IPhone programmable caps

capacitorprogramming

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…

Best Answer

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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