I am designing a circuit which will amplify a few millivolt input (~+/-10mV full range) to a few volts (~0-8V output, centered at 4V). I have spec'd out an instrumentation amplifier with a voltage reference to give the desired center voltage, and now am designing the voltage reference part.
The option I am considering is the AS431 shunt voltage reference. I like the simplicity, but I am worried about output impedance as the INA126 specifies(ish) that ~8ohm output impedance will reduce the CMRR to ~80dB, and to prevent the AS431 from getting fried, I would prefer to limit current to the AS431 to ~10mA.
Reference voltage regulator circuit (taken from the datasheet):
Vin=8V, thus the desired R3 is 4V/10mA = 400 ohms (assuming R1+R2 >> R3). If the output impedance is 400 ohms, the CMRR is completely toast. However, being a voltage reference I don't know if the output impedance is necessarily the same as the input impedance. Even playing it fast and loose by drawing ~100mA through the AS431 requires ~40ohm shunt resistor.
Would I have to add something like a voltage follower to the output to get the desired low-impedance reference the INA126 needs? Or is there something I'm not quite understanding about how shunt regulators/voltage references work?
If I do need the voltage follower after the voltage reference circuit, why would I even bother using the shunt reference in the first place? I could just as easily feed the voltage-divided reference directly into the voltage follower to get a low-impedance reference output.