In a practical operational amplifier (op-amp) circuit, how does the input impedance affect the power supply rejection ratio (PSRR) and thermal drift performance of the system?

A high input impedance means the op‑amp draws little current from the signal source, so the source sees almost no loading and the bias network stays more stable, which helps keep the supply‑noise voltage that can sneak through the bias resistors small. Because the bias network is less stressed, the voltage drop across it changes less with supply ripple, giving a higher PSRR. Conversely, a low input impedance forces more bias current through the bias network, making the bias resistors a stronger conduit for supply noise and lowering PSRR. Thermal drift is also tied to input impedance: a very high impedance reduces bias‑current variations with temperature, so the offset voltage stays steadier as the device heats or cools. For example, an op‑amp with a 10 MΩ input impedance will see less supply‑noise coupling and a smaller temperature‑dependent offset than one with only 1 MΩ, improving both PSRR and thermal drift performance.