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

--- Quote from: nctnico on May 12, 2018, 10:50:08 pm ---
--- Quote from: KrudyZ on May 12, 2018, 10:11:03 pm ---You could use a Tektronix P6247 or P6248 if you're looking For a TekProbe interface.
Common mode range of +-7V

To zoom in on the ripple, apply a constant 5V from a reference to the negative input of the probe.
Make sure the ground side of the reference is hooked up to the ground of the cap on the DUT supply that you are measuring your ripple across. Add a local bypass cap for the reference and either twist your leads or use coax for the feed.
--- End quote ---

I'm wondering if this isn't an overly obfustigated way of measuring ripple. One of the problems I see is that the 5V offset must be ripple free as well over a large frequency band. Just look at the Jim Williams appnote linked to above which uses a DC blocking capacitor and a coax cable soldered directly to the point of interest.
--- End quote ---

The advantage of this method, differential comparison, is that the low frequency response extends to DC.  Most oscilloscopes support this to a limited extent using their position control and to a greater extent using an offset adjustment if available.  The previously mentioned 7A13 and the differential amplifiers from Preamble (now LeCroy) support this to an extreme extent and so do the Keysight power measurement probes.

Noise from the comparison voltage is not a problem; the reference it is just too easy to filter and low frequency drift is not an issue at 1mV/div.

The problem with this method is that the difference in potential and resulting common mode current between the DUT (device under test) ground and oscilloscope grounds can corrupt the measurement.  Jim Williams mentioned this in his application note:

GROUND POTENTIAL DIFFERENCES PROMOTE OUTPUT HIGH FREQUENCY CONTENT AND CORRUPT MEASUREMENT.

Using a continuous coaxial connection as Jim Williams recommends definitely helps and is sufficient in many applications.  I do it by soldering a short coaxial BNC pigtail to the test point and then using a BNC to probe tip adapter to connect a x1 probe.  Then the oscilloscope's AC coupling capacitor can remove the DC offset or a differential comparison can be made if the offset range is sufficient.

A differential measurement removes the ground imbalance but coaxial connections are still required (or possibly twisted pair as KrudyZ suggested) for best performance.  The ATX power supply specifications do not even bother recommending a single ended coaxial test setup.
mtdoc:
If you’re on a tight budget, why not go old school and pick up a Tek p6046?
BravoV:

--- Quote from: mtdoc on May 13, 2018, 04:25:58 am ---If you’re on a tight budget, why not go old school and pick up a Tek p6046?

--- End quote ---

+1 on P6046 if the OP only on hobbyist budget.

Its also the example probe in the official standard ATX document.

David Hess:

--- Quote from: BravoV on May 13, 2018, 04:47:42 am ---
--- Quote from: mtdoc on May 13, 2018, 04:25:58 am ---If you’re on a tight budget, why not go old school and pick up a Tek p6046?
--- End quote ---

+1 on P6046 if the OP only on hobbyist budget.

Its also the example probe in the official standard ATX document.
--- End quote ---

If you can find one which is undamaged; the input JFETs are unprotected.  It is just barely possible to replace them but calibration is difficult.

A modern implementation of the P6046 is one of the projects I have considered but so far I have managed to get by using other means.
icpart:
Another possibility is to use very low noise preamplifier for oscilloscope measurements of noise and ripples of power supplies. Just look these another great AN83 from Jim Williams http://www.analog.com/media/en/technical-documentation/application-notes/an83f.pdf
Also I find another great improved version of that amplifier int that AN http://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf
I also have two p6046 probes but I am not tested how they are perform for low noise measurements. I am also very interested in measurements of parameters of DC/DC converters.
P.S.
 :-+ WOW That AN159 is great source for power supplies measurements and characterization.
Also another possible alternative is that new differential amplifier from Picotest if you have to spend 1500$ https://www.picotest.com/products_J2113A.html or https://www.picotest.com/products_J2180A.html
Picotest have a great products for power supplies measurements but there instruments are not very cheap.
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