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Strain measurement ADS1248

VT
Genius 3300 points
Other Parts Discussed in Thread: ADS1248, THS4524

I have to measure strain (quarter bridge three wire), using 350ohm and 1K.

Problem is signal change is so small . I have attached the circuit.
I am using internal IDAC of ADS1248. 

1. For 1K strain , voltage range = -24.39mV to 25.641mV. with 0.5uV step (approx)
2. For 350 strain , voltage range = -12.805mV to 13.462mV. with 0.13125uV step (approx)

Now either:
1. I will use external higher current source for high voltage change.
2. Or I will external amplifier. I am searching for such amplifier. I think THS4524 will do the job.

3. Any other suggestions on how I can measure such small changes using ADS1248.

4. My application is battery operated. 

ckt.pdf

  • 0
    TI__Guru* 97905 points
    Vindhyachal,


    Is there something wrong with using the on-board PGA? With a PGA=64, and a reference of 2.048V, you would get +/-32mV full scale range, which would be fine for the 1k strain. With a PGA=128, you should be able to get a full scale range of +/-16mV which would be fine for the 350 strain.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Genius 3300 points
    @Joseph Wu

    I will use PGA= 64 & internal reference.
    My concern was will ADS1248 will be able to discern such small changes .
    As noise tables in datasheet are very high compared to small step size that I need.
  • 0 in reply to Joseph Wu
    Genius 3300 points

    @Joseph Wu,

    I have attached the circuit. Addition I had made is adding a small resistor below quarter bridge, to put the device in correct Vcm range.

    This also created 0.6V reference in case or I can use 2.048V internal reference.

    My circuit had AVDD = DVDD = 3.3V & AVSS = DVSS = 0V.

    1. For 1K strain , voltage range = -24.39mV to 25.641mV. with 0.5uV step (approx)
    2. For 350 strain , voltage range = -12.805mV to 13.462mV. with 0.13125uV step (approx)

    My concern was will ADS1248 will be able to discern such small changes ?

    t.pdf

  • 0 in reply to VT
    TI__Guru* 97905 points
    Vindhyachal,


    There's not much information on your schematic. It only shows a few resistors making two bridges.

    As for the noise performance, if you look at the Table 1 in the datasheet, it outlines the noise that you would get with different PGA and data rates. The noise that you want may be difficult, but should be achievable if you are willing to go to lower data rates. Note that the noise performance is for a low noise reference at 2.5V. Also, it shows the noise when the input is 0V and there is the least contribution from the noise of the reference.

    For the 1kOhm strain, you would be in a gain of 64, running at 5SPS, the noise would be 0.27uVp-p. I believe this is what you're asking for.

    For the 350 Ohm strain, you would be in a gain of 128, running at 5SPS, the noise would be 0.21uVp-p. While this isn't quite where you want to be, you could average several data points and the noise should drop by the square root of the number of averages. With four averages, you could be near 0.1uVp-p which is what you want.

    I would note that the performance depends heavily on the reference that you are using. You need to have a reference that has a very good noise performance. I don't know if the 0.6V reference is up to the task. Even the on-board reference for the ADS1248 is marginal to this performance and you may need to go to a filtered REF5020 for the performance that you need. I would definitely add space for another low noise reference just so you have access to it on your circuit. Even then, I'm not sure what noise performance you would get as you moved through the full scale range of the input.


    Joseph Wu