Products > Test Equipment
Looking for a Probe to Use for 5V DC Power Supply Ripple Measurement
precaud:
--- Quote from: David Hess on May 12, 2018, 07:59:18 pm ---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...
--- End quote ---
I can't recall ever making a realtime noise diff measurement above 1MHz but what you're saying makes sense.
--- Quote from: NiHaoMike on May 12, 2018, 06:24:59 pm ---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.
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I played around with that once, those xfmrs roll off the low end pretty early, the one I tried was only good down to 1kHz or so.
MarkL:
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
KrudyZ:
--- Quote from: bendras on May 12, 2018, 11:34:13 am ---
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?
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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.
bendras:
--- Quote from: MarkL on May 12, 2018, 09:49:18 pm ---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
--- End quote ---
Having looked through the specs. of this device it seems that it would not only meet my current needs but that it would also be a safe bet for the future :-+
--- Quote from: KrudyZ on May 12, 2018, 10:11:03 pm ---
--- Quote from: bendras on May 12, 2018, 11:34:13 am ---
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?
--- End quote ---
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.
--- End quote ---
The Tektronix P6246 definitely seems suitable :-+. Having said that, since I do not have a compatible scope at the moment it would workout cheaper for me to get the Lecroy DA1855A.
nctnico:
--- Quote from: KrudyZ on May 12, 2018, 10:11:03 pm ---
--- Quote from: bendras on May 12, 2018, 11:34:13 am ---
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?
--- End quote ---
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.
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