Human Machine Interface
ASICs: the touching point between man and machine
Thanks to today’s touch-screen technology, interaction with a computer can be as instinctive as a conversation between two humans. Most people use touch-screens daily, but few stop to wonder about the electronics that makes them possible. Here, Dr Mike Coulson, analogue design manager at Swindon Silicon Systems, discusses how Application Specific Integrated Circuits (ASICs) play an integral role in uniting man and machine
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n the last 15 years, the world has witnessed a touchscreen revolution. Touchscreens now feature extensively in our daily lives, not least within our mobile devices, but also in retail, healthcare and industrial settings. More niche applications call for specialised touchscreens that are sufficiently sensitive to detect a human finger through thick glass, through gloves, or even at a distance above the screen’s surface. This Human-Machine Interface (HMI)
32 May 2022
technology is still evolving, driven largely by advances in the supporting electronics and signal processing.
Electronic challenges
Almost all modern touchscreens use the ‘projected capacitance’ technique, where a matrix of invisible conductors is embedded within the display. At each crossing point, the conductors form coupling capacitances between one another, at least partially through the air above the display.
Components in Electronics
A finger in close proximity reduces this coupling, albeit by a tiny fraction. The job of the touchscreen controller is to measure the numerous capacitances, usually by driving each wire in turn with a voltage stimulus and thus pushing vanishingly small currents through them. By measuring the many tiny currents simultaneously, and at fantastic resolution, it is possible to perceive the effect of any touches and calculate where they occur. Upon entering the controller, each
wire’s signal is first passed through filtering components to reject interference — perhaps unsurprisingly, given that each wire resembles a long antenna. The signal is then amplified and sampled before being digitised by an analogue-to-digital converter (ADC). The replication of all this componentry, to serve the numerous wires in the panel, represents a significant challenge to the cost and form factor of the electronics.
Because the changes in capacitance are
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