Problem with Instrumentation amplifier Gain INA826

I have built a circuit to measure temperature using 2 wire RTD interface as shown below:


I am effectively using one half of the dual current source (100uA) as 2nd half is connected to 0V-GND (via R41 & R81 0 ohm links). The idea is to use this circuit with Pt-100 & Pt-1000 type devices. The output of amp is going to an ADC. The voltage divider at output (R44 & R45) is to cater for broad range of temperatures using Pt-100 & Pt-1000 with one circuit. I started observing some non-linearity so decided to do some test by connecting a variable resistor (1K) between RT_Pos & RTD_Neg termianls (RTD_Neg is effectively GND)
Using a DVM I took measurements for voltage drops and got the following results:


As you can see than somewhere between 812.5ohms & 830.3 ohms, the gain of amplifier started to drop and it carries on like this up till 1K+ input resistance . I am struggling to understand the reason for this behaviour of the amplifier. Am I overlooking any parameter in datasheet of the amplifier here? It is rated for Single Supply operation from 3V to 36V DC. I am using 5V DC as Rail. The amp has 1uF decoupling cap across its Rail (not shown on schematic) Any help would be greatly appreciated

Please review datasheet Figure 12. It shows that the common mode range doesn't include the negative rail for output voltages above 1.5V and Vref = 0.

Thanks for the feedback. Learned something new today . I have found an app note explaining these graphs,
https://www.ti.com/lit/an/slyt647/slyt647.pdf


will start reading it now. I might have to ask some questions on this topic if I struggle to understand.

Hi there, thanks for the feedback. What I understand from the graphs is that my graph would be somewhere between the graph of Gain = 0 & graph for Gain = 100; as Gain for my circuit is apprx 20.
In any case my differential mode input signal is sitting very close to 0V, which means with Vref = 0V, the maximum voltage I could get at the output is apprx 1V (for linear operation of this Instrumentation amplifier)
The way I understand my circuit and from definition of COMMON Mode voltage, the common mode voltage of my circuit is apprx 0V. so I am sitting on the bottom blue line of the graph.

Is there a way to raise this common mode voltage to say 1.5V (with Vref still connected to 0V) so that I could get wider range on Vout. Would this be a reasonable approach to get wider linear operation at Vout? It would be greatly appreciated if you could suggest a way to achieve this on the existing circuit.