the output and maybe the inputs as well. Would that make any difference? The multimeter, when connected across the output or input, stabilised the output (maybe because of the capacitors in the DC measurement mode). Thanks for the help ;). \$\endgroup\$ – Ohbhatt Feb 6 '19 at 5:11

If your sure you have proper grounding and shielding techniques then try a filter like this:

enter image description here

enter image description here

Source: AD8420 Datasheet

I would set the pole lower than 60Hz because in an area with AC mains (and higher frequencies from the VFD's) this would block out most of your noise.

The board needs to be shielded also and the probe cable. Make sure the probe is shielded in a manner that prevents currents on the shield (tie it to ground only on one end, preferably the amplifier end).

Another thing is shielding can only block electric fields, not magnetic. If the sensor or amplifier is placed in close proximity to magnetic fields (or run next to cables with high currents in them) then that will most likely create noise.

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