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CCS/ADS1248: Changing the ADS1248 PGA value doesn't match the amplification ratio written on the datasheet. (Additionally How to use SYSOCAL, SYSGCAL commands.)

La Elya
Prodigy 10 points
Part Number: ADS1248

Tool/software: Code Composer Studio

Hello.

I'm trying to create a design using the ADS1248, and have questions during the test.

There is a problem with the output value.

First, I checked that the input data was enterd correctly in the register. I want this data is amplified through the OP-AMP inside ADS1248.

However, even if I modify the program according to the amplification ratio written on the datasheet, the output value does not match the amplifiction ratio.

Ex) PGA = 1 : Vin = 0.5 > Output Voltage 0.5

      PGA = 2 : Vin = 0.5 > Output Voltage 0.7 (Expected Value : 1.0)

I'm changed the amplification ratio of the PGA by changing the value of the SYS0 register.

So, I wonder why the amplification ratio written on the datasheet isn't correct.

Additionally, What is the procedure for using both commands, SYSOCAL and SYSGCAL?

I carried out the procedures written on the datasheet for both commands. But the results were normal when not in use, but were not normal after use.

Thanks

Elya

  • 0
    TI__Guru* 97725 points
    Hi,


    One thing that may affect the output of the PGA is the input range of the PGA itself. The PGA is constructed like the input of an instrumentation amplifier.

    First, note that if the supply is unipolar (from 0V to 5V). The input should not be grounded. The input does not extend to ground, because like an amplifier, the output can't be pulled all the way to either rail. Also, the input signal common mode must be the same as the output signal common mode of the PGA. The input range will be more limited as the gain gets higher.

    If you go to the ADS1248 datasheet, the PGA and the PGA input range is discussed in detail starting on page 26. Let me know if this clears up your PGA problem.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 97725 points
    Hi,


    I was just checking to see if you had resolved the problems you were having with the PGA on the ADS1248. Again, I think this may be a problem caused by the input not being in the range of the PGA. Again, you can review the input range of the PGA in the datasheet.

    I'll close this post for now, but if you continue to have issues, post back and we can work on this problem again.


    Joseph Wu