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TLA2022: Designing a weight scale to measure 50 lbs with a 0.02 lbs accuracy.

Noaman M
Intellectual 350 points
Part Number: TLA2022
Other Parts Discussed in Thread: ADS1120, ADS1231, ADS1131

I am trying to design a weight scale to measure parcels with a 0.02 lbs accuracy, to measure upto 50 lbs. 

I am using weight/load sensors from Aliexpress which are half bridge each (i believe 2mV/V - data is missing).

Is there an application note I can follow. is TLA2022 even the right part for this purpose?

  • 0
    TI__Guru* 92715 points

    Noaman,


    With a full scale of 50 lbs, and an accuracy of 0.02lbs, that would be a desired resolution of about 2500 codes. With a 12-bit ADC, that would be difficult when the positive full scale is only about 2^11 codes (2048). Also, you can't make a ratiometric measurement with the TLA2022, where the reference is internal, and not brought out of the device.

    Instead, I think something like the 16-bit ADS1120 is better suited to the application. Using that device, you can set up a ratiometric measurement with an external reference input to the device. I don't know of any particular application notes to follow for that application. However, the method for a full-bridge measurement is shown in the datasheet starting on page 58.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 350 points

    Hi Joseph,

    Thanks for the quick response.

    ADS1120 is a bit pricey for the application. Would ADS1231 also work? 

    What you said about ratiometric measurement is very clear. What about temperature and time drift? I have been reading and have come across unity gain zero-drift amplifiers that would take the bridge input and supply the ADC (bridge -> zero-drift amp -> ADC). I am not sure if either ADS1120 or ADC1231 have zero-drift like functionality built in. 

    Application: The device will be permanently place outdoor and see temperatures from -25'C to 45'C. The weight will be placed on the device for long periods of time (days or even months) and could deplete or be added to from time to time. I have to be able to tell minor additions/subtractions apart from temperature/time drifts.

    Thanks again for your help. Looking forward to learning from you.

    noaman

  • 0 in reply to Noaman M
    TI__Guru* 92715 points
    Noaman,


    I think the ADS1231 (or even the ADS1131) could work in your application. These devices were designed with weigh scale applications in mind. There is even a evaluation module (ADS1X31REF) available. At the minimum, it would be worth downloading the user guide to look at it. You can find a link to it here:

    www.ti.com/.../sbau175a.pdf

    Both the ADS1120 and ADS1231 do have circuitry to reduce the drift of the offset. However, you would have to determine if that is enough for you. For example, the offset of the ADS1120 is listed as a typical of ±4uV, while the offset drift is typically 0.08uV/°C for the full range of temperature. At the same time, the offset of the ADS1131 is listed as a typical of 10uV, while the offset drift is typically 15nV/°C. In addition to offset, you'll also need to be concerned with the gain error drift if the device is to be used in a wide range of temperatures.


    Joseph Wu