Building A Digital-To-Analog Converter (Dac) - Arduino Uno Quick Start Manual

Building a Digital-to-Analog Converter (DAC)

The Arduino doesn t natively support analog output signals, so we need to
find a way around this limitation. The solution is a digital-to-analog converter
1
(DAC). As the name suggests, such a circuit turns a digital input into an
analog output. You can find DACs in a lot of consumer devices—for example,
in your MP3 player. It turns your digitally encoded music into an analog
sound wave.
One of the most important characteristics of a DAC is its resolution. In our
case we need to generate four different voltage levels to create a video signal.
To encode four voltage levels, we need two bits—that is, our DAC has a 2-bit
resolution. The following table shows how we could map the four binary input
values to their voltage levels.
Binary input:
00
Output voltage: 0.0V
We can use two of the Arduino s digital pins to control the DAC s input value,
but we still have to find a way to generate different voltages depending on the
pins values.
There are several ways to achieve this, but one of the easiest is by using a
binary-weighted DAC. It has the following characteristics:
• You need a resistor for every input bit.
• All resistors have to be in parallel.
• Resistor value for bit #0 is R. For bit #1 it s 2R, for bit #3 it s 4R, and so
on.
Let s say we use the Arduino s digital pins D7 and D9 to control the DAC s
input value. In the following figure, you can see our DAC s circuit. You have
to add the 470Ω and 1kΩ resistors yourself, but you get the 75Ω resistor for
free, because it s part of your TV set s input connector.
In principle, the binary-weighted DAC is a voltage divider
an input voltage into a smaller voltage. The output voltage depends on the
1. http://en.wikipedia.org/wiki/Digital-to-analog_converter
2. http://en.wikipedia.org/wiki/Voltage_divider
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