Electronic – Is a buffer needed to measure the output of a power supply using an ADC

The typical way to protect ADC inputs (and many other high impedance inputs) is to have a current limiting resistor and clamping diodes at the pin. Many chips already have clamping diodes inside the chip, but it is often useful to have some outside as well (especially when the integrated clamping diodes do not have proper documentation on their specs, which is most of the time).

Your resistor-divider is already going to function as a current limiting resistor. As a general rule, you want your resistor values to be as high as is practical-- with something in the neighborhood of 10K to 500K being reasonable.

As the resistors go higher in value you start to get weird effects from the parasitic capacitance (lowers your frequency response) and the leakage current of the ADC and diodes (messes up your accuracy). If you are unsure about these things, then pick something on the low side of 10K to 500K. If you have already figured out the math or are willing to just try it and see if it works then go with something higher.

There would be one diode between GND and the input. And another diode between the input and VCC. If you have a signal that goes + and -, then connect one diode to the V+ rail and the other to the V- rail. Orient the diodes in such a way that it works (turn the diodes around and nothing will work). Put the diodes between your ADC and the resistor divider.

It works like this... If you get a voltage spike coming in, the diodes will redirect it to a power rail and not directly into the chip. The current limiting resistor will, duh, limit the current to something reasonable. You want to current to be limited for several reasons: 1. You don't want so much current that the diodes blow up. 2. You don't want so much current going into the power rails that it effects the voltage regulation of that rail.

With this setup, the maximum voltage on the input pin will be VCC + the forward voltage drop of the diode. And the minimum voltage on the input will be GND - the forward voltage drop.

There are hundreds or thousands of appropriate diodes for this. The BAV99 is just one of many, but is a good one to start looking at.

That is correct.

A capacitor is needed because such ADC are SAR and as such the switching can cause an upset to the part in question. Typically a SAR type ADC requires any input pin capacitance to be a minimum of 10x the Cia to ensure that when it switches the actual voltage on the input pin does not droop (before said capacitance can be trickle charged from any sensor).

4.7nF satisfies this and is a typical value I would use when connecting Allegro devices directly to an ADC. The only times I consider placing a buffer between the Allegro and an ADC is if I require a larger capacitance (usually due to filter time constant concerns)

Not to sure about explicitly placing an Rl in parallel with the sensor, in series with the output to produce an RC filter may be of more benefit

Once piece of advice. The benefit of these devices over others is their ratiometric gain w.r.t. supply voltage. Ensure such a device is as physically close to the ADC as possible to ensure the ADC and the HES share the same rail so the ratiometric nature of the ADC & HES can be taken advantage of (by same rail I don't mean 5V powerplane on a 1m x 1m PCB... I mean their power pins as close as a via and decent local tracing will allow)