You can do this easily in your schematics: just locate the component you need for your capacitor, and then bridge the ground nets with a direct connection. The typical place to do this in the PCB layout is close to the transformer.
The grounds come together at the point G, where the chassis is also connected. Where there are a few inches of wire tying the individual grounds together, it is a good idea to insert fast signal diodes and a capacitor as shown between the separate ground runs.
A solution is to create a circuit board that establishes a ground with the characteristics of node_G. The principle is simple—the circuit trace from the input ground terminal to the ground side of R1 should be a clear path with no connections to contaminating sources of current along the way (figure 2).
The input ground connection should not connect to equipment chassis at an input connector. This would create an opportunity for other interfering ground noise (such as AC mains ground currents) from impressing current on the clean input ground trace. A single blog cannot begin to cover all the issues relating to the art of grounding.
The fundamental rule for grounding is depicted in Figure 1. By “ground” I mean the common 0 V potential to which signals are referenced. The “chassis ground”, if grounding conductors had 0 Ω impedance, would also be 0 V—but, unfortunately, it never is. Yet there are still systems that are sufficiently insensitive to ground potential differences.
The “chassis ground”, if grounding conductors had 0 Ω impedance, would also be 0 V—but, unfortunately, it never is. Yet there are still systems that are sufficiently insensitive to ground potential differences. They use the chassis for the signal and power returns. At one time, this was the way cars had been wired.