Power rail probe

[smarley]

Hi, I was thinking in the usage of the power rail probe, to measure noise on my low voltage power supplies circuits.
I was studying the concept of it and the reasons to be used, I understood that the idea is to not use the 1Mega ohms input (AC) from the scope that can catch noise from the environment or the input circuit of scope itself, and use the 50ohms input.
And also to not connect the 50ohms input directly to the power rail to not load the power supply.

But I had some questions:
- Looking at the datasheet of N7020A from Keysight and also a teardown video that I found on youtube seems that the probe has high impedance in low frequency, and after 1Mhz it drop to 50ohms.
Like if you had just add a capacitor in series with your power rail and attached to the 50ohms input of the scope.
With this impedance behavior, this 50ohms will not load under the high frequency noise attenuating it?
- There is an advantage or need in having the DC offset control, instead of just do an AC probe with high input impedance, strength to drive the 50ohms and low noise output?

Thanks!

[tszaboo]

Yes, the probe will load the power rail in AC. But what is the source impedance of a power delivery network? Let's say it's a regular microcontroller board, with a bunch of capacitors on the power rail. Ceramic SMD capacitors have ESR in the order of tens of milliohms in the same frequency range. So the 50 Ohm loading is not that important, it doesn't really change the behavior of the circuit.
If you just AC couple, your low frequency response is not ideal. You will see the noise but you will not see brownouts for example. Also, you will temporarily overload your scope when the power rail comes online, because it will generate equally large opposite direction spikes. So more circuitry is needed. Bear in mind most power rail probes will be able to 12V or even 24V supplies, while your scope input is only rated to 5V in 50Ohm input.
Oh and scopes don't really have AC coupled 50Ohm inputs, it's almost always DC coupled. So that's why the probe.
I'm working on a power rail probe BTW, I hope to order it soon-ish.

[smarley]

Yes, the probe will load the power rail in AC. But what is the source impedance of a power delivery network? Let's say it's a regular microcontroller board, with a bunch of capacitors on the power rail. Ceramic SMD capacitors have ESR in the order of tens of milliohms in the same frequency range. So the 50 Ohm loading is not that important, it doesn't really change the behavior of the circuit.
If you just AC couple, your low frequency response is not ideal. You will see the noise but you will not see brownouts for example. Also, you will temporarily overload your scope when the power rail comes online, because it will generate equally large opposite direction spikes. So more circuitry is needed. Bear in mind most power rail probes will be able to 12V or even 24V supplies, while your scope input is only rated to 5V in 50Ohm input.
Oh and scopes don't really have AC coupled 50Ohm inputs, it's almost always DC coupled. So that's why the probe.
I'm working on a power rail probe BTW, I hope to order it soon-ish.

Thank you for the explanation,  so the idea is really to have all the bandwidth passing, just with the offset, make sense.
The tricky so is to design the low frequency with low noise and the offset, because the high frequency you can pass-through it with a capacitor.

[jonpaul]

Helpful to know your power supply being measured and test setup,

eg a very different technique  and probes, 
15V 100 mA linear lab supply or a
or a 1000W off line SMPS?
Suggest you first research, read and learn before proceeding.

HP app notes Power Supply measurements
https://www.hpmemoryproject.org/ressources/resrc_an_01.htm

Tektronix Circuits Concepts series books (1980s)
https://w140.com/tekwiki/wiki/Concepts_Series

Oscilloscope probe circuits
https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

Probe Measuements
https://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Enjoy!

Jon