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

user4977325
Prodigy 90 points
Part Number: ADS1256
Other Parts Discussed in Thread: ADS1234, ADS1232, , ADS1241, ADS1235, ADS1248, ADS124S08, ADS1261

Hi!

I am redesigning an old board that uses ADS1232/ADS1234, using a faster processor to read scale weight.
One thing that today is a problem is that ADS1232/1234 is very slow and it makes practically impossible for us to acquire more samples to use any existing algorithm to measure moving animals over the platform. 
And it's necessary to increase the stability of signals measured on load cells.

Is ADS1256 desireable to be used as a Weight scale AD converter?
if not, or if there is an AD converter faster and suitable to be used as Weight scale converter, what is recommended?
I see another component, ADS1241, and about it?

I Intent to place booth used today and the new one at the same PCB (for compatibility reason). 

What is the best recommendation?

Sincerely

Cleber Magalhães 

  • 0
    TI__Guru** 113765 points

    Hi Cleber,

    The ADS1235 is the follow-on device for the ADS1232 for higher speed applications.  The ADS1235 would be the better choice.  The ADS1256 is a low noise device but it will not allow you to truly measure ratiometrically as the reference is 2.5V (when using excitation of 5V).

    The ADS1241 has similar issues, not as good for noise performance and is an older device.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 90 points

    Thanks for attention, Bob!

    But I still have a problem that doesn't allow me to use ADS1235: As an Upgrade from ADS1234, my project must use 4 ADC channels.
    Do you know any other IC that fills this requirement?

    Best Regards,

    Cleber Magalhães

  • 0 in reply to user4977325
    Prodigy 90 points

    continuing...

    Searching on Texas documentation, I found ADS1248, that is very similar to ADS1234, only a few pins are different in function, analog pins are similar and adding some resistors I think that I can multiplex pins to mount ADS1234 or ADS1248. The Datasheet tells that I can use Bridge sensors, but have nothing about weigh scale.

    I'm still looking for alternatives to use in my product. If you any other that fills better, put it here, I will really be thankful.

    Cleber Magalhães



  • 0 in reply to user4977325
    TI__Guru** 113765 points

    Hi Cleber,

    It would be helpful from the start to know your specific requirements.  The ADS1232/34 runs at slow data rates to achieve the lowest level of noise.  The ADS1232/34 also uses a full-scale range that is +/- 0.5Vref/GAIN which allows better dynamic range for load cell applications. 

    Low noise is a requirement for precision/resolution of your load cell measurement.  When you increase the data rate, you will also increase the noise.  The ADS1232/34 also improves noise by the increased dynamic range and the digital sinc4 filter that is used.  The disadvantage of the ADS1232/34 is you end up with a trade-off where the performance comes at the cost of the speed that the data becomes available.  When using a multiplexer this becomes even worse as you must wait for 5 complete conversions before the data are valid.

    Another way you can improve noise in the measurement (as well as excitation drift) is to make the measurement ratiometric where the reference and the excitation of the load cell are the same.  In this way the noise of the excitation source cancels.

    When deciding what converter you choose, you cannot just look at the number of bits that the converter can resolve as the actual weight resolution will depend on the value of 1 bit (LSB) and the best achievable noise.

    Using a device like the ADS1256 does offer low noise comparable to the ADS1232 but a disadvantage is the reference which is limited to 2.5V.  The dynamic range is also lower as the full-scale range is +/-2Vref/GAIN.  So the value of 1 bit is 4 times larger than that of the ADS1232 using comparable gain settings.  So even if the number of noise-free bits are the same between the ADS1234 and the ADS1256, the scale resolution is not the same.  You need to determine the noise-free voltage in each case and determine the resolution based on the noise voltage.  And as I stated previously, the noise and stability will be worse for a non-ratiometric measurement.  When picking a device it is very important to know the desired end result.

    The ADS1248 is not a good choice for load cell applications because your reference is limited and cannot range to the AVDD supply.  This makes it very difficult to make a ratiometric measurement with good scale resolution.  The ADS124S08 is an updated version of the ADS1248 and would be a much better choice.  However, you need to consider the noise at the desired data rates required to achieve valid measurements within the scale resolution needed.

    A better choice would be the ADS1261 which is very similar to the ADS1235 but will more device input channels.

    Best regards,

    Bob B