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Add CJC channel for thermocouples

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Hi!

 

I'm looking for an appropriate temperature measuring device for to add a temperature channel that I will use as a reference channel for my multifunctional X-Series DAQ board (USB-6353).  Ideally something that was small and had 3 or 4 terms (power and output) would be nice.  Like a small TOC packaged IC.  Something that can be easily wired to the terminal block connections and output the temperature around these connection is what I'm after.  Perhaps an RTD?  Mega kudos for part numbers.

 

Thank you for your input,

-Nic

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Solution
Accepted by topic author hecmar.arreola

Nic:

 

An RTD would need a precision current source, here are a few chip based sensors:

 

http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FM50.pdf

 

http://www.analog.com/static/imported-files/data_sheets/AD22100.pdf

 

http://datasheets.maximintegrated.com/en/ds/DS600.pdf

 

-AK2DM

 

 

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"It’s the questions that drive us.”
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Message 2 of 10
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Thanks for your reply!  I purchased a two terminal zener diode temperature sensor that is sensitive to ~20degC and then it gets bad error away from that point.  I'm not sure how well it will work...

 

The FM50 looks like a good low-profile device.

 

I wonder if I can mount it between my terminal blocks and that would be sufficient.  I've generally purchased specific TC reading devices in the past, but I find NI devices with integrated CJC to be a little over priced.  Their SCC type solutions are also a bit out of budget as well.  Don't get me started on 5B modules...

 

Measurement Computing have a bit better feature sets on their multifunctional DAQ boards as most come with integral CJC, however I much prefer the performance and drivers of NI multifunctional DAQ.

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Glad to be of assistance.

 

As long as there are no large thermal gradients in the area, I think mounting it to the connector block should suffice.

 

 

Was the Zener unit of the LM135 series? Those need to be trimmed for calibration, but should be linear even if not.

http://www.ti.com/lit/ds/snis160c/snis160c.pdf

 

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
~~~~~~~~~~~~~~~~~~~~~~~~~~
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Yup.  You are a good guesser.

 

LM335AZ to be percise.  Should come in today.  I hope I can get it to work properly.  I'm thinking of using a Fluke TC calibrator to give a constant 0 deg C to a channel so I can just subtract the offset later if I can't get it to work for this project.

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This LM335AZ works great on my benchtop with a seperate power supply using 1kohm resistor connected to a 5V DC source. In this setup I get about 3V at ambient temperature and it readily responds to me squeezing the diode (increasing the temperature0.

 

When I use the 5V off my DAQ boards digital +5V going through a 1k ohm resistor and then to DGND I get 5V across the zener diode :(.  If I go up to 10k ohm, I get 4.578 (something) V at ambient but when I squeeze it, it is far less responsive.  I can't figure out what would be different from my DAQ device 5V source versus my benchtop 5V source.

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I have gotten the LM335AZ to work for my DAQ applications, but I find it to be rather noisy.  The noisiness of the device can be observed on the thermocouple channels that are using it as a termination temperature reference. 20 - 40 deg C (room temperature plus some wiggle room) is all that I'm after as far as range goes.

 

I could design my own filtering for the LM335AZ but I think there might be a better solution out there.

 

Thanks for your help.

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Nic,

 

You said you were able to get your LM335AZ to work for your DAQ (USB-6353) applications. As you said, the only difference between the 5V sources was that one was from the DAQ device and the other was from your benchtop power source.  

 

What did you do to get the setup with the 5V from the DAQ to work?  Or did you stick with the benchtop power supply?

 

And what kind of noise are you getting? Is it sinusoidal or random?

 

 

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Of course  the CJC noise dircetly add to TC noise. (well, nearly 😉 )

 

Looks like some EMC issue, since CJC temperature usually change slowly, some (vitamine) C migth help.

I don't have the specs of your DAQ/ Diode , just to avoid a to high load on power up, a 1k R as current limiter and 470nF at the AI to GND ??

 

Better find the source of that noise. 

(Analog) scope to 5V, Vout ??

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 don't remember how I fixed this problem, but was able to... still using this part. The +5V was from the DAQ board itself. No analog low-pass filters, but I do utilize a lot of digital filtering and oversampled averages. I find in my applications the noise level when related to °C is nominally ±0.5, which is great considering my thermocouples are ±2.2°C or greater.

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