I would be careful with the use of a level shifter. Make sure that it does not introduce in-band noise for your reference. For best noise performance, keep the number of devices that can contribute noise low. The level shifter can be one of several options:
- Place matching devices between M1 and R1 and between M2 and Q2. Take the voltages for the amplifier inputs from the more positive connection. Device can be a resistor, a diode connected NMOS, PMOS or bipolar. Since the currents in each will be the same, they will provide the same voltage drop, and will increase the voltage input to the NMOS input pair on your opamp.
- Use a follower device before the input to the opamp.
Depending upon your supply voltage, p-channel inputs to the amplifier might be the best solution, since it will probably not add any new devices for noise contribution.
Bipolar inputs to your amplifier might work, but I am guessing that your process will not support it?