TC Filtering

[wyked]

I've seen a couple posts where people have mentioned using capacitors or RC filter circuits for filtering thermocouples.  I didn't see any listing specifics though.  Does anyone have a suggested capacitor size or RC circuit they can offer up that has worked for filtering K-type TCs that are being read in as raw mV signals?

[Logan_K]

If you use a device that is design for thermocouple measurements, filtering is usually built in. What device are you using to measure the thermocouple? What sort of noise or frequencies are you trying to filter out? What is the bandwidth of your measurement device? Typically 50Hz or 60Hz are the most troublesome frequencies that people try to filter out. Filter design is a complicated subject (probably too complicated for a discussion forum like this), so I am not going to presume that a simple RC filter is the right choice for your design. However, in order to get about 40dB of attentuation at 60Hz with an RC filter, you will need to position the 3db point of your filter at 2Hz. So just solve this equation for your resistance and capacitance values: Fc = 1/(2*pi*R*C) where Fc is the cutoff frequency (2hz). If you have large bias currents in your circuit, then the resistor may generate offset errors if it is too large.

Good Luck!

-Logan K

[wyked]

Thanks Logan,

I'm running the TC channels to a PXI-6259 through a SCB-68 connector box so no filtering is built in that I'm aware of?  The 60Hz noise is exactly what I'm trying to eliminate initially to determine if that is all I have going on or if I have other problems I need to address.   Thanks again for your help.

Dan

[Mark_E]

Hi Dan,

Correct, there is no built-in filtering for the PXI-6259, as it’s not designed specifically for temperature measurements. But the great news is that your SCB-68 includes a temperature sensor and breadboard so you can have a reference temperature and add any RC filtering you want.

The entire SCB-68 68-Pin Shielded Connector Block User Manual is great, and starting on page 5-7 (52) you’ll find lowpass filtering theory, component selection, etc.

Some other relevant links are below:

How Can I Read the Value of the Cold Junction Compensation of the SCB-68 (SCB-100)?

How Do I Configure the Switches for CJC Sensor Input on the SCB-68?

How Do I Use the Cold Junction Compensation on the SCB-68?

Hope this helps.