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ADS1191: CM input calculation

Part Number: ADS1191

Hi,

I am trying to use the ADS1191 in a signal chain the has a load-cell bridge and INA333. I am trying to calculate the required CM of the device and am confused by the equation and example in the ADS1191 datasheet:

If I take AVDD =3V, gain =6, Vmax_diff = 350mV, and AVSS = 0V.

 3V - 0.4V - 1.05V > CM >0V + 0.4V + 1.05V

1.55V > CM > 1.45V

In the ADS1298 datasheet the value of 0.4V is changed to 0.2V and the example and equation work out. I wanted to check if  ADS1298 is correct or if the equation in the ADS1191 is correct but has the wrong answer in the example.

I am trying to understand how I need to set the CM input of the ADS1191 with respect to setting the CM output of the INA333.

I will be using AVDD =5V and a gain of 12.

The max-diff voltage will be 336mV calculated from equation 4 in the ADS1191 datasheet. Given internal 4.033Vref  and Gain =12

Max(INP-INN) < (Vref/Gain) ---> Max(INP-INN) < 4.033V/12V   ----> Max(INP-INN) = Vmax_Diff = 336mV 



Taking that number and plugging into the CM equation found in ADS1298 datasheet:

If I take AVDD =5V, gain =12, Vmax_diff = 336mV, and AVSS = 0V.

 3V - 0.2V - 1.05V > CM >0V + 0.2V + 1.05V

 2.784V > CM > 2.216V



This means I can set my CM for a single ended implementation at mid supply (2.5V) and have a signal swing of 336mV about the CM and satisfy the requirements.



Is this understanding correct?

Regards,

Dakotah

  • Hi Dakotah,

    The ADS1292 is in the same family as the 1191, and it also has 0.2 V in the datasheet. After speaking with another engineer we believe 0.2 V to be correct. We are in the process of verifying it, we need to speak with the design team.

    Regards,

    Matt Mendez

  • In reply to Matt Mendez:

    Matt,

    Thanks. Looking forward to finding out the correct answer. 

    Then I can stop scratching my head over why I couldn't make the simple equation work out.

    Regards,

    Dakotah

  • In reply to Dakotah:

    Hi Dakotah,

    Just got confirmation that 0.2 V is correct. There is no analog change between ADS1291 and ADS1191. Hope this helps!

    Regards,

    Matt Mendez

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