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LTC2984 2 wire and 3 wire RTD interchangeable connection

I want to generic hardware for RTD to connect both 2 wire and 3 wire types. However I should connect 4 RTDs to one IC. I want to use PT100 and PT1000 RTD types on the same configuration. What can you advise me? Should I use analog switch IC(if it's, which one?), or different connection method? Addition of that, I want to ask a question about Rsense, should I use Rsense sharing to yield max channel number and interchangeability?

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  • Hi Obione,

    LTC2984 is a complete temperature measurement system that can measure multiple sensors.  It has all necessary active circuitry, switches, measurement algorithms, and mathematical conversions to determine the temperature for each sensor type. So, you don’t need to add a switch IC to your design. You can use one sense resistor for multiple RTDs. RTDs possible per LTC2984 is 9 for a 3-wire and a sense resistor sharing configuration. 

    Please refer to page 32-49 in the LTC2984 datasheet for different RTD measurement applications.



  • Thanks for reply marymae but I didn't ask a question for 2 wire support, sorry for misunderstanding. I will make a pcb for RTD measurement but I want to use same ports both 2 wire and 3 wire sensor types without change the hardware. Briefly, there will be 3 pins and this 3 pins can connect with 2 wire and 3 wire types and my hardware will sense both types with only software configuration not hardware changing.

  • Hi,

    If you want to design a generic 2-wire or 3-wire sensor using the same connection port, you don’t need an external switch IC to do it. The LTC2984 input pin requirements for 2-wire and 3-wire pins are the same. See the example below. Take note that RTD and SENSE resistor channel assignment follows a general convention shown in Figure 14 in the datasheet. You just need to program the device sensor config bits B20 and B21 to determine if the RTD is a 2 or 3 wire type. To maximize the number of RTDs per LTC2984, use RSENSE sharing capability.

    The example below is an interchangeable 2wire or 3wire config with shared RSENE generated from LTC298x evaluation software. You can use this to get a good idea of how to configure and program LTC298x. 

  • Thank you. How can I make this generic hardware without using Rsense sharing? because individual Rsense circuits looks different in datasheet.

    According to software that you mention, I think there are some mistakes. For instance, I can select the Ch3 to RTD in your picture. However it is not correct right? or is it possible?

  • The example below is a 5 sets interchangeable 2wire and 3wire configuration. Each of these sets has a separate sense resistor.

    As mentioned, the RTD channel assignments follow the general convention. For example, in a 3-wire RTD application, RTD terminal1 is tied to device CHRTD (tied to Ch4 in the example provided), terminal2 to CHRTD-1 (Ch3), and terminal 3 to CHRSENSE (Ch2). The sense resistor terminals are tied between CHRSENSE (Ch2) and CHRSENSE-1 (CH1), where CHRSENSE (Ch2) is also tied to the 3rd terminal of the RTD. You need two adjacent channels to connect your RTDs terminal1 and 2. For more information, please refer to page 40 in LTC2984 datasheet.

    I hope I answered your questions. 

  • Thanks MHermoso. When I connect the 2 wire RTD, CH2 and CH3 should be connected with external conductor and CH4 will be tied to GND (according the datasheet page 37) with this solution. Also, I need to connect the CH1 to GND for 3 wire RTD because we are not using Rsense sharing(page 40) but this software doesn't show that GND. What is exact mean of "OPTIONAL" term in page 40?

    Thank in advance.

  • Could you give me an answer for my last question?

  • Hi  ,

    The term "OPTIONAL" in the page 40 of the LTC2984 datasheet means that the pin is either grounded or left floating depending on the configuration of the Rsense. If the Rsense is shared, the pin should be left floating. The pin should be grounded otherwise.

    I hope this helps.