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I made a post a while ago about having large excursions when reading from my MAX6675. Despite using adum5401 digital isolator to float the MAX6675 as suggested in the original post, I still see a lot of noise on the MAX6675 thermocouple input using a scope.

I did notice on the MAX6675 board there's only one bypass cap on the the VCC rail. There is absolutely no filtering on the thermocouple input.

Large excursions in thermocouple readings with MAX6675

Questions: 1) Can I apply some filtering on there?

2) I read on a different forum that the thermocouple amp should ideally be connected to analogue GND and that the AVR microcontrollers don't have any. By using a ADUM5401 to float the MAX6675, have I effectively created an analogue GND? If so, it's still no go.

Any help would be very much appreciated!

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  • \$\begingroup\$ Thermocouples exhibit extremely low impedance (less than an ohm), so a capacitor simply doesn't do any good. Yes, it acts like a low-pass filter, but (assuming, for instance, a 1 ohm impedance) a 1 uF cap gives a frequency of more than 150 kHz. And thermocouples are actually lower resistance, so the cutoff frequency is even higher, probably limited by the self-resonance of the cap. \$\endgroup\$ – WhatRoughBeast Feb 4 '17 at 5:03
  • \$\begingroup\$ Bypass BOTH leads of the thermocouple to Analog GND. This reduces the need for the Interface to handle large RF and large spike energy. \$\endgroup\$ – analogsystemsrf May 23 '17 at 5:13
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If you have a reference junction then you could use a scheme like this.

enter image description here

If it's high frequency noise (+100Mhz rectifying at the amplifier) you might want to try simply adding a clamp on ferrite on the cable or wrapping the thermocouple cable a few times around a ferrite core if you have one lying around. Make sure the thermocouple wire is not making a loop and twist it if its long.

Another option would be to filter the output of the MAX6675 and add a passive or active low pass filter

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