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I get how mutual capacitance touch screens work, by changing the dielectric around the pairs of electrode and "stealing" electric field lines. Connection to earth of the object (a finger usually) isn't required.

What I don't get with self capacitance measurement is that the finger-electrode pair forms a capacitor connected to earth through the capacitance between the human body and earth. How could the smartphone measure that capacitance without itself being connected to earth? How could it load this newly formed capacitor in any way without "completing the circuit"?

Is that because the smartphone is usually also connected to ground through capacitive coupling (as is the human body)? It troubles me because it's never mentionned in any of the explanation I've seen about this technology.

My preferred ad hoc explanation of this phenomena is that the circuit is closed through capacitive coupling between the human body and smartphone ground, thus eliminating the need of earth. But I'm not really convinced since the capacitance of the coupling between the human body and the smartphone is extremely variable and largely unknown.

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  • \$\begingroup\$ What does earth have to do with anything? You are holding the phone so you have galvanic and/or capacitive connection to it. Both you and phone also have capacitive coupling to everything around you, not just earth. \$\endgroup\$
    – Justme
    Jun 12, 2022 at 11:12
  • \$\begingroup\$ Maybe of some help \$\endgroup\$
    – Andy aka
    Jun 12, 2022 at 11:19
  • \$\begingroup\$ Put two capacitors in series. The segment in the middle need not be connected to anything else. Now move the parts around until the schematic looks like two T's with a bar over both of them. The bar is the finger \$\endgroup\$
    – Abel
    Jun 12, 2022 at 11:28
  • \$\begingroup\$ @Justme It does not need indeed to be earth, just a common environment as you said. But I don't agree on galvanic coupling. You don't need to be touching the screen for it to work, and capacitive coupling has the problem I mentionned which is that it's highly unknown. \$\endgroup\$ Jun 12, 2022 at 12:38
  • \$\begingroup\$ @Andyaka Oh, yes, it helped a bit. The answer could be that the circuit is closed through the common environment, and because this environment is so far away, we can use the approximation of self capacitance. \$\endgroup\$ Jun 12, 2022 at 12:42

2 Answers 2

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For the size of a normal cell-phone its self capacitance to earth is usually about 4pF.

The capacitance of a finger touch to the screen for "self-capacitance" detection (as opposed to mutual capacitance mode) is about 1pF.

Mutual capacitance detection, as used by most cell-phones to detect the position on the screen, requires capacitance detection with a resolution of 1-10 fF to get adequate physical position sensitivity.

When the phone is being held in the hand the body to phone capacitance is much higher, maybe 100pF, even if there is no direct galvanic connection.

Similarly the human body to ground capacitance is in the region of 100pF (depending upon size of person, type of footwear and floor etc).

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Some ascii because touch devices are not allowed to use the schematic editor:

Two capacitors in series look like:

-||-||-

Capacitors are drawn as plates with no electrical contact through them. Notice that the middle part (|-|) needs no electrical connection to ground.

The same schematic can be drawn with the two caps positioned vertically and with the wire between them brought down to the level of the plates.

____
T  T

Now you have a circuit equivalent to two capacitors in series with a plate not connected to ground.

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  • \$\begingroup\$ I think what you've described here is mutual capacititance sensing, whereas I was asking specifically about self capacitance sensing. \$\endgroup\$ Jun 12, 2022 at 17:01
  • \$\begingroup\$ If you understand this, apply that understanding to all. There is a capacitance between any two entities where current cannot flow between them unrestricted. Human to earth, phone to earth, phone to human... just add as many capacitors/plates in series and parallel as you need in order to accurately describe the situation. You may realize that there are conditions where observability suffers greatly as noise in certain capacitors can easily overshadow one you seek to measure \$\endgroup\$
    – Abel
    Jun 12, 2022 at 19:20

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