Depending on budget and oscilloscope, a lot of people are using power rail probes for this type of measurement. Something like the our N7020A:
https://www.keysight.com/en/pd-2471132-pn-N7020A/power-rail-probe?cc=US&lc=eng
It was designed to handle offsets higher than 5V, but the power rail probe has been the go-to for a couple years (at least on the enterprise side).
Not sure how helpful it will be for your specific application, but this blog post also talks about making single ended measurements with a differential probe:
https://community.keysight.com/community/keysight-blogs/oscilloscopes/blog/2018/04/17/use-your-differential-probe-where-you-never-thought-possible
The easiest way is to solder a piece of coax to the board and use a DC blocker and 50 Ohm termination at the oscilloscope.
"Ripple" suggests low-freq AC, so why not just use a 1:1 probe into a 1M Ohm AC coupled scope input? The probe's capacitive loading won't be a problem here.
Depending on budget and oscilloscope, a lot of people are using power rail probes for this type of measurement. Something like the our N7020A:
https://www.keysight.com/en/pd-2471132-pn-N7020A/power-rail-probe?cc=US&lc=eng
It was designed to handle offsets higher than 5V, but the power rail probe has been the go-to for a couple years (at least on the enterprise side).
You could also go for a preamp like a stanford resaerch SR560 (http://www.thinksrs.com/products/sr560.html) These have a differential input, high input impedance, ac coupling together with very high gain...
These pop-up regurarly on Ebay for about 750€, not cheap, but very versatile in many applications, and also universal usable on any scope...
"Ripple" suggests low-freq AC, so why not just use a 1:1 probe into a 1M Ohm AC coupled scope input? The probe's capacitive loading won't be a problem here.
You need a differential probe or all the common mode noise that's generally slopping about will completely swamp the noise that's actually being generated by the power supply and that's all you'll see.
Depending on budget and oscilloscope, a lot of people are using power rail probes for this type of measurement. Something like the our N7020A:
https://www.keysight.com/en/pd-2471132-pn-N7020A/power-rail-probe?cc=US&lc=eng
It was designed to handle offsets higher than 5V, but the power rail probe has been the go-to for a couple years (at least on the enterprise side).
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?
You need a differential probe or all the common mode noise that's generally slopping about will completely swamp the noise that's actually being generated by the power supply and that's all you'll see.This is exactly the reason why I am looking for a way to do a differential measurement.
You need a differential probe or all the common mode noise that's generally slopping about will completely swamp the noise that's actually being generated by the power supply and that's all you'll see.This is exactly the reason why I am looking for a way to do a differential measurement.
OK. How about a Tek AM502 diff preamp? PAR made a similar standalone unit, the 113. Signal Recovery has the 5113, that will be in the same price range used as the SR560, though.
The easiest way is to solder a piece of coax to the board and use a DC blocker and 50 Ohm termination at the oscilloscope.I have investigated this technique but at 5V it seems to be loading the circuit with a few tens of mA of current draw. That is the reason why I am looking for a way to do differential measurements.
OK. How about a Tek AM502 diff preamp? PAR made a similar standalone unit, the 113. Signal Recovery has the 5113, that will be in the same price range used as the SR560, though.
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?
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?
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.
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?
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?
Not offhand. I think you're in diff probe territory to get useful CMRR at that bandwidth.
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...
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.
The suggested probe seems to be suitable for the task but as far as I can tell it is only compatible with Keysight oscilloscopes. Does anybody know of a similar probe compatible with bog standard oscilloscopes or at least the ones with the "TekProbe Level II" interface?
Another possibility is the Lecroy DA1855A differential amplifier (which is a distant offspring of the 7A13 that David Hess is eluding to).
It has a switchable gain of x1 and x10, and a BW of 100MHz. There are selectable filters to reduce the BW to 20MHz, 1MHz, or 100kHz. CM range is +/-15.5V (x1) or +/-155V (x10).
They can be found on ebay for exorbitant prices, or you can hang out and wait for a deal. There was one that just sold for USD$330 with a beat up front panel. More commonly they can be had for around $600 - $800. It's a good tool to have if your budget allows.
Additional data:
http://teledynelecroy.com/probes/differential-amplifiers/da1855a
The suggested probe seems to be suitable for the task but as far as I can tell it is only compatible with Keysight oscilloscopes. Does anybody know of a similar probe compatible with bog standard oscilloscopes or at least the ones with the "TekProbe Level II" interface?
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.
In 1:1 mode these probes have a 850 mV range and they are FAST (> 1GHz). This setup will give you DC readings and you can switch the probe to 10:1 to look at the turn-on and turn-off of the regulator.
The P6246 is a slower model at 400 MHz and there is also a P6250 with higher voltage ratings, but it's less common to find and does not have a 1:1 range.
The suggested probe seems to be suitable for the task but as far as I can tell it is only compatible with Keysight oscilloscopes. Does anybody know of a similar probe compatible with bog standard oscilloscopes or at least the ones with the "TekProbe Level II" interface?
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.