External Power Supply Grounding Scheme
Somewhere in all that circuitry you must have an ADC converter. That is the point where analog and digital signal grounds meet. Some ADC's even have separate gnd pins to handle this issue. The digital gnd then has a trace to connect it to power gnd. This way all signal grounds are on the circuit board and power gnd follows its normal path from the board to the power supply (large can capacitors and/or voltage regulators). Star grounds are still the best for multiple analog channels. That point has a trace going to digital ground. The key point here is to keep your analog and digital grounds on the board. At some point they need to connect to power gnd and the large electrolytic bypass (can) capacitors on the board is the best place with the lowest noise. Your off-board power source and ground would connect here as well, avoiding ground loops.At this point you could add a chassis gnd or use a 10k 1/2watt resistor if a direct connection inserts noise.