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Anyone have any details on how to make a programmable capacitor? Is it some fancy op amp trick?

EDIT: I suppose I should be more clear. The new iPhone 4 is having severe antenna problems, because the antenna is exposed to the user's touch, and some users are bridging two ends of the antenna while holding the phone, altering the capacitance of the antenna so significantly that calls get dropped with even 4 bars.

Allegedly, the iPhone 4 also uses programmable capacitors, and there is some noise about whether the iPhone OS 4 will be able to tune the caps to compensate for the user's hand grip. When I heard this, I was not sure exactly how one might create a programmable capacitor...

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  • \$\begingroup\$ What are you asking? Are you wanting an iPhone to control the value of a capacitor? \$\endgroup\$ – Kellenjb Jul 1 '10 at 20:46
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    \$\begingroup\$ Can you provide a reference for this "programmable capacitor" used in the iPhone? \$\endgroup\$ – endolith Jul 2 '10 at 0:42
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Here's such a device, with some diagrams: the MAX1474.

The MAX1474 is a fine-line (geometry) electronically trimmable capacitor (FLECAP) programmable through a simple digital interface. There are 32 programmable capacitance values ranging from 6.4pF to 13.3pF in 0.22pF increments. The quartz dielectric capacitance is highly stable and exhibits a very low voltage coefficient. It has virtually no dielectric absorption and has a very low temperature drift coefficient (<33ppm/°C). The MAX1474 is programmed through two digital interface pins, which have Schmidt triggers and pulldown resistors to secure capacitance programming.

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  • \$\begingroup\$ I'm not really looking to build one, I'm just wondering how they do it. The block diagram you linked to is a good example, though. Multiple caps of varying values in parallel \$\endgroup\$ – ajs410 Jul 1 '10 at 20:40
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You can simulate the response of a variable capacitor with an opamp. See this site: http://www.falstad.com/circuit/e-capmult.html

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  • \$\begingroup\$ can you bring over the info so if the link dies we still have it. \$\endgroup\$ – Kortuk May 17 '11 at 8:05
  • \$\begingroup\$ That's exceedingly clever. \$\endgroup\$ – drxzcl May 17 '11 at 8:27
  • \$\begingroup\$ The op-amp circuit simulates one capacitance using a different capacitance values, but changing the cap values on the fly would require changing some resistors. I think in practice the best way to simulate variable capacitance is to switch various capacitors into and out of a circuit using FET switches. \$\endgroup\$ – supercat May 21 '11 at 2:45
  • \$\begingroup\$ You're going to need a lot of bandwidth on that op amp. At least the 100X that you need with an active filter. \$\endgroup\$ – Mike DeSimone Dec 7 '11 at 5:16
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A normal Varicap (or Varactor) diode is a component that change his capacity by varying the voltage across the junction. Just control that voltage with a DAC and the varicap becomes "programmable" (its capacity can be changed via software). Or by "programmable" you mean something else?

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The most innovative implementation of digitally-controlled variable capacitance was done by none other than AfroTechMods. I know - with a name like AfroTechMods he doesn't seem like he'd be on the level, but it's rather ingenious.

In short: This method only works if one end of the variable capacitance needs to be tied to ground. You take several different values of capacitors and short one terminal of them all together - connect this to the point in the circuit where normally you'd connect the capacitor to ground. The other end of each capacitor gets connected to a digital I/O pin on a microcontroller. If you set the pin to sink current then it's the same as if that end of the capacitor was tied to ground. By changing the state of the pins that each capacitor is connected to you can essentially put one or more capacitors in parallel to achieve almost any value capacitance - controlled digitally. It's rather neat.

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  • \$\begingroup\$ You need to make sure to use a small-size microcontroller and route ground and I/O lines as short as possible. Otherwise you'll be putting tens-of-nH inductors in series with some of the caps, changing the phase angle and effective capacitance. A ground plane is a must. \$\endgroup\$ – Mike DeSimone Dec 7 '11 at 5:20

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