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Affordable <200MHz PDN analysis / impedance measurement hardware?

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inevitableavoidance:
I've been playing with- and modifying my Analog Discovery 2 today, messing with the input attenuation and gain circuitry of the scope inputs to give me in total 26dB extra gain, giving me a great improvement in usable impedance measurement range. Here's a stack of five 10uF 0805 capacitors without any averaging:



Problem however with this new milliohm range extension is... inductance. Even at 1MHz a little 10mm back-and-forth of wire has an AC impedance of around 200mΩ, severely limiting my usable frequency range now I can measure down to the milliohms. For the image above I soldered the capacitors on the AD2 PCB itself. Can't really solder an entire PDN on there...

So I believe it's back to the NanoVNA-H4!

nctnico:
No. Your cabling is 50 Ohm so there is no wire inductance. What you can do is add some attenuation (like 6dB) at where you inject and measure the signal to improve impedance matching a bit but it should affect the measurements much. See the source's output resistor as a current source. When calculating the impedance of the board, the current from the source is used together with the measured voltage level to calculate the impedance (R = V / I).

inevitableavoidance:
@nctnico you're right! Turns out I was accidentally measuring the inductance / impedance of a shared ground path. I messed up the wiring of one of the differential inputs. Measurements are starting to make a lot more sense now I've got that sorted!

Here's the impedance plot of a 3.3V LDO, both when on and off:



I'm getting 60mΩ at 1MHz there - 10nH - I'd say that makes sense based on how I wired it.

Here's one soldered directly over an 0805, that also has 200uF of bulk capacitance on the same net:



Giving 30mΩ at 10MHz - 500pH - sounds quite appropriate for an 0805. :)

Glad I got that sorted, thanks for the push nctnico. I can't really use it as a scope anymore with all the modifications, but am pretty happy with the little device it has become.

I'm still getting back to the NanoVNA-H4, to dive more into the higher frequencies, but as an addition instead of a replacement now. :)

inevitableavoidance:
I ended up designing a little adapter board for the Analog Discovery series!

Here's the board measuring a low impedance capacitor array:





Here's a bunch of verification resistors (with varying inductances):



I realized I didn't need to measure higher than ~25MHz - once I see the inductance slope at the end I'll known enough about what happens afterwards. The board has a 200mA output current buffer, and two fully differential opamps. This gives me both more current and enough resolution for all the low stuff. The bottom limit is about 100uΩ. :)

The flat tinned bars are spaceholders for tantalum capacitors, to be able to measure with a DC Bias as well. Works great so far.

Thanks for all the tips along the way!




axb:

--- Quote from: inevitableavoidance on October 28, 2024, 02:57:41 pm ---I ended up designing a little adapter board for the Analog Discovery series!

Here's the board measuring a low impedance capacitor array:

--- End quote ---

Hi,
Could you please share the schematic?

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