How do I remove low pass filter in the scxi-1102 module?

[hhvu]

I am using the SXCI-1102 module for thermocouple measurements but I need to remove the 2 Hz low-pass filter in order to observe very fast changes on the order of milliseconds or less. I was told that this is possible by removing a resistor. How do I do this?

[Bill B]

Hello,

Thank you for contacting National Instruments.

In order to remove or modify the 2Hz filter, you would need to modify the circuitry of the SCXI-1102. We do not support modifications to our hardware. Once the hardware has been modified, you will have voided your warranty and lost accuracy specifications of the device.

I understand your intentions and I find them to be reasonable, however, this is something we do not support.

Regards,
Bill B.
Applications Engineer
National Instruments

[guqunlin]

I am use scxi-1102b measuring 26 channels thermalcouple and 6 channels voltage .Can i cut-off the scxi-1102b filter frequency from100HZ to 50HZ for filter againest the electrical power noisy.

[Buechsenschuetz]

If you have 50Hz noise on your inputs you first should check for ground loops and other possible sources of 'mains hum'. It is better to avoid such unwanted signals than to filter them.

[guqunlin]

i make a lockrotor motor testing ,the T type thermocouple welding to the motor frame .and i sure the noise from electrical power , because the nois go away when power shutdown. but i wonder that noise will be generated when i touch the thermocouple in my hand (not touch on thermocouple tip).i not sure the motor whether grounding .i used blockterminal is 1303.

[guqunlin]

by the way , my signal conditioning card is scxi-1102b, acquiring data card is scxi-1600. i touch the thermocouple when the power off.but the noise still occur.

[Logan_K]

The 2Hz filter on the SCXI-1102 was designed specifically with 50Hz/60Hz noise in mind because it has a very large attenuation at 60Hz. If you want to filter out noise from power lines I would recommend the SCXI-1102.

The type of noise that you are seeing, however, appears to be a large common-mode noise on motor's frame. The SCXI-1102/C isn't designed to work with non-floating thermocouples, so the 2Hz filter of the SCXI-1102 might help a little bit, but in a situation like yours I would highly recommend using an isolated amplifier for your measurements. The SCXI-1125 with the SCXI-1328 terminal block is the ideal solution for making isolated thermocouple measurements, and I am certain that it would help eliminate the large noise you are seeing.

-Logan K

[guqunlin]

but i wonder that there is no exceed 3HZ singal in the spectral measurement.i don't know if the thermocouple will disturb by magnetic field .if it's sure ,how about the frequency of the noise.

[Logan_K]

Real-world filters are not ideal and do not have a sharp cutoff at their nominal cutoff frequency. So, the 2Hz filter on the SCXI-1102 will attenuate much of the signal at 2Hz, but higher frequency signals will still pass through. There is a lot of material out there on filters if you want to know more.

So, how large is your noise? If the noise is 50Hz, like you say, and it is only a few degrees in amplitude, then it is possible that the 2Hz filter on the SCXI-1102 will solve the problem. Although, I have seen several noise issues with the SCXI-1102 when the thermocouples are welded directly to a large piece of conductive metal, and in all of those cases the problems were solved by using an isolated board instead (like the SCXI-1125).

Regards,
Logan K

[guqunlin]

thanks for your help.

my terminal block is scxi-1303 so the bias resistor set is 10 ohm by factory default ,i check the scxi-1303 installlation manul i notice that the recommend 10M ohm bias resistor for scxi1102/B/C when the signal is floating signal ,as you know thermocouple is floating signal.i don't know whether the problem caused from this one.

regards