ADS1248 internal temperature diode

Attila Eredics

Other Parts Discussed in Thread: ADS1248, ADS1118, ADS1216

Dear all,

I’d like to use the ADS1248 to measure 7 copper-constantan thermocouples (the constantan wire is common for all the thermocouples, so basically I have only one cold junction). My question is about the cold-junction compensation:

Can I use the internal temperature monitor for this purpose? In an other forum post (http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/51686/182869.aspx#182869) this use was discouraged by Joseph Wu, stating that sensor error may be as big as +/-6 C. However the datasheet says nothing about the accuracy and repeatability of this sensor, only a nominal output at 25 C (+118 mV) and drift (405 uV/C).

After custom calibration of each device can I use this onboard diode for cold-junction compensation (assuring good thermal contact between the IC and the junction)?

Inside the IC where the temperature diode is situated? I’d like to place the cold junction as close as possible.

What is the repeatability and aging of the onboard temperature diode?

Thank you for your help,

Attila

over 14 years ago

0 Bob Benjamin over 14 years ago

TI__Guru** 113765 points

Attila,

I agree with Joe. This temp monitor for the ADS1248 was not designed as method for CJC. To the best of my knowledge, the temp diodes where not characterized over temperature, so we can not identify the linearity over temperature nor repeatability over time. This differs from parts like the ADS1118.

That said, this doesn't mean you can't characterize the diode yourself and calibrate your own system. There may be some other alternatives however. You may want to review the following documents for some further ideas.

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=SBAA073

http://www.ti.com/lit/an/sbaa134/sbaa134.pdf

Best regards,

Bob B

0 Attila Eredics over 14 years ago in reply to Bob Benjamin

Prodigy 195 points

Bob,

Thank you for the information! It's a pity that the ADS1248's temperature diode is not as flexible as the ADS1216...

I think I'll use a standalone temperature sensor with SPI interfce for the CJC, and I'll totally shut it down (including the SPI lines) during conversion, to avoid the interference with the analog signals.

Best regards,

Attila

Data converters

Data converters forum

ADS1248 internal temperature diode