I am about to build the circuit that you have here: – Millivolt Meter – Bargraph
I am pretty sure that I understand all of it, except for the symbols that you have at the grounds which are little triangles with the letter F next to them. Please excuse my ignorance, but what does that mean? Is that the ground from the tested millivolt source?
One Ground carries the LED Currents and that track has to be thick and go directly to the Voltage Regulator 7805 which provides 5V supply. If this track is looped with the signal ground, millivolts of error may occur in reading and if tracks are thin it may lead to oscillations.
We have a High Gain Amp in this circuit that will amplify even these small jumps in mV due to track Resistance in ground loops.
Analog Ground, Digital Ground, Supply Ground, RF Ground and Earthing have to be carefully designed to get accurate performance. PCB Autorouting may ignore these subtle requirements.