Inside A Thriller Eighties Filter IC That includes Switched Capacitors

Recently [Ken Shirriff] obtained handed a thriller IC by [CuriousMarc] who had discovered stated IC in a field of assorted previous chips. Labelled solely with a date code of the forty seventh week of 1985 and a mysterious F1-10-5 alongside a emblem that didn’t result in any hits within the typical IC databases, it was as much as [Ken] to figure out what lurks inside this CDIP chip.

As it seems, it’s really a reasonably attention-grabbing IC with a straightforward to determine die as soon as the ceramic package deal had been cracked open with a surgical hammer-and-chisel strike.

With the die beneath a microscope it was recognized as a Harris HF10, itself a variant of the National Semiconductor MF10. This is a twin switched capacitor filter, with the fashionable model being e.g. the TI MF10-N in each PDIP and SOIC, however sadly no CDIP any extra. Availability appears scarce, so the MF10 will not be lengthy for this international stage.

These filter ICs can tune their capacitors to behave as a low-, band- or high-pass filter. How this works is defined by [Ken], with switched capacitors changing the resistors of a typical filter circuit. The important causes for doing this over simply utilizing a resistor embrace how giant resistors can get in comparison with the comparatively small capacitor, to not point out their mounted worth versus the adjustable resistance from the switchable capacitor.

The the rest of the circuit is usually there to allow the switching and general operation, offering an interesting glimpse at a chip that will have been extremely helpful within the Eighties, together with in synthesizers the place this adjustable filter function was most handy.