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Original question:

ADS1256: Is ADS1256 suitable to be a substitute for ADS1234 in Weight scale applications?

ADS1232: ADS1232

wagre prashant
Prodigy 60 points
Part Number: ADS1232

Hi,

We are designing a 50,000 KG measuring weigh bridge, the load cell will be required to excited 10V or 15V power supply.

the load cell is 2mV/V, is it possible to achieve least count of 1KG, with ADS1232 chip.

at the input can we use a INA to amplify the signal and give the amplified out to ADS1232.

I would prefer 10V excitation with no signal processing using INA and feed the inputs directly to ADS1232 but the accuracy of 1KG needs to be achieved

Thanks

  • 0
    TI__Guru** 113765 points

    Hi Wagre,

    The ADS1232 is designed to operate using a 5V excitation for a ratiometric measurement. This allows for a common-mode of 5V and minimizes noise and drift as the reference and excitation voltages are the same.  You should be able to achieve 50000 counts of resolution using this method with a 2mV/V load cell sensitivity (see the example on page 24 of the ADS1232 datasheet.)

    If you use a voltage greater than 5V for excitation, then you will be required to separate voltages for both the excitation and the reference to the ADS1232. You need to be careful as the output for the load cell will be outside of the input range of the ADS1232 if you are using 10-15V excitation when using unipolar supplies referenced to the same analog ground. 

    When exciting with 10-15V, you will need to use bipolar supplies to maintain the common-mode input to the ADS1232 PGA at 2.5V (for a 5V analog supply).  For 10V excitation you could use +7.5/-2.5 or for 15V you could use +10V/-5V.  The reference could be established using a voltage divider from the excitation source.  However, the result will be influenced by the accuracy/drift of the voltage divider resistors.  In this configuration it is difficult to tell if the resolution will be better or worse even though the dynamic range increases due to increased noise/drift.

    To achieve 50000 counts of resolution external noise is often the limiting factor due to EMI/RFI pickup on connectors and cabling.  Proper shielding on cables and good PCB layout are key factors.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 60 points

    Hi Bob,

    Thanks for the answer.

     I am thinking of two options

    1. Excite the bridge using 10V excitation, full load output of load cell will be 20mV, use an INA provide gain of 125, this will boost the analog signal to 2.5V.

    configure the ADS1232 for Uni polar operation ie AINP = 2.5V, AINA = 0V, set the PGA gain to 1, this will satisfy the full scale input voltage condition (AINP -AINA)= 0.5 * Vref / Gain.

    VREF = 5V (to be derived using precision resistor divider from 10V) , AVDD = 5V, DVDD  = 3.3V.

    2. Second option using +5V & -5V as excitation voltage, using the VREF=AVDD =Bridge excitation = 5V , rest signal processing using INA (Having gain of 125), PGA gain = 1 same as option 1

    Thanks

  • 0 in reply to wagre prashant
    TI__Guru** 113765 points

    Hi Wagre,

    The suggestions I was making is not using an external INA.  You can certainly use an external INA and adjust the common-mode accordingly.  However, you will add noise of the INA and the measurement will not be a ratiometric measurement and will be subject to additional drift.  Also there is a noise advantage of using the PGA on the ADS1232 as the input-referred noise lowers with gain applied.  You will not see this advantage using an external gain stage.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 60 points

    Hello Bob,

    Thanks for reply.

    Can you please elaborate the 50000 counts comes from, the noise free counts shown in the datasheet is 177385.

    Secondly using +/-5V excitation for bridge & AVDD=VREF=+ Bridge excitation = 5V & PGA as 128, can 1KG resolution be achieved?

    Thanks

  • 0 in reply to wagre prashant
    TI__Guru** 113765 points

    Hi Wagre,

    50000 comes from the stated requirement of 50000kg and 1kg resolution.  The datasheet example shows that you should be able to achieve much more than that, so you should be able to meet the 50000 desired noise free counts needed in your measurement results by the ADS1232.

    However, you cannot achieve the desired response directly using +/-5V excitation for the bridge without a level shift of some sort. The reason is the output of the load cell will have an output common-mode of EXC/2, or  0V (AGND).  The input restriction for the PGA of the ADS1232 is AVDD-1.5V to AVSS+1.5V.  For 5V AVDD, this means that the common-mode input range is limited to 1.5V to 3.5V.  So you would need to use unbalanced supplies for excitation greater than 5V.  To target the common-mode to 1/2 of AVDD (which is the most ideal range) you would need +7.5V and -2.5V for 10V excitation.  This sets the bridge output at ((EXC+) + (EXC-)) / 2 to (7.5 + (-2.5)) / 2 = 2.5V.

    You could use other combinations of supply voltage as long as the output from the load cell (balanced state with no load applied) is within the common-mode input range of 1.5V to 3.5V.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 60 points

    Hi Bob,

    Thanks for the elaborate explanation.

    the reason for choosing 10V excitation supply is weight bridge platform uses 6 load cell cables of 10000 kg each connected in parallel, and each load cell has cable of 10 meter length or more, as the platform itself is more than 10 meter long.

    using 5V extrication, drops have been observed over such long cable length.

    will try to attempt both the designs ie with INA & unbalanced excitation to check the performances....

    Regards,

    Prashant