I am new to precision designing and thus to noise analysis.

Amplifier noise principles for practical engineer by Matt Duff was enlignhtning

I also watched

EEVblog #528 - Opamp Input Noise Voltage Tutorial - YouTube

Engineer It -- How to select an op amp based on datasheet noise specs - YouTube

and read

But I also have additional questions espesially about near-DC aplication

Do I assume correctly that I would get full noise spectrum, unless I cut it out with the filter?

Does noise adhere to Ohm's law? so if 1k resistor has 4 nV/rtHz NSD then at 100 Hz bandwidth it produces 400 nV noise rms which coresponds to 400nV/1kOhm=400 pA rms current noise?

How about current noise in active filters? wouldn't large resistors typically used in them lead to increase in noise?

To test how well (or perhaps how bad ^_^) my understanding is, here is an abstract example calculation:

So assume I have an AD5780 DAC, that has constant voltage outputs for prolonged periods. Let's imagine I need relatively large current output that's why I put an AD8655. With a corner frequency at 1k it s not an ideal opamp for DC, but there is no alternative.

So the DAC has ca 70 nV/rtHz at ~0 Hz. Th op amp 100 nV/rtHz

Additionally, the DAC has output inpedance of 3.4 kOhm, so current noise needs to be accounted too. The spec doesn't provide the number, so I just wild guess it to be 50 pA/rtHz which gives 170 nV/rtHz

Also at full scale ther is probably noise coming from voltage reference (ADR4550) at approx 280 nV/rtHz

The total noise: rt(70^2+100^2+170^2+280^2)=350 nV/rtHz

Hope this calculation makes at least parttially sense

Now assume the same, but DAC changes output value 100 times per second. Then the noise is 350 nV/rtHz * 100 Hz=35µV rms. I supect that Ive made here a nonsense calculation but as Chinese proverb says "to ask is a minute shame, not to know is a lifelong shame"

I have mor questions:

Does a filter on the input of an amplifier (let's say a simple RC first order low pass filter) has influence on the badwidth of the noise of the amplifier itself?

The videos that i watched almost unanimously ignored 1/f noise under assumption that is has negligble contribution compared to white noise. But what if the signal bandwidth does not exceed the corner frequency, (let's take DC as an extreme example)? How do I calculate the 1/f noise of any device from graph and specification.