I am doing ECG using INA128. It's working good when +5V applied to V+, -5V applied to V- and Grounding the REF. I have constraint of having single +5V supply in my project, so by using +5V applied to V+, Grounding V- and +2.5V(using voltage divider as shown in the figure) applied to REF, the circuit should work(as simulations are running fine on Proteus) but its clipping the negative half of the input signal. Can anyone explain the function REF pin, I suppose this will offset the output voltage with the voltage applied to it.
Attached is the circuit I used for simulations (AD620 is analogous to INA128)

 

This is from the INA128 datasheet.
7.4.2 Input Common-Mode Range

The linear input voltage range of the input circuitry of the INA12x is from approximately 1.4 V below the positive supply voltage to 1.7 V above the negative supply. As a differential input voltage causes the output voltage increase, however, the linear input range is limited by the output voltage swing of amplifiers A1 and A2. Thus the linear common-mode input range is related to the output voltage of the complete amplifier. This behavior also depends on supply voltage (see performance curve Figure 6).

Input-overload can produce an output voltage that appears normal. For example, if an input overload condition drives both input amplifiers to their positive output swing limit, the difference voltage measured by the output amplifier will be near zero. The output of A3 will be near 0 V even though both inputs are overloaded.


Without seeing the scope trace of the problem, it appears you may be bumping into the supply limits.