EEVblog 1379 - What's all this NPLC Stuff Anyhow?

Why does a handheld multimeter read zero volts and a bench meter reads several volts?
It's all about the NPLC's man...

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[WizardTim]

I love demoing this trickery to people however I usually take it a bit further and put my feet on a power board or hold a mains cable which capacitively couple huge amounts (for 10 MΩ) of signal onto a bench DMM or oscilloscope (sometimes 50+ V and enough to dimly light an LED), this way you can also demo how grounding yourself almost completely removes the signal and that those capacitively coupled 'ghost' voltages (especially those from isolated switching supplies) aren’t something to be afraid of (but might give you a small tingle).

[fanplant]

Dave finds the free energy test leads and still doesn't believe in it

[mcconkeyb]

Most people outside of the electronics industry have no idea how big and ubiquitous the electric fields are that surround us all the time.
When I was just starting out in electronics design, I was working on a switching power supply, and was measuring the switching frequency which changes with load on a very old (1950 - 1960) scope. I was doing this in the basement of the house where I was living. Depending on the day, I was seeing both the expected switching frequency but also a signal at 1.160 MHz. I chased that 1160 KHz signal for weeks, trying to understand what was going on. Finally 1 day the signal was so big that dominated the measurement to such and extent that it was clear that this signal had some sort of modulation on it. So I ran it through a diode and then into my stereo input, and presto I was listening to a local AM radio station on 1160 KHz. Turns out that the basement dimensions were just the right size to form a resonant cavity, and that AM station would somedays be up to 1.5 Vpp on open leads. Even just moving to an upstairs room would cause the radio signal to be reduced to more normal radio signal level of uV or nV, un-measurable on my ancient scope.
Just a little story about how experience is important when designing anything. :-)

[deadlylover]

Fun fact: The integration time adjustment on many meters maxes out at 10 NPLC, and the 100 NPLC setting is just an average of ten 10 NPLC measurements for example. This is because of the 1/f noise in the input amps and integrator which becomes problematic with longer integration times.

I think the Keithley 7510 that Dave shows in the video performs best in the 1-5 NPLC range, Keithley might have shown a graph of noise vs NPLC in the manual somewhere, very interesting stuff!

[_adamalfath]

Free energy community: "That's free 2Vpp form empty lead! Gonna harvest that"

[tiborherman]

These little fragments of new knowledge is what I love most about this channel. :)

[TonyBarr]

Thank you Dave! I have a 5.5 digit HP bench multimeter. I bought it used on eBay and the 1 VDC reading had me concerned. I did a google on the subject and read it was due to the high impedance. I let it go at that. Now watching this great video enlightened me so much deeper on the subject. I had not idea the multimeter was integrating. I never knew about the powerline cycles etc. Now I know, thanks to you. I am now going back through my multimeter manuals to understand how to vary the number of PLCs etc.

[gblargg]

That Keithley has an amazing refresh speed, looks like at least 50x a normal multimeter.

[redtails]

3:30 the stock market uses this as their input for deciding the prices of most stocks lol

[PeterMilanovski]

These are the videos that both make this channel what it is and underrate it at the same time! But I believe that these will be the videos that people will always come back to in time because the content is so valuable...
Great job 👌

[danielm]

Excelent video!! Welcome to the difference between deterministic noise and random noise. Averaging ( low pass filter after sampling depending on implementation ) is mainly designed to kill high freq random noise. A moving average ( different from averaging frames) can cancel out a deterministic noise if the averaging period exactly matches the signal period( to kill a 50hz signal with a Fs of 100KHz you need 2k samples) . However as you said, "Integration" is effectively anti-alliaisng low pass filter before sampling is performed. By increasing the the value of integration you move the role off frequency under 60Hz killing the deterministic noise.

[sdp]

I always thought my meter was at fault since it was a dumpster find. Good to know it's working just fine. Thanks.

[juancarlossandoval]

Wow Dave your meter looks so much better than the OEM red meter. Looks good in blue.

[gregfeneis]

When sampling a signal, you can reject local interference by sampling at a frequency equal to the lowest common multiple of the frequencies to reject, or harmonics of the LCM of the frequencies to reject. EG Sample at 300 SPS to reject both 60 and 50 Hz.

[Pops]

Committed to memory. As always, thank you Dave

[prathikprashanth]

wow that was interesting! I never expected such a phenomena to occur!

[LutzSchafer]

Dave have you ever wondered why picked up PowerLine noise is never even close to sinusoidal and rather triangular? Where is the non-linearity? ???

[MyAvitech]

Dave must have found those leads in the Over Unity section on ebay.

[dingolovethrob]

I miss Bob Pease, what a star he was.

[xDevscom_EE]

Internally for many DMM aperture time is not always equal NPLC, so saying its a symptom is not quite right. For example settings NPLC100 on 3458A is not same as aperture to same time, as meter internally takes NPLC10 readings and filter them together.