• Resolved

ADS1191: CM input calculation

Part Number: ADS1191


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?