Hi Vishal,
One thing to keep in mind when using the ADS1148 is that there is a common mode input range restriction. You cannot use a single supply voltage for the analog supply and then connect any of the inputs to ground as a part of the measurment. See the information in this section:
So the statement about reading proper data is incorrect when one of the inputs is tied to ground. You must bias the sensor (whether it be a resistor, thermocouple or RTD) into the correct common mode input range. If you are using the IDACs for sensor excitation, then you must also add a resistor between the sensor and ground so that the measurement is within the correct common mode range. Be careful with the value as there is also a IDAC voltage compliance requirement (about 1V below the analog supply) where the total voltage drop created by the sensor/bias resistor combination meets the compliance requirement.
As a side note, the bias resistor is often used for establishing an external reference so that the conversion result becomes a ratiometric measurement.
Your original schematic that you posted shows no input filtering ( you most likely will want to add at least anti-aliasing). If this is still the case, and you connect a 100 ohm resistor across the AIN0 and AIN1 inputs you will need some place for the current to travel. So also connected to the AIN1 input is a 2k resistor where the other end connects to analog ground. This completes a current path for the current source. If the IDAC is turned on to 1mA output, and is sent out AIN0, then you will see 100mV dropped across the AIN0 and AIN1 and 2V dropped across the 2k bias resistor. With a 5V analog supply this will set the common mode close to mid-supply and also the total voltage drop will be within the IDAC compliance voltage.
If you turn off the IDAC current and turn on the burnout sources, you will see a voltage drop across the 100 ohm resistor somewhat relative to the value of current chosen, but the measurement is not totally valid as it is now referred to ground (now outside the common mode range). You should see no voltage drop across the 2k bias resistor as the current source will pull the current to ground through the AINN input that is sourced from the AINP input. If you short the resistor, you should see about 0V input (plus noise and the non-linearity of the input still referred to ground) and for an open you should see full-scale as there is no current path. This happens because of the open that pulls one source toward AVDD and the other that is pulling the source toward AGND. This actually is overdriving the input beyond the reference value and that is why it becomes full-scale. If you do not see full-scale in this condition, then you have some sort of leakage path which might be created by your input protection diodes.
Best regards,
Bob B