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I am now wondering how a laptop is powered when it is on battery. Let me give you some context: I am using a Mac Book Pro (with an aluminium case) for measurements with a very low signal to noise ratio. The signals are acquired through an amplifier, run on a battery, and grounded with an electrode on my body (impedance below 10 kOhm). The amplifier is linked through USB to the Mac Book Pro.

I noticed a very strong noise on the measurements, spot on 50 Hz, and very similar to a power line noise. The noise can only come from the Mac Book Pro for 2 reasons:

  1. I'm inside a faraday cage with the macbook pro and the probes.
  2. If I touch the casing of the laptop, i.e. if I GND the laptop, the noise disappear instantly.
  3. Obvously, if I let go of the contact with the laptop, the noise come back.

I always thought that when on battery, the laptop was feed with DC current. Is this true, or does the laptop internally convert the DC signal back to an AC signal?

To solve my noise problem, I will obviously ground the laptop. However, I would like to know exactly what is causing this noise.

EDIT: A bit more context

To be fair, the application is still in development. Within the faraday cage were 3 laptops (one shut down) and about 4 phones. The amplifier and the amplifier battery were obviously within too.

The laptop used for recordings + the battery and amplifier are placed on a table.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Nov 17, 2020 at 16:30

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Yes, inside a laptop and almost any other battery-powered electronic devices today there are power supply circuits that convert the battery voltage to other voltages. But these typically operate at frequencies of tens or hundreds of kHz. I'd bet a great deal of money that the laptop is not generating any 50 Hz noise. If I had to guess based on my past experience with 60 Hz noise (here in the US), I'd guess that YOU are the source of the 50 Hz noise. Even if your left wrist is well grounded, your right hand will supply lots of noise to anything you touch (or even put your hand near).

To test this, try grounding the laptop not by touching it, but by some other means. And then try variations on how YOU are grounded.

Edit for additional info:

The power supplies I mentioned use high frequency oscillators, coils, and capacitors to step up and/or down the battery voltage, to get 5V, 3.3V, 1.5V, or whatever. They use very high frequencies to allow the coils to be very small. Note that these are all in use whether your laptop is plugged in or not. For example, if you want to convert from 12V down to 5V, the popular way to do that is a DC-DC converter that uses intermediate AC, rather than a simple linear regulator that drops the voltage by dissipating power (waste heat) in a transistor.

As for my claim that you are the source of the 50 Hz signal, I'm not suggesting that you are generating it, but rather that the human body is a surprisingly good antenna in that frequency range, and that your Faraday cage might not be quite as good as you hope. (The human body does use signals in that frequency range, and that's the reason that a 50 Hz shock is more dangerous than a 60 Hz shock. But if you're seeing a solid 50 Hz signal, your body is receiving it, not generating it.)

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  • \$\begingroup\$ The fact that the battery voltage is converted to other voltages doesn't surprise me. There are obviously different power rails within, for instance, I believe MacBook uses a 1.8V power rail. But is it AC or DC? I'm not sold on the second part. I'm sitting on a chair, within a faraday cage in which there are no power lines at all (obviously). Within the cage are the laptop and the measurement sys (probes and amplifier). I'm not touching anything, and I don't expect humans to generate spontaneously a 50 Hz noise nor do I expect thenoise to be completely gone if I touch the laptop with 1 finger. \$\endgroup\$
    – Mathieu
    Nov 16, 2020 at 18:13
  • \$\begingroup\$ Thank you for the edit. Now the second part can make sense to me. But how do you explain that the noise completely disappear when touching the alumiun casing of the laptop connected via USB to the amplifier? \$\endgroup\$
    – Mathieu
    Nov 17, 2020 at 8:23

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