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Noisey 9226 and Common Mode

Hello,

I am running into some problems with the PT1000 card. I am seeing offsets (10s degC) and significant noise on the signals (10s degC). The noise only appears when I energise certain noise sources on the rig but, unusually, it only appears above certain temperatures

Admittedly the environment is noisey and there could be some common mode on the RTD input. 

Does anyone have much experience debugging noise issues on these cards? If say prehaps there was too much common mode, what would this look like on the measurement?

 

Many Thanks

Nick
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the 9226 measure in DC mode ... so it is prone to DC errors like thermovoltage .. any wire changes/connectors with temperature gradient?

What is the temperature range ?

do you use 4 wire connection down to the sensing element?

type of wire?

 

 

You can try an additional foil capacitor (my first try would be in the range of 470nF) between the sense wires.

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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Hello,

 

I somehow missed this reply which is frustrating!

Anyway,

Yes we can see wires with a temperature gradient, probably 100 degC on the wires.

 

Can you expand at all why this might be a problem?

 

How would this cause DC common mode?

Connections are only 2 wire.

Yea some additional filter caps could be sensible.

Thanks

Nick
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Switch to a 3 or better 4 wire connection/sensor

What sensor are you using? (Pt1000 is not the answer;) )

What type of cable?

Length?

 

 

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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I'm not sure I'd go w/ the temp gradient as a first issue.  First issue is 2 wire gives much poorer accuracy and isn't officially supported.  We have 2 wire RTDs in aerospace application simply because designers didn't want the extra weight of more conductors. For test sets I'll always go w/ 3 or 4 wire.  

 

If the RTD is in contact w/ chassis (film RTD on surface or a probe that is in contact w/ the probe sheath, in contact w/ chassis or plumbing) then ground noise voltage will create common mode (noise on both wires). I've seen inverter drives often have capacitive coupling into chassis that can mess up all kinds of readings as soon as you energize the inverter.  Likewise they can radiate and if you don't have a shield properly drained on one end only, a 1000 ohm RTD is more vulnerable than 100 ohm since it's higher impedance.  The filters on the 9226 are really for 60 Hz noise and are not as good in high speed mode if you are using it.

 

If it is common mode, you can look at each sense line with a scope and see the same noise signal appear when the noise source turns on.  If you have multiple inputs, I'd disconnect one at a time and see if only one is the issue.  It's a multiplexed A/D so noise on one can bleed onto the others, in my experience w/ similar modules (haven't used the 9226 itself).  And last, yeah, capacitors across the sense lines will help normal mode noise.  If you know the noise source frequency, you can tailor the cap value to make a low pass filter that addresses the frequency but minimizes impact on the real signal.  Higher frequency common mode noise can be reduced with ferrite clamps, too.

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