Your provided source material explains does not say it is required to use a resistor. All it says is that you must not leave the pin floating. It then suggests that, if desired, you can use a resistor and then it tells you why: To keep the functional test possible.
That means it is saying that connecting through a pull resistor lets you override it and use the pin if required such as during testing.
If the input is a BJT base, connecting to ground through a resistor is necessary to limit current through PN base-emitter junction. Connecting the base directly to ground in this case would be producing a short via diode if there is a high-side PNP transistor inside the input. The same thing is true for pull-up resistors and low-side NPN transistors.
But if the input is a CMOS this isn't a problem because the gate is insulated and cannot pass DC.
But on top of this, there are many reasons why you might not want to connect an unused pin directly to a power or ground rail, and sometimes you can't. It depends on whether the pin is an input or output and whether it is analog or digital.
- Directly connecting unused outputs to a ground or power rail is very
bad. Outputs are driving a signal voltage so you don't want to short them out. You could connect them to a power rail through a resistor but there is often little reason to since they are not floating and doing so would just result in increased current and potentially excessive or even damaging power dissipation.
That generally works for digital inputs and outputs. But analog inputs and outputs can have other considerations. You might not want to connect the inputs or outputs to the power rail, even through a resistor. You might not even be able to safely do so.
See for more details:
https://www.ti.com/lit/an/sboa204a/sboa204a.pdf
But basically:
- Opamp inputs don't always have a common mode range that swings the entire range of the power rails in which case you don't want to tie the input to a ground or power rail with or without a resistor.
- Even if the opamp could safely tolerate being tied to a rail you might not want to since it would cause the output to saturate and result in increased current consumption and potentially overheat. It can also cause cross-channel effects.
- So the safest thing to do is wire up the unused opamp as a buffer that feeds to nothing and then bias the input of the buffer to mid-supply.
