This is a very complex issue, since it deals with EMI/RFI, ESD, and safety stuff. As you've noticed, there are many ways do handle chassis and digital grounds-- everybody has an opinion and everybody thinks that the other people are wrong. Just so you know, they are all wrong and I'm right. Honest! :)
I've done it several ways, but the way that seems to work best for me is the same way that PC motherboards do it. Every mounting hole on the PCB connects signal gnd (a.k.a. digital ground) directly to the metal chassis through a screw and metal stand-off.
For connectors with a shield, that shield is connected to the metal chassis through as short of a connection as possible. Ideally the connector shield would be touching the chassis, otherwise there would be a mounting screw on the PCB as close to the connector as possible. The idea here is that any noise or static discharge would stay on the shield/chassis and never make it inside the box or onto the PCB. Sometimes that's not possible, so if it does make it to the PCB you want to get it off of the PCB as quickly as possible.
Let me make this clear: For a PCB with connectors, signal GND is connected to the metal case using mounting holes. Chassis GND is connected to the metal case using mounting holes. Chassis GND and Signal GND are NOT connected together on the PCB, but instead use the metal case for that connection.
The metal chassis is then eventually connected to the GND pin on the 3-prong AC power connector, NOT the neutral pin. There are more safety issues when we're talking about 2-prong AC power connectors-- and you'll have to look those up as I'm not as well versed in those regulations/laws.
Tie them together at a single point with a 0 Ohm resistor near the power supply
Don't do that. Doing this would assure that any noise on the cable has to travel THROUGH your circuit to get to GND. This could disrupt your circuit. The reason for the 0-Ohm resistor is because this doesn't always work and having the resistor there gives you an easy way to remove the connection or replace the resistor with a cap.
Tie them together with a single 0.01uF/2kV capacitor at near the power supply
Don't do that. This is a variation of the 0-ohm resistor thing. Same idea, but the thought is that the cap will allow AC signals to pass but not DC. Seems silly to me, as you want DC (or at least 60 Hz) signals to pass so that the circuit breaker will pop if there was a bad failure.
Tie them together with a 1M resistor and a 0.1uF capacitor in parallel
Don't do that. The problem with the previous "solution" is that the chassis is now floating, relative to GND, and could collect a charge enough to cause minor issues. The 1M ohm resistor is supposed to prevent that. Otherwise this is identical to the previous solution.
Short them together with a 0 Ohm resistor and a 0.1uF capacitor in parallel
Don't do that. If there is a 0 Ohm resistor, why bother with the cap? This is just a variation on the others, but with more things on the PCB to allow you to change things up until it works.
Tie them together with multiple 0.01uF capacitors in parallel near the I/O
Closer. Near the I/O is better than near the power connector, as noise wouldn't travel through the circuit. Multiple caps are used to reduce the impedance and to connect things where it counts. But this is not as good as what I do.
Short them together directly via the mounting holes on the PCB
As mentioned, I like this approach. Very low impedance, everywhere.
Tie them together with capacitors between digital GND and the mounting holes
Not as good as just shorting them together, since the impedance is higher and you're blocking DC.
Tie them together via multiple low inductance connections near the I/O connectors
Variations on the same thing. Might as well call the "multiple low inductance connections" things like "ground planes" and "mounting holes"
Leave them totally isolated (not connected together anywhere)
This is basically what is done when you don't have a metal chassis (like, an all plastic enclosure). This gets tricky and requires careful circuit design and PCB layout to do right, and still pass all EMI regulatory testing. It can be done, but as I said, it's tricky.
It depends on final equipment ground policy which may differ. In all systems i designed i use GND star connection, which means the following:
- All isolated* grounds are connected.
- There is only one point of connection, near the power supplies.
- Chassis is connected in the same place with a thick wire.
*there are sometimes grounds that somehow depend on other systems and cannot be connected. Most common: minus of diode bridge in AC rectifier. Another possibility- minus of isolated supply for current sensing, which is connected to a motor phase. But since you are not asking about it, we will not discuss it.
Also on each board near each mounting hole there are capacitors that connect the chassis to grounds in AC domain. This is important, because being thick (in relation to PCB copper planes) the case usually has very low inductance and resistance and may be considered perfect ground for noises of all kinds. But on the other hands you don't want any power to pass through the case, so the connection is by caps.
Speaking of ground policy, nice to see that you use optocouplers. Hope you did not forget power filters.
Best Answer
If you need to follow FDA rules (or similar) because this is a product, then there are requirements that should be followed if the design is to be used in a product.
In many designs the MAX86150 is subject to these rules here and IEC 601-1, “Medical Electrical Equipment - Part 1: General Requirements for Safety.
Source: https://www.analog.com/en/technical-articles/mitigation-strategies-for-ecg-design-challenges.html
It might also mean that the MAX86150 could be subject to these rules below:
Because of these rules many designs might employ separate analog and digital grounding systems, or even isolated ones. In the datasheet it is suggested that isolated digital and analog grounds be used (I don't know how the isolation is to be maintained in the IC)
So they break out the grounds to give the designer options to satisfy the possible isolation requirements.
If you don't need the isolation (or separate analog and digital grounds) and the design isn't subject to these rules, then the grounds could be tied together.
One problem will be reducing noise and connecting electrodes from a circuit to another body or circuit forms a kind of AC current loop (if you've ever touched an oscilloscope and not grounded yourself you'll find the human body can be a great antenna for 60Hz). A good recommendation on isolation, separate grounds and the like is Electromagnetic Compatibility Engineering.