Post

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.

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.

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.

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.