Teardown of a 1980s Harris Switched-Capacitor Filter Chip

Identifying the Mystery Chip

While sorting a box of old ICs, CuriousMarc came across Harris ICs labeled "F1-10-5", a part number that didn't show up in any databooks. Since the chip was in a ceramic package, it could be opened with a quick tap from a chisel. Under the microscope, the die showed a grid of square capacitors, suggesting a switched-capacitor filter. The die marking "HF-10" provided the final clue: the chip was Harris's version of the standard MF10 switched-capacitor filter, introduced by National Semiconductor in 1981. The package carried a 1985 date code.

Inside the Die

The MF10 provides two flexible filters on a chip, each configurable as low-pass, band-pass, or high-pass, controlled with a few external resistors. The die's top and bottom halves are mostly mirror images, corresponding to the two filters. The distinctive reddish squares in the middle are 72 tiny capacitors built from polysilicon, estimated at 5 picofarads each. Above the capacitors, CMOS switches toggle at the clock frequency. Each filter uses three operational amplifiers, with high-pass, band-pass, and low-pass outputs. Control circuitry handles clock level shifting, shaping, frequency ratios, and startup. Twenty hair-thin bond wires connect the die to its external pins. The die also carries chip art: a Harris logo, an outline of Florida where Harris was headquartered in Melbourne, and the initials of the design engineers.

How Switched-Capacitor Filters Work

A switched capacitor can replace a resistor in certain circuits. Switches controlled by a clock alternately charge the capacitor to the input voltage and pass charge to the output. The circuit behaves like a resistor with resistance of 1/(fC), so higher capacitance or frequency means lower resistance. On-chip resistors are inaccurate and large, while capacitors save space and allow the filter frequency to be tuned simply by changing the clock frequency. Although absolute capacitance varies by about 20% from chip to chip, the MF10 depends on capacitance ratios. Some capacitors are separate while others are connected in groups of 8, yielding a stable 8:1 ratio.

The chip also inverts voltage while acting as a resistor, using four switches so the top plate grounds during phase 2, which cancels some parasitic capacitances and improves accuracy. The filters use a state variable topology invented in 1967, with three op-amps providing summing and two integrations, allowing frequency, gain, and Q to be varied independently. CMOS switches combine NMOS and PMOS transistors, and the MF10 surrounds transistor pairs with isolation rings and a grounded polysilicon shield to reduce clock noise. A non-overlapping clock generator ensures both switches are never closed at once.

Harris and the MF10 Today

Harris began in 1895 as a printing press company, moved into high technology in the 1950s and 1960s, and entered the IC business in 1967 by acquiring Radiation, Inc., later renamed Harris Semiconductor. Harris spun off Intersil in 1999, which was later acquired by Renesas, and merged with L3 Technologies in 2019 to become L3Harris. Switched-capacitor filters have lost popularity as filtering moved into the digital domain. Texas Instruments acquired National Semiconductor in 2011 and lists the MF10 as active but expensive and out of stock, suggesting it is probably no longer manufactured. State variable filters remain in use in the synthesizer world for their flexibility.

Sources: Lobsters