PS 4444: Common-mode overrange ?

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Harald Koenig
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PS 4444: Common-mode overrange ?

Post by Harald Koenig »

Hi,

I'm new to PicoScope, trying to evaluate which PS is the right device for my needs.
for testing I got a PS 4444 from other department where specs look good, but...

I'm trying to measure current/power (200-500 mA) for a uC with 12V power supply,
running measurements with 1+ MS for 60+ seconds.
there is a 0.01 Ohm shunt in 12V line, so signal on shunt is ~2-5 mV.

PS 4444 with differential probe and 12/14 bit for +/- 10mV looks great -- in theory?!
BUT only range +/- 1V and higher work (though has miserable SNR and dynamics),
for smaller max. range I always get blinking "Common-mode overrange" and
mostly constant 7-8 mV with high noise instead of expected -2 to -5 mV (which I roughly see for +/- 1V range).

is PS 4444 with differential probe not able to measure few mV with 12V DC offset ?
which other device shall I try/buy?

or what's wrong with my setup ?

I'm running Linux (Ubuntu 20.04.2) and I've tried both picoscope version 6.14.47.5899 and 7.0.77.8819.

thanks for any help!!
Harald

Harald Koenig
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Re: PS 4444 differential probe problem: "Common-mode overrange" ?

Post by Harald Koenig »

today I did many more tests -- and found "some constellations" where the planned measurement works well with no "Common-mode overrange". but still I do not understand root cause of my problem and how to really "solve" it, or why it's an issue at all with that DB9 differential probe for PS 4444 ?!

details -- with basic idea "there are some ground problems" (why every this is an issue for differential probe ?!):

if the uC devkit to be measured has *no* connections other than 12V lab power supply (HMC 8043) and PS 4444 diff. probe, "Common-mode overrange" is gone, and +/- 10 mV range is fine for small signals.

if I connect either USB (serial console) to notebook, or HDMI cable to external 20" monitor (HP E242), error is back!

with no USB/HDMI connection, I measure 11.4V between lab power supply uC GND and shield of either USB from notebook or shield from HDMI from monitor.

notebook is Lenovo T480 with docking station, power supply to docking station is 20V connector with "rectangular USB-A shape plug) (the one before Lenovo switched to USB-C).

from lab supply and uC GND to Lenovo power supply outer connector, it's 10.4V.

if I change output voltage from lab supply to 12 / 11 / 10 / 9 V then the voltage to notebook USB shield is 11.40 / 10.44 / 9.49 / 8.54 V -- so it's always round about Vbat - 0.6V. hmmm...
but I cannot measure any current flowing between lab supply GND and USB/HDMI shields.
if USB/HDMI is connected, "all ground levels are the same" (no surprise, is it?).

my problem: for the scenario I have to test I *need* both USB and HDMI connections.
so right now PS 4444 with that differential probe does not work for me:-(
looks like I have to check other power supplies...

BUT WHY IS THIS AN ISSUE AT ALL ???
why does this influence measurements with that differential probe?



BTW power supply: I got a 5V 3.5A power supply plug with the PS 4444.
if I connect that to the PS, and notebook is *not* connected to it's power supply (runs on battery) and no USB/HDMI connection, then I see *huge* distortions with 50 Hz ~300 mV in measurement ranges smaller than +/- 1V (see attached screen shot), but the 1V range is fine again...

what's the problem with that power supply?
this seems to be easy to avoid, as at least 1 differential probe works fine with USB3 port (later I'll need at least 2 probes, and 1-2 single ended BNCs -- will that still work??)


thanks for any insights, thoughts, help and SOLUTIONS;)

Harald

Harald Koenig
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Re: PS 4444: Common-mode overrange ?

Post by Harald Koenig »

measurement with 5V PS4444 supply and no USB/HDMI ground -- everything connected to the same 230V power hub btw.
measurement with 5V PS4444 supply and no USB/HDMI ground -- everything connected to the same 230V power hub btw.

bennog
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Re: PS 4444: Common-mode overrange ?

Post by bennog »

from the specification page

https://www.picotech.com/oscilloscope/4 ... ifications

Input common mode range
±5 V on ±10 mV to ±500 mV ranges
±50 V on ±1 V to ±50 V ranges

so max 5V for the lower ranges.

I Use a PicoConnect 442 for this kind of measurement (also if you want to do measurements on the mains)

Benno

Harald Koenig
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Re: PS 4444: Common-mode overrange ?

Post by Harald Koenig »

Hi Benno,
thanks for your answer! I did not understand that these "ranges" are not only relevant for my signal (< 10 mV for 1A current = 12W power @ 12V) but also for the offset/bias(?) voltage. guess I have to talk with my electronics experts again;)

reading the specs for the PicoConnect 442, the best I can get is 14 bit for +/- 2.5 V differential range,
so 1 LSB = 0.305 mV ~= 30 mA which isn't that great anymore (without resolution enhancement, will try that too of course;)

anyway thanks for pointing me in that direction -- will try to get a PicoConnect 442 for evaluation too,
and I will try to get rid of that offset voltage (so understand all details of all power supplies etc. :-(

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