I've read in several sources that you should etch away the ground plane under the input and output pins of an opamp to avoid 'capacitive coupling to ground." This doesn't make sense for a lot of reasons, especially in wideband surface mount circuits.
1. The entire signal trace has capacitive coupling to ground. Why would that last bit to the input pin be any different? In fact the opposite seems true. Coupling to ground is a lot better than coupling between pins. Pin-to-pin coupling is far worse without a ground plane directly underneath to confine the field lines. Pin-to-pin coupling will cause instabilities. The worst thing capacitive coupling to ground might do is cut into your bandwidth.
2. A signal trace without a ground plane is just a loose wire, with all the inductance of a loose wire near the worst possible place - the juncture of the feedback line & input of the opamp (for an inverting amp.) Stray inductance is an invitation to instability.
3. I'm from the RF world and ground planes everywhere are critical, especially in microstrip (signal trace over a ground plane) circuits. That's how you control the impedance of the line, by balancing the lines capacitance with it's inductance. I know you don't usually worry about impedance matching at the input of the op amp but at least the mismatch is real (resistive.) Removing the last bit of ground plane will only make the situation worse by introducing a little reactance.
Everything about this rule is counter-intuitive.
So why do it?
1. The entire signal trace has capacitive coupling to ground. Why would that last bit to the input pin be any different? In fact the opposite seems true. Coupling to ground is a lot better than coupling between pins. Pin-to-pin coupling is far worse without a ground plane directly underneath to confine the field lines. Pin-to-pin coupling will cause instabilities. The worst thing capacitive coupling to ground might do is cut into your bandwidth.
2. A signal trace without a ground plane is just a loose wire, with all the inductance of a loose wire near the worst possible place - the juncture of the feedback line & input of the opamp (for an inverting amp.) Stray inductance is an invitation to instability.
3. I'm from the RF world and ground planes everywhere are critical, especially in microstrip (signal trace over a ground plane) circuits. That's how you control the impedance of the line, by balancing the lines capacitance with it's inductance. I know you don't usually worry about impedance matching at the input of the op amp but at least the mismatch is real (resistive.) Removing the last bit of ground plane will only make the situation worse by introducing a little reactance.
Everything about this rule is counter-intuitive.
So why do it?