Active differential power rail probe...

[whit3rd]

On Friday, May 6, 2022 at 11:50:27 AM UTC-7, John Larkin wrote:

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.

 

[Phil Hobbs]

John Larkin wrote:

On Thu, 5 May 2022 21:14:45 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

Dominic Chan wrote:
On Thursday, May 5, 2022 at 2:42:53 AM UTC+10, Phil Hobbs wrote:
Mike Monett wrote:
Dominic Chan <dom...@arista.com> wrote:

On Wednesday, May 4, 2022 at 2:49:41 PM UTC+10, whit3rd wrote:
On Tuesday, May 3, 2022 at 8:48:08 PM UTC-7, Dominic Chan
wrote:
I\'ve been doing some power rail noise/ripple measurements
lately. ... This has got me wondering though, why isn\'t an
active differential prob
e optimized for power rail measurements a thing? Perhaps it
does exist? - I f so please let me know!
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 )
https://www.youtube.com/watch?v=9JinSfCKuNQ

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:

http://www.fnirsi.cn/productinfo/556152.html

It is available at

https://www.aliexpress.com/i/4000861098295.html

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

https://www.youtube.com/watch?v=5iwtDwJlbWk

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

https://www.ti.com/lit/ds/symlink/opa846.pdf

It is under $10 at Octopart:

https://octopart.com/search?q=opa846&currency=USD&specs=0




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.

https://www.minicircuits.com/pdfs/T1-6-X65+.pdf



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.


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.

Sure, they\'re used a fair amount for low-Z resistive stuff. I have a
bunch of old mu-metal shielded transformers for ribbon mic preamps that
I\'ve never actually used for anything.

Win and Paul made a pretty cool low-noise front end for a ribbon mic in
AOE3.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

whit3rd wrote:

On Friday, May 6, 2022 at 11:50:27 AM UTC-7, John Larkin wrote:

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?

With a \"feedbeside\" network, i.e. you DC couple the DC and AC couple the AC.

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.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Fri, 6 May 2022 13:03:22 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, May 6, 2022 at 11:50:27 AM UTC-7, John Larkin wrote:

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?

Er... use a capacitor.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[John Doe]

AOE3 = The Art of Electronics 3rd Edition