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ANALOG SYSTEM DESIGN

by George Martin

Start ı Considerations ı Transducer Interfaces ı Thermocouples ı ADCs ı Almost Thereı ı Sources and PDF

A man walks into an electronics store and asks the clerk for a new computer game. He wants one thatıs filled with lots of graphics, is challenging, and full of twists and turns. The clerk pauses for a moment and suggests Windows98! Iım the proud owner of WIN98 SE, so I believe I qualify to poke fun at Microsoftıs expense. In the last few weeks, Microsoft stopped selling Windows95. Itıs time to move on.

This month, instead of going down a software or digital path, Iıd like to talk about how I recently consulted on an analog design project. A customer called and told me he was putting together a custom car. Itıs actually a rally car for the street. If you ever come upon a Subaru with a rear wing, be careful, one in the country has about 300 hp and can fly. The car story is long, but his request has a lot of application.

He is a highly qualified software and computer engineer and would like to add a panel that displays all of the sensor readings. He starts asking about thermocouples and how to get them into the ADC. This is a good question but there are so many options to consider before I get into that.

So, letıs take a step back for a moment. Any analog system will contain transducers that represent a real-world values such as temperature or oil pressure as voltage, current, or perhaps resistance. Easy-to-use transducers take no input power and have their outputs scaled to a value that can be connected to an ADC (i.e., an oil pressure transducer with 0 to 120 psi as 0 to 10 V). These transducers are convenient to design with but probably expensive.

What you are more likely to find is a transducer that has resistance output. The resistance is 1000 ohms at 0 psi and nonlinearly changes to 900 ohms at 120 psi. Or, a thermocouple that makes 0 V at freezing and 40 ıV/ıC.

When non-analog designers come up with inputs such as these, they are stumped. And, I usually find that they take an over-complicated path to a solution. So, letıs try to layout a framework for analog system design.

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