I am thinking about using an AD8237 in a biopotential amplifier. I would like use a common mode feedback scheme to improve CMRR. I have prevously obtained the CM voltage from the mid point of two series connected resistors across Rgain. This is simple with a "3 amplifier" type INA as shown below. Where would I get a CM signal from in this new Gm topology- other than by adding resistors across the input?