Page 100 - The Art and Science of Analog Circuit Design

Steve Roach

are not involved in the high-frequency signal path, they too can be built
with slow, high-voltage semiconductors.
The complete circuit is now too involved to show in one piece on the
page of a book, so please use your imagination. We have eliminated all
electromechanical switches and have a solid-state oscilloscope front-
end. Although I had a great deal of fun inventing this circuit, I do not
think it points the direction to future oscilloscope front-ends. Already
research is under way on microscopic relays built with semiconductor
micro-machining techniques (Hackett 1991). These relays are built on
the surface of silicon or gallium arsenide wafers, using photolithography
techniques, and measure only 0.5mm in their largest dimension. The
contacts open only a few microns, but they maintain high breakdown
voltages (100s of volts) because the breakdown voltages of neutral gases
are highly nonlinear and not even monotonic for extremely small spac-
ing. The contacts are so small that the inter-contact capacitance in the
open state is only a few femtofarads (a femtofarad is 0.001 picofarads).
Thus the isolation of the relays is extraordinary! Perhaps best of all, they
are electrostatically actuated and consume near zero power. I believe
micro-machined relays are a revolution in the wings for oscilloscope
front-ends, I eagerly anticipate that they will dramatically improve the
performance of analog switches in many applications. Apparently, even
a device as old as the electromechanical relay is still fertile ground for
a few ambitious inventors!

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