I'm trying to use a type-J thermocouple with the ADS1248, but I'm having some difficulties. I know i have to enable the VBIAS on the negative channel, but I'm not sure about IDAC. If i enable it it overflows, if i disable it the values get very unstable. Is there anything I'm missing? I know the hardware should be fine since I'm using this design with a PT100.
Hi Ramiro,Welcome to the forum. For ungrounded/floating thermocouples, the VBIAS function could be used to bias the thermocouples at the optimal common-mode voltage in the middle of the supplies. As you mentioned, one of the inputs of the thermocouples will need to be connected to VBIAS, and this works well provided the thermocouple sensor is floating.Thermocouple sensors produce their own differential voltage accross their terminals as a function of temperature and they do not require a voltage or current excitation. RTD's and Thermistors are resistive elements and they require the IDAC current excitation to produce a voltage output. Are you using the IDAC excitation current for another RTD sensor in the application? Where are you applying the IDAC current? Do you have an schematic? Attached is an application note of the ADS1118 measuring thermocouples that may give you useful background information on thermocouple measurements.
Thank you for the application note.
I'm not sure I understand your question. When I want to read a RTD I apply IDAC current on both the negative and the positive input.
Hi,On the post above, you have mentioned that you are trying to measure a J-type thermocouple with the ADS1248 applying VBIAS on the negative channel; however, it mentions that you are not sure about the IDAC.One possible way to set up the device to measure an ungrounded thermocouple is to apply VBIAS in one of the terminals and set up the device to perform the conversion using the 2.048V internal reference of the ADS1248 device. When performing the RTD measurement, the user would typically use the IDAC current to excite the RTD and also place an RBIAS resistor in series between REFP and REFN to generate an external reference in a ratiometric configuration. The RBIAS resistor is also used in this configuration to bias the RTD at the ideal common-mode voltage close to the middle of the supplies. Please refer to the diagram below.Please refer to the link below to an application note where the ADS1248 is performing thermocouples and RTD measurements.http://www.ti.com/lit/an/sbaa180/sbaa180.pdfPlease let me know if you having issues with the RTD or thermocouple measurement; if you have an schematic available, we are happy to review it.Thank you and Best Regards,Luis
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