AD7124 internal temperature reading error

Hello Precision ADC Support Team,

My customer a precision temperature measuring device using ad7124-4, the analog schematic is attached as well as code and registers values. In case of connected RTD they provide 2 measurements – first with source current through RTD and second with sink current through RTD. They switch a current direction using a multiplexor.

They have an issue: 

They measure a temperature of ad7124 using an internal temp sensor after each measurement. In case of forwarding AIN0 and AIN1 polarity (source current) when they measure internal temperature, everything is ok. In case of sink current, the temperature reading (ADC code) grows 3 times. It is very strange because when temperature measurement occurs, AIN0 and AIN1 are switched off. Also, they noticed that temperature reading grows when the voltage on AIN0 or AIN1 is close to 2.45V.

 

Could you please give the advice on how they can resolve that issue?

 

Please ask me in case you need additional information.

Best regards, Dmitry Serbinov
FAE, Analog Devices, Inc. REP in Russia

/cfs-file/__key/communityserver-discussions-components-files/425/MKTFM.PDF

/cfs-file/__key/communityserver-discussions-components-files/425/5076.Code.txt

/cfs-file/__key/communityserver-discussions-components-files/425/AD7124_5F00_register.h.txt



attached files corrected
[edited by: rotax at 8:43 AM (GMT -4) on 12 Jul 2021]

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  • 0
    •  Analog Employees 
    on Jul 12, 2021 10:52 AM

    Hi, 

    Apologies. I am not sure if I understand the issue correctly, you have mentioned RTD and also mentioned internal temperature sensor. Does the issue concerning RTD measurements or just internal temperature measurements or both measurements? I meant does the customer perform RTD measurement using AIN0-AIN1 channel then followed by measuring internal temperature? 

    In terms of the attached schematic it looks like incomplete (missing pages). If customer can't provide the actual schematic or at least if they can provide us a block diagram (schematic and setup) on what are they trying to do so it will be easy for us to capture it. 

    For example, what is meant by sourcing and sinking current? Where does the current come from and what value? or how the RTD is connected for the two measurements? If a simplified version of how the sensors are connected can explain here then that would be helpful. I also noticed on the attached code that IOUT is output on AIN7 but in the schematic AIN7 is connected to AIN6 and connected to GND?

    If you would like to take this offline, that would be an option as well. Have you contacted the product owner directly? 

    Thanks,

    Jellenie

  • 0
    •  Analog Employees 
    on Jul 16, 2021 12:56 PM in reply to JellenieR

    Hi Jellenie,

    Apologies for so long delay, I clarified technical details. 

    The issue concerning only internal temperature measurement error. The customer performs internal temp measurement each time after RTD measurement. The attached schematic accommodates all analog schematic.

    RTD is connected as follows:

    Using analog MUXes on the board, the customer switches the current direction between I and IO pins. It's necessary for errors compensation. The current values are 0.2mA, 1 mA, 2mA. 

    AIN7 was used previously for thermocouple measurements. Now it's disabled. 

    Best regards,

    Dmitry 

  • 0
    •  Analog Employees 
    on Jul 19, 2021 11:38 AM in reply to rotax

    Hi, 

    Thanks for clarifying. Though I forgot to ask what is meant by grow 3x. It is in hex or decimal code reference? Can you give us the exact data for this? 

    When the current is switched, does the customer probe the inputs or reference inputs? On the attached schematic can you explain where or which part there show the switching or where exactly there is the RTD? I only see IN+ and IN- but I do not see that name in any other part of the circuit. The reference is also coming from ADR4525 and the same reference was used in the resistors at DA5 and I am not sure where will this be use as the output of the last mux does not show where the ND node be connected. 

    My thought here is that the RTD is excited by voltage and not a current source, but It is not clear based from a quick look. So I find it really hard to understand the flow Apologies, but if you can explain this or show the complete connected then that would be helpful. 

    But if the customer can probe the inputs, and reference and see that it is all within specifications in both conditions (switching current), then I think that is also helpful for debugging.  

    I believe this issue is being handled offline. So you if you wish to continue this offline then we can close this thread and just come back here to share how we resolved the issue. 

    Thanks,

    Jellenie

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  • 0
    •  Analog Employees 
    on Jul 19, 2021 11:38 AM in reply to rotax

    Hi, 

    Thanks for clarifying. Though I forgot to ask what is meant by grow 3x. It is in hex or decimal code reference? Can you give us the exact data for this? 

    When the current is switched, does the customer probe the inputs or reference inputs? On the attached schematic can you explain where or which part there show the switching or where exactly there is the RTD? I only see IN+ and IN- but I do not see that name in any other part of the circuit. The reference is also coming from ADR4525 and the same reference was used in the resistors at DA5 and I am not sure where will this be use as the output of the last mux does not show where the ND node be connected. 

    My thought here is that the RTD is excited by voltage and not a current source, but It is not clear based from a quick look. So I find it really hard to understand the flow Apologies, but if you can explain this or show the complete connected then that would be helpful. 

    But if the customer can probe the inputs, and reference and see that it is all within specifications in both conditions (switching current), then I think that is also helpful for debugging.  

    I believe this issue is being handled offline. So you if you wish to continue this offline then we can close this thread and just come back here to share how we resolved the issue. 

    Thanks,

    Jellenie

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