ci wrote on Aug 18th, 2009, 4:13pm:Ground bounce may affect the bandgap output since this output is a voltage referenced to ground. Ground electric potential changes across the circuit and these differences may couple into the bandgap reference distribution circuitry.
There is another more insidious effect. AC currents in bipolar are nonlinear, so the DC value of the current changes with the amplitude of the AC current. The net effect is that the DC output of the bandgap will change depending on the amplitude of the AC noise. The AC current can be coupled in from the base/collector capacitance of the PNP, or from other sources. This is in addition to the effect you describe. It is frequency dependent.
Many an engineer have freaked out upon seeing their dear bandgap voltage drop 50 mV when the noisy circuitry is switched on.