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:
- I'm inside a faraday cage with the macbook pro and the probes.
- If I touch the casing of the laptop, i.e. if I GND the laptop, the noise disappear instantly.
- 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.