• Active differential power rail probe

    From Dominic Chan@21:1/5 to All on Tue May 3 20:48:04 2022
    I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid
    underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

    Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently
    small for good measurements.

    This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

    Cheers, Dominic

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  • From olaf@21:1/5 to Dominic Chan on Wed May 4 06:13:16 2022
    Dominic Chan <dominic@arista.com> wrote:


    This has got me wondering though, why isn't an active differential
    probe optimized for power rail measurements a thing? Perhaps it does
    exist? - If so please let me know!

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

    https://www.tek.com/en/datasheet/active-power-rail-probes

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

    Olaf

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  • From whit3rd@21:1/5 to Dominic Chan on Tue May 3 21:49:37 2022
    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 probe optimized for power rail measurements a thing? Perhaps it does exist? - If 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.

    --- SoupGate-Win32 v1.05
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  • From Dominic Chan@21:1/5 to olaf on Tue May 3 21:46:05 2022
    On Wednesday, May 4, 2022 at 2:15:09 PM UTC+10, olaf wrote:
    Dominic Chan <dom...@arista.com> wrote:


    This has got me wondering though, why isn't an active differential
    probe optimized for power rail measurements a thing? Perhaps it does
    exist? - If so please let me know!
    Tek offers the TPR4000. :-D And I think there is one from Agilent
    in the same price range, too.

    https://www.tek.com/en/datasheet/active-power-rail-probes

    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.

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  • From Dominic Chan@21:1/5 to All on Tue May 3 22:58:57 2022
    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 probe optimized for power rail measurements a thing? Perhaps it does exist? - If 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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Mike Monett@21:1/5 to Dominic Chan on Wed May 4 08:30:22 2022
    Dominic Chan <dominic@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




    --
    MRM

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    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to dominic@arista.com on Wed May 4 06:44:34 2022
    On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
    <dominic@arista.com> wrote:

    I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid
    underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

    Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is
    sufficiently small for good measurements.

    This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

    Cheers, Dominic

    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.

    https://www.tek.com/en/search?keywords=isolated



    --

    Anybody can count to one.

    - Robert Widlar

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From olaf@21:1/5 to Dominic Chan on Wed May 4 16:08:20 2022
    Dominic Chan <dominic@arista.com> wrote:


    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.

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

    Olaf

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  • From Chris Jones@21:1/5 to Dominic Chan on Thu May 5 00:39:02 2022
    On 04/05/2022 13:48, Dominic Chan wrote:
    I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid
    underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

    Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is
    sufficiently small for good measurements.

    This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

    Cheers, Dominic

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

    https://twitter.com/chrisgj198/status/330590675806605312

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From legg@21:1/5 to dominic@arista.com on Wed May 4 10:17:24 2022
    On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
    <dominic@arista.com> wrote:

    I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid
    underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

    Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is
    sufficiently small for good measurements.

    This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

    Cheers, Dominic

    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

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  • From Phil Hobbs@21:1/5 to Mike Monett on Wed May 4 12:42:39 2022
    Mike Monett wrote:
    Dominic Chan <dominic@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>

    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

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  • From piglet@21:1/5 to Phil Hobbs on Thu May 5 06:35:57 2022
    On 04/05/2022 17:42, Phil Hobbs wrote:
    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>


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

    piglet

    --- SoupGate-Win32 v1.05
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  • From Jeroen Belleman@21:1/5 to piglet on Thu May 5 09:54:19 2022
    piglet wrote:
    On 04/05/2022 17:42, Phil Hobbs wrote:
    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>


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

    piglet

    About 13pF.

    Jeroen Belleman

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Thu May 5 09:03:56 2022
    On Wed, 4 May 2022 12:42:39 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

    Mike Monett wrote:
    Dominic Chan <dominic@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>

    Cheers

    Phil Hobbs

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





    --

    Anybody can count to one.

    - Robert Widlar

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Dominic Chan@21:1/5 to jla...@highlandsniptechnology.com on Thu May 5 15:13:39 2022
    On Wednesday, May 4, 2022 at 11:44:50 PM UTC+10, jla...@highlandsniptechnology.com wrote:
    On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
    <dom...@arista.com> wrote:

    I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid
    underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

    Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is
    sufficiently small for good measurements.

    This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

    Cheers, Dominic
    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.

    https://www.tek.com/en/search?keywords=isolated



    --

    Anybody can count to one.

    - Robert Widlar
    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?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Dominic Chan@21:1/5 to Phil Hobbs on Thu May 5 15:19:10 2022
    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>

    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

    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).

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Dominic Chan on Thu May 5 21:14:45 2022
    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.

    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Doe@21:1/5 to All on Fri May 6 01:31:23 2022
    TIA = Transimpedance Amplifier

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to dominic@arista.com on Fri May 6 07:26:35 2022
    On Thu, 5 May 2022 15:13:39 -0700 (PDT), Dominic Chan
    <dominic@arista.com> wrote:

    On Wednesday, May 4, 2022 at 11:44:50 PM UTC+10, jla...@highlandsniptechnology.com wrote:
    On Tue, 3 May 2022 20:48:04 -0700 (PDT), Dominic Chan
    <dom...@arista.com> wrote:

    I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid
    underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input.

    Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is
    sufficiently small for good measurements.

    This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!

    Cheers, Dominic
    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.

    https://www.tek.com/en/search?keywords=isolated



    --

    Anybody can count to one.

    - Robert Widlar
    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?

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

    https://www.amazon.com/gp/product/B081Q2KDJT/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1



    --

    Anybody can count to one.

    - Robert Widlar

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri May 6 11:50:14 2022
    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.

    Cheers

    Phil Hobbs

    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.

    --

    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From whit3rd@21:1/5 to John Larkin on Fri May 6 13:03:22 2022
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to John Larkin on Fri May 6 16:09:30 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to All on Fri May 6 16:45:58 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to All on Fri May 6 14:48:34 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Doe@21:1/5 to All on Sat May 7 23:36:52 2022
    AOE3 = The Art of Electronics 3rd Edition

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)