Microscope Teardown Reveals Internals of 1980s MF10 Filter IC

The internal structure of a 1980s analog filter IC has been revealed through microscopic die analysis. The device is a mysterious IC believed to have been made by Harris, identified from on-die markings as an MF10-compatible switched-capacitor filter. Reporting via Lobsters from righto.com via calvin documents the capacitor array and amplifier layout in detail. The process from identifying the physical part through decapsulation to microscope observation illustrates the design philosophy of analog integrated circuits.

Sometimes it’s easier to identify an IC with a microscope.

That is the key insight offered by the original author. For parts that cannot be resolved through part-number searches, microscope observation proved to be an effective tool.

Mystery Marking F1-10-5 Identified

It began with an effort to sort old ICs. A collector known as CuriousMarc found an IC marked “F1-10-5” made by Harris. No reference listed the part number. Because unidentifiable ICs have little practical use, he provided the sample to an analyst. The package was ceramic-sealed and could be opened with a single chisel strike. Square capacitor elements were arranged in a grid on the die surface. This feature suggested it could be a switched-capacitor filter. The marking on the die read “HF-10.” Using this clue, it was confirmed to be the Harris version of the MF10. The MF10 is a general-purpose filter IC introduced by National Semiconductor in 1981. It is notable for integrating two tunable filters on a single chip.

Ceramic Decapsulation and Microscope

Observation Process

Ceramic packages are easy to open. In this case as well, mechanical impact exposed the die. In the microscope image, white wiring indicates the metal layer. Below it, two layers of polysilicon appear reddish. The gray area at the lowest level is the silicon substrate. The top and bottom halves of the chip are nearly mirror images. This reflects the symmetrical layout of the two filters. Twenty fine bond wires were observed around the die. They connect the die to each pin of the 20-pin package. Control circuitry was concentrated on the left side of the die. This section handles clock level shifting and signal shaping. Frequency-ratio processing and startup circuitry were also placed in the same area. It also includes current sources that supply constant current to other circuits.

72-Capacitor Array Reveals Circuit Structure

The reddish squares in the center of the die are the capacitor array. Seventy-two tiny polysilicon capacitors are arranged in rows. CMOS switches are placed above them. They repeatedly turn on and off at the clock signal frequency. Switching capacitors in and out creates an equivalent resistance. Each filter uses three operational amplifiers. The areas outlined in red correspond to those sections. Three outputs are lined up on the right side. They are high-pass, band-pass, and low-pass outputs. The design allows characteristics to be set with just a few external resistors. It can switch between low-pass, band-pass, and high-pass operation. Spreadsheet array operations are effective for organizing measurements obtained from the analysis, and the techniques explained in How to Use Excel Dynamic Array FILTER, SORT, and UNIQUE with Practical Applications can be applied. Decorative figures were also found on the die. They are the Harris logo and the outline of the state of Florida. Harris was headquartered in Melbourne, Florida. Markings believed to be the designer’s initials also remain.

How Switched-Capacitor Operation Works

Switched-capacitor circuits are a technique for replacing resistors. Two switches close alternately in response to a clock signal. The amount of charge transferred corresponds to current. The equivalent resistance is determined by the capacitance value and clock frequency. This is why the approach is well suited to integration. Forming high-precision resistors is difficult in integrated circuits. Capacitance ratios can be maintained with relatively high accuracy. There is also the advantage of controlling characteristics via the clock frequency. This is the background behind the spread of single-chip filters in the 1980s. They were valued for combining ease of use with performance. They also helped reduce the number of external components. It was a technology that promoted more compact analog signal processing.

Position as an MF10-Compatible Device

The MF10 is known for its versatility. Each filter can operate as low-pass, band-pass, or high-pass. External resistors set the center frequency and selectivity. Harris’s HF-10 is a compatible version of this device. The “F1-10-5” marking includes a 1985 date code. It may also read as “FI-10-5.” In either case, it comes from the same design lineage. At the time, multiple companies supplied compatible parts as second sources. It is an example of inherited design assets. Reevaluating obsolete parts also benefits maintenance and education. It provides clues for considering replacements for discontinued products.

Editorial Opinion

On short-term impact. This analysis is seen as increasing interest in vintage analog ICs. Switched capacitors are expected to become a teaching topic again in circuit education at universities and technical colleges. Inquiries about MF10-series parts in the secondhand market are seen as likely to increase. Stock checks for maintenance use and selection of replacements may move forward. It is considered valuable as reference material in repair work for measuring instruments and audio equipment.

On the long-term perspective. Mixed analog-digital design is seen as continuing in the future. The technique of defining characteristics with a clock is regarded as a prototype of programmable analog. Decorations and initials on the die are seen as records of design culture. Accumulating such analyses is considered to contribute to passing on technology. It is seen as worth preserving as primary material for semiconductor history.

Questions from the editorial team. How should insights from analyzing vintage ICs be applied to current designs. Will reliance on capacitor-ratio accuracy remain effective in the era of miniaturization. How should destructive decapsulation and non-destructive inspection be used appropriately. What information should be recorded from the standpoint of passing on technology. Readers’ hands-on experience is seen as key.

References

• “Inside a 1980s filter chip that uses switched capacitors”, by righto.com via calvin — Lobsters, 2026-10-10T21:28:51.000Z (ARR)
• Source URL: http://www.righto.com/2026/10/ML10-switched-capacitor-filter.html