Question about the bottom plate and parasitic capacitance in this DAC capacitor

[hsinli]

Hi all
I have two questions
1. Why the circuit is insensitive to parasitic capacitance?
2. The bottom plate of the capacitor is connecting to the opamp, is it ok?

thanks

 

[hung_wai_ming@hotmail.com]

About DAC capacitor

Bottom plate should be connected to VREF+/-.
As bottom plate is always connected to VREF+/-, so impedance is low, relatively compared to some floating nodes, so insensitve to noise.
Remember all floating nodes are sensitve to noise as impedance is not well defined and any charges from noise injected to that node will accumulate and cannot skip away.
Upper plate is at 1 metal layer or poly layer normally higher than the bottom plate, so intrinsically less noise sensitve, also it is at the output of the op-amp, which is also a low impedance node, so noise is insensitive