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Ken Shirriff
@righto.com

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…

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Ken Shirriff
@righto.com

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.

Ken Shirriff
@righto.com

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.

Ken Shirriff
@righto.com

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.

Ken Shirriff
@righto.com

Here's a close-up of one of the electronic switches. It uses four transistors and acts as a toggle switch. To prevent interference, the transistors are surrounded by two isolation rings. For the second photo, I removed the metal layer with acid to reveal the polysilicon and silicon underneath.

Bret Victor
@worrydream.com

Switched-cap filters occupy a rare middle ground between analog and digital -- they are discrete-time but continuous-value! You design them using digital filter theory (in the z-transform domain), but implement them as analog circuits. They always felt fascinatingly chimeric to me.