It says isolated In data sheet. Can I put an other ground in non inverting input as a ground for feed current. How do the integrated inputs in the circuit look like ?
Could you be more specific which part of the LT1990 datasheet that says isolated? If I understand it correctly, you want to ground the non-inverting input is that correct?
For the last question, the LT1990 is an integrated difference amplifier as seen on the block diagram in the datasheet. The input resistor network composed by the 1M and 40k on chip resistor creates the internal common-mode voltage divider to allow the part to accept a wider input voltage range compared to the power supply used.