Measuring voltage drop on power rail

[bhj99]

Hello,

I would like to measure the voltage drops for the 3.3V_M.2 power rail on a eval board design and wanted to get some thought on the best approach. The 3.3V rail is an output from an LDO and that is being filtering using a ferrite bead to generate the 3.3V_M.2 rail used for a connector that hosts an M.2 SSD (see schematics attached). Would a fair approach be to use a differential probe to measure across both the pads of the ferrite bead E7, or would it be easier at the decoupling caps? Or would just using a multimeter be easier?

[Manul]

Differential probe is not needed. Scope in general is not for accurate measurements. Use multimeter. I'm not sure what exactly you try to measure and what is your goal, but in this case I would measure voltage drop from LDO output up to the IC pins of interest. Don't forget that there will be some drop on the return path too (GND). Return path drop is usually less, because ground is typically distributed by massive copper planes and has less resistance. So total supply drop will be the sum of those.

[bhj99]

Differential probe is not needed. Scope in general is not for accurate measurements. Use multimeter. I'm not sure what exactly you try to measure and what is your goal, but in this case I would measure voltage drop from LDO output up to the IC pins of interest. Don't forget that there will be some drop on the return path too (GND). Return path drop is usually less, because ground is typically distributed by massive copper planes and has less resistance. So total supply drop will be the sum of those.

Appreciate the feedback. Part of the intent to is validate DCIR drop power analysis.

Sounds like I should put my red probe on one side of the ferrite (LDO output) and the black probe on the other (connector) side to make measurements easier. The IC at the connector is not easily accessible so it would be best to measure at larger components like the caps.

[DavidAlfa]

It will be millivolts at most. WIth a scope you'll only get noise. And you need a really good DMM to measure sub-mV.
Normally you amplyfi it by 100, 200, whatever, so you get something much more usable.

[Terry Bites]

You could make handy widget with an instrumentation amp.
A micropower like a $3 INA33 it has an input offset of 10uV, an error of 1% in 1mV.
Set gain to 100 and chuck it in the draw. It will run for thousands of hours from a couple of AAs .
A practical circuit is attached.

[wasedadoc]

You could make handy widget with an instrumentation amp.
A micropower like a $3 INA33 it has an input offset of 10uV, an error of 1% in 1mV.
Set gain to 100 and chuck it in the draw. It will run for thousands of hours from a couple of AAs .
A practical circuit is attached.

A practical circuit was not attached.

(And the word is "drawer", not "draw".)

[PartialDischarge]

To measure the steady drop use a multimeter
To measure switchings drop use a x1 probe with scope set at AC, easy to see mV changes

[Terry Bites]

Errata the input resistors are 10k not 10meg. Ooops, drawers at half mast again.