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I opened up a mystery chip; it turned out to be a "switched capacitor filter" from 1984. Filters are important circuits, for example, adjusting the bass or treble in an audio signal. This chip, the MF10, was popular because it combined two filters into an easy-to-use chip. Let's take a closer look…

At first, we couldn't identify this mystery chip from Harris. Since it had a ceramic package, I could easily open it by tapping it along the seam with a chisel, popping the lid off and revealing the tiny silicon die inside.

Under the microscope, a tiny map of Florida is visible. Harris, the manufacturer, was in Melbourne, Florida; the stylized H is the logo. The chip has various initials, presumably the names of the designers of the chip.

The chip contains two filters; the top and bottom filters are mirror images. The chip contains 72 tiny capacitors, the pink squares in the middle. Above the capacitors are electronic switches, which switch the capacitors in and out of the circuit at high speed.

The switched capacitor is a bit bizarre. By opening and closing the switches at high frequency, charge moves in and out of the capacitor at a controlled rate, just like a resistor. Why replace a resistor with a complicated circuit? Because resistors are large and very inaccurate on chips.

The heart of most analog circuits is the operational amplifier, or op-amp. Op-amps are very flexible; in this chip, they integrate signals. The op-amp is interesting under the microscope; its transistors have various sizes and patterns.