Specifications

Chapter 6 - Samples

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12-bit Analog to Digital converter

Since everything in the microcontroller world is represented with "0's" and "1's", how do we cater

for a signal that is 0.5 or 0.77?

Most of the world outside a computer consists of analogue signals. Apart from speech and music,

there are many quantities that need to be fed into a computer. Humidity, temperature, air

pressure, colour, turbidity, and methane levels, are just a few.

The answer is to take a number of digital lines and combine them so they can "read" an analogue

value. An analogue value is any value between 0 and 1. You can also call it a "fractional value."

All the above quantities must now be converted to a value between 0 and 1 so they can be fed

into a computer.

This is the broad concept. It becomes a little more complex in application.

If we take 8 lines and arrange than so they accept binary values, the total count will be 256 (this

is obtained by a count to 255 plus the value 0).

If we connect these 8 lines into a "black box," they will be called output lines and so we must

provide a single input line. With this arrangement we can detect up to 255 increments between

"0" and "1." This black box is called a CONVERTER and since we are converting from Analogue to

Digital, the converter is called an A-to-D converter or ADC.

AD converters can be classified according to different parameters. The most important

parameters are precision and mode of data transfer. As to precision, the range is: 8-bit, 10-

bit, 12-bit, 14-bit, 16-bit. Since 12-bit conversion is an industrial standard, the example we have

provided below was done with a 12-bit ADC. The other important parameter is the way data is

transferred to a microcontroller. It can be parallel or serial. Parallel transmission is faster.

However, these converters are usually more expensive. Serial transmission is slower, but in terms

of cost and fewer input lines to a microcontroller, it is the favourite for many applications.

Analogue signals can sometimes go above the allowed input limit of an ADC. This may damage the

converter. To protect the input, two diodes are connected as shown in the diagram. This will

protect from voltages above 5V and below 0V.

In our example we used a LTC1286 12-bit ADC (Linear Technology). The converter is connected to

the microcontroller via three lines: data, clock and CS (Chip Select). The CS line is used to select

an input device as it is possible to connect other input devices (eg: input shift register, output shift

The circuit below shows how to connect an ADC, reference and LCD display to a micro. The LCD

display has been added to show the result of the AD conversion.

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