AD7172-2 Precision Weigh Scale Issues

I´m using AD7172-2 converter, with external amplifier, in bipolar mode input. The load cell is 30kg capacity and 2mV/V, and the excitation voltage is +5VDC (REF is +5VDC too to do ratiometric system). I attach the scheme.

1- I have configured the Gain of amplier to 426.5 (Rg=470R). When I put about 27-28kg in load cell, AIN1_1 respect to AGND is 0V (it´s saturated). I don´t understand this. I attach table. Any suggestions? Actually, I have configurated the Gain of amplifier to 295.1 (Rg=680R), but I get less points.   

2- What is the procedure to compensate temperature drifts by software, on a weighing scale? Using AD7172-2 with DC excitation of load cell, It generates temperature drifts of the electronic components, which must be compensated, for legal trading, or to meet metrological standards.

Regards.

Parents
  • +1
    •  Analog Employees 
    on Sep 13, 2019 3:44 AM

    Hi,

    1. I would like to clarify something in your schematic and setup. I'm confused with the naming convention, does IN+/IN- your differential output? Are you probing this values on your table or reading it directly from ADC? You are reading the positive and negative input to GND, I assumed that the common mode voltage should be 2.5V? Why it looks like its dropping? Have you calibrated your system? A load cell has an offset or TARE around 50% and as well as a gain error up to 20% that's why ADCs full range is not typically used in weigh system application and load cell always need calibration. Can you lower your gain let say for example to 128 so that the offset and gain error do not overload the ADC. 

    In terms of calibration,  

    System zero scale calibration: empty pan. Ask ADC to perform the system offset calibration.

    Full scale calibration: place full scale weight on pan. This generates a FS voltage. With Vref = 5V, the FS signal that the ADC can accept is +/-40mV. The part can only perform system FS calibrations if the applied input is at least 80% of the ADC’s FS value. For this reason, the calibration needs to be performed in the uC. With the full scale weight applied to the loadcell, perform a conversion. The result is equivalent to the max weight applied. So, all further conversions need to be corrected in the uC (for FS correction).

    Lets say

    Conversion FS code = 30kg

    Conversion ZS code = 0g (assuming bipolar mode and a system offset cal has been performed).

    All further conversions are then scaled

    Conversion 0x?????? = ((30kg)/(FS code – ZS code))*(0x?????? – ZS code) kg

     2. You can use this as your reference for temperature compensation. https://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0355.html#rd-description

    Thanks,

    Jellenie  

Reply
  • +1
    •  Analog Employees 
    on Sep 13, 2019 3:44 AM

    Hi,

    1. I would like to clarify something in your schematic and setup. I'm confused with the naming convention, does IN+/IN- your differential output? Are you probing this values on your table or reading it directly from ADC? You are reading the positive and negative input to GND, I assumed that the common mode voltage should be 2.5V? Why it looks like its dropping? Have you calibrated your system? A load cell has an offset or TARE around 50% and as well as a gain error up to 20% that's why ADCs full range is not typically used in weigh system application and load cell always need calibration. Can you lower your gain let say for example to 128 so that the offset and gain error do not overload the ADC. 

    In terms of calibration,  

    System zero scale calibration: empty pan. Ask ADC to perform the system offset calibration.

    Full scale calibration: place full scale weight on pan. This generates a FS voltage. With Vref = 5V, the FS signal that the ADC can accept is +/-40mV. The part can only perform system FS calibrations if the applied input is at least 80% of the ADC’s FS value. For this reason, the calibration needs to be performed in the uC. With the full scale weight applied to the loadcell, perform a conversion. The result is equivalent to the max weight applied. So, all further conversions need to be corrected in the uC (for FS correction).

    Lets say

    Conversion FS code = 30kg

    Conversion ZS code = 0g (assuming bipolar mode and a system offset cal has been performed).

    All further conversions are then scaled

    Conversion 0x?????? = ((30kg)/(FS code – ZS code))*(0x?????? – ZS code) kg

     2. You can use this as your reference for temperature compensation. https://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0355.html#rd-description

    Thanks,

    Jellenie  

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