Active differential power rail probe

Tek offers the TPR4000. :-D And I think there is one from Agilent in the same price range, too.

These probes are AC-coupled, so offering a direct connection for High frequency and add the DC-Level to the output again.

Olaf

Olaf those are the classic power rail probes - the DC level circuitry is 'active' but the AC path is purely passive, and of course it is a single ended probe.

For a quick check, I just use two channels, with x10 passive probes, and difference 'em. It's not terrific for accuracy, but power isn't required to be accurate. Biggest drawback: you need both hands to hold the probes.

Yep that works for a quick check, I've done the same for jank current measurements through a resistor. The issue with x10 probes for power rail measurement is the measurement noise. E.g. Xilinx transceivers guidelines allow for 10mVpp from 10kHz to 80MHz. Using active differential probes on rails with even more stringent requirements also runs into the same measurement noise issue.

Since input cap / loading of a probe is pretty much irrelevant for power rail measurements, it seems to me that an active diff probe that is designed intentionally for power rail measurements could obtain much better noise performance.

E.g. the RT-ZD40 active diff probe has 3mVrms input referred noise, while 50 ohm oscilloscope frontends easily achieve sub 1mVrms.

For interest, Marco Reps shows how to get 0.001pf isolation on line power:

eXtReMe iSoLaTiOn ( Low Leakage Power in Precision Electronics )

For even more isolation, a number of battery powered oscilloscopes are available for under $200. For example, the FNIRSI-1013D is a 100 MHz dual trace touch tablet:

It is available at

Dave Jones, EEVblog doesn't think much of it, but whadda gonna do for CAD $168.74 ?

The lowest range is 50mV, but you could easily add a differential opamp for low level work, such as the opa846:

It is under $10 at Octopart:

Tek has a number of fully isolated scopes and outboard isolated probes. We have a 4-channel isolated scope, TPS2024 I recall, that's wonderful for working on power things. Clip the probe ground things anywhere.

But the DC level allow more shifting than the scope alone, so for most power supply it is enought.

Olaf

Noise, by definition, is not a DC thing.

For PARD (Peak And Random Deviation), you would include DC.

Differential and common-mode (AC) noise can be measured/contrasted using passive or active combination, over a specific bandwidth.

Wideband diff probes are available for lower frequencies, but reduce with BW and accuracy as DC maximum withstand levels increase.

Is somebody asking for this data? Ask for the measurement standards being referenced. This will usually include a test set-up figure and equipment requirements.

I built a 2-pin, battery-operated Ppk measurement probe for quick 'relative' PSU measurements that gave a digital reading of % in the ancient past. This used the commodity ICL7106 to do a ratiometric comparison between the DC (ref) and an ECL-type biased AC peak detector, though the peak could be obtained as a number, as well.

It's not common test gear because it has very low volume demand. Indications are sufficient, because real world testing usually requires a >>2:1 performance margin on the test limit, so tolerances are not a great issue - just repeatability.

Aim is to avoid trouble, not to track it down after the fact; though it can be used as prediction/symptom of failure elsewhere. Physical noise : Same thing.

Hence strict test setup and usefulness of field experience, in tracking down measurement or application errors / physical configuration pitfalls. RL

I put some connectors on an AD830 because I have lots of them:

Another approach is to put a Mini-Circuits transformer across the current sense resistor, and use a normal scope on the secondary. Needs a ~1 uF coupling cap, of course, but that'll get you down to the low kilohertz. Coupling is very high--their gizmos typically measure over

0.999, often about 0.9997.

Try the T1-6.

Cheers

Phil Hobbs

What is the pri-sec capacitance of that transformer the datasheet is silent?

piglet

About 13pF.

Jeroen Belleman

The DRQ series dual-winding inductors have crazy high coupling and are good for low frequency cases. They come up to 1 mH.

Neat! Their isolated probes with 50 ohm probe input impedance is along the lines of what I was thinking of. Though I don't think true isolation is necessary, just some CMRR. Why isn't this a more common thing?

I'm curious if you've ever tried using a transformer in front of a TIA to boost the photocurrent from a PD for effectively lower TIA input noise? (understandably the impedance looking into the transformer + TIA would be greater than the TIA alone).

I've occasionally used reactive matching networks for narrowband UHFish things. At ordinary frequencies transformers don't help, because in dim light the limiting noise source is the internal series resistance of the photodiode.

Getting to that point requires a decent TIA design, of course--barefoot op amps generally won't cut it.

Cheers

Phil Hobbs

Some of the cheap ebay/amazon battery-powered scopes are actually not bad. That does single-channel-isolated measurement.

Somebody, SRS I think, makes a lab amplifier with a transformer in the front end. The noise level is way sub 1 nv/rthz. A good (well shielded!) transformer is noiseless voltage gain.

I recall that ribbon microphones like to have a transformer. They are very low voltage, very low impedance sources.

Given a low impedance source, a DC-coupled transformer-enhanced amp wouldn't be difficult.

Er... if it's a power supply being probed, how do you transformer-enhance without putting a short on the power supply?

For low impedance at high ground-relative voltages, the current-sense instrument amps like INA283 more-or-less fill that niche. Input current is in the dozens-of-microamps range.