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Looking for a Probe to Use for 5V DC Power Supply Ripple Measurement
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_Wim_:

--- Quote from: bendras on May 12, 2018, 01:46:46 pm ---
This particular preamp seems to be limited to 3Vpp inputs signals, hence measuring 5V DC rail would be out of spec. (please correct my if I am wrong). Also as far as I understand its 1MHz bandwidth would not allow measurement of high frequency ripple.


--- End quote ---

AC coupled these can withstand 100Vdc according to the user manual. Yes, bandwidth is limited, but high bandwidth AND low noise is not possible. For me "ripple" seemed like a low frequency thing, I was not aware you wanted to measure at least up to 20Mhz.

As suggested, the probes designed for the job are off course ideal, but expensive and typically dedicated to a specific scope brand.

Edit: quote formatting
NiHaoMike:
Since only AC is important, salvage the coupling transformer from an Ethernet device or DSL modem, connect the primary to the voltage rail with a DC blocking capacitor, and connect the secondary to the scope with a parallel resistor selected to get as flat a frequency response as possible.
_Wim_:

--- Quote from: Cerebus on May 12, 2018, 12:57:28 pm ---The N2792A (200MHz ±20V differential ±60V common mode) is probably a better fit than the 2GHz N7020A (which at a guess is probably $2k a pop). That or the N2791A (25 MHz ±70V differential @ 10:1  or ±700V differential @ 100:1, ±700V common mode).

Trying to get him to spend more money Daniel?  :)

--- End quote ---

The N2792A and N2791A have much higher noise (6mVrms and 50mVrms) making them not usefull for this kind of application. The N7020A suggested by Daniel achieves 90uVrms combined with an  S-series scope, a top solution if money is not an issue...
Kosmic:
There's a interesting application note from linear talking about DC DC converter noise and how to measure it.

http://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf

At page 10 you will find : APPENDIX C Probing Technique for Sub-Millivolt, Wideband Signal
Integrity

There is also a short YouTube video  :)
https://youtu.be/WxhjLIu-vPg
David Hess:

--- Quote from: precaud on May 12, 2018, 05:56:51 pm ---
--- Quote from: bendras on May 12, 2018, 04:14:52 pm ---All of these preamplifiers seem to have maximum bandwidth of 1MHz. As far as I know the standard for measuring power supplies is 20MHz. Do you know of any models with 20MHz or higher bandwidth?
--- End quote ---

Not offhand. I think you're in diff probe territory to get useful CMRR at that bandwidth.
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In my experience only a moderate amount of common mode rejection is required for these measurements.  The old Tektronix 7A13 achieve a minimum of 300:1 or 50dB at 20MHz from 1mV/div to 20mV/div where despite no input dividers being used, it still allows for a +/-10 volt DC input range.  Since it supports AC coupled inputs, higher voltage measurements are feasible and indeed, switching between AC and DC coupling when measuring the ripple on a 5 volt supply makes no difference unless something is broken.

I have made the same measurement using analog and analog input DSOs in add and invert mode as long as the common mode rejection is adjusted with the variable volts/div control.  The reason I do not do this all the time is simply because none of my oscilloscopes have the native 1mV/div sensitivity that the 7A13 provides although the 7A13 is pretty noisy if its full bandwidth is used.

Ideally what is needed is a low voltage 20MHz differential probe with a wide input common mode range or AC coupling but offhand I do not know of any.  The old Tektronix P6046 sort of meets these requirements since it supports AC coupling.
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