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ADS1118: How to set Gain =8 &FSR=0.256V in ADS1118

Betty Guo
Expert 7300 points
Part Number: ADS1118


Hi team,

customer use ADS1118 for thermocouple measurement, and have a question is that how to set the Gain=8 and FSR=0.256V.

I found that the change PGA[2:0] can set FSR, but can't set the Gain, right?

also, the revision history of datasheet show that deleted "Gain=2/3,1,2,4,8,or 16" from function block diagram. does it mean that the ADS1118 can't change the Gain?

could you help give some ideas? thanks.

  • 0
    TI__Guru* 98135 points
    Betty,


    I'm sorry I missed this question yesterday.

    The PGA, Gain, and FSR are all the same thing. With the changes in FSR, you scale the maximum of what you can measure in the ADC. PGA changes the full scale size against a virtual reference.

    I think the reason we stopped using the term "gain", is because we don't list a nominal reference size, although we used to call 4.096V the gain=1 setting. In reality, the reference is virtual, and inputs are capacitively scaled to change the gain/full scale range.

    Regardless, the full-scale range is what is important. If you are using the ±0.256V FSR, that is the equivalent to the highest gain setting.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Expert 7300 points

    Hi Joseph,

    I'd like to know what's the gain of the default. I found an application note SBAA189, please see the below.

    the gain=16 and the FS=256mV. could you confirm that the FS=256mV is the same with gain=16. we can set the FS=256mV, but can't set gain=16 at the same time, right?

    thanks.

  • 0 in reply to Betty Guo
    TI__Guru* 98135 points

    Betty,


    The gain setting and FSR are the same thing. Changing the FSR changes the equivalent gain of the ADC. As you mentioned, you can't set the FSR to ±0.256V and the gain to 16 to have a range of ±0.016V.

    In a list, the following are the equivalent FSR and gain:

    FSR=±6.144V is the same as Gain=2/3
    FSR=±4.096V is the same as Gain=1
    FSR=±2.048V is the same as Gain=2
    FSR=±1.024V is the same as Gain=4
    FSR=±0.512V is the same as Gain=8
    FSR=±0.256V is the same as Gain=16

    For the ADS1118, the default setting is FSR= ±2.048V.


    Joseph Wu