Page 121 - Embedded Microprocessor Systems Real World Design

If the three switches are treated as a %bit digital word with ON = 1 and
OFF = 0, the output voltage is an analog representation of a digital input. This
three-switch digital-teanalog converter (DAC) has eight possible states, and each
voltage step is VR/8.
We could add more R/2R pairs and switches for more resolution. Four switches
would give us 16 steps of VR/16 volts each. Eight switches would give us 256 steps
of VR/256 volts each. If we replace the mechanical switches in the schematic with
electronic switches, we have a true DAC.

Analog-todigital converters (ADCs) do the exact opposite of DACs-they output a
binary word that is a digital representation of an analog voltage or current. An
&bit ADC converts an input into 256 steps. A 10-bit ADC produces 1024 steps.
DACs and ADCs interface to a microprocessor just like other peripheral ICs.
Parts are available with different bus interface types, including SPI and 1%. While
the microprocessor side of a DAC or an ADC is the same as other parts, there are
some special considerations when dealing with these analog devices, which we’ll
discuss in this section.

Reference Voltage
The reference voltage is the maximum value that the ADC or DAC can convert. An
&bit ADC can convert values from OV to the reference voltage. This voltage range
is divided into 256 values, or steps. The size of the step is given by the following
equation:

5V
Reference Voltage - -- = .0195V, or 19.5mV
256 256

This is the step size of the converter. It also defines the converter’s resolution. Note
that no ADC or DAC can be more accurate than its reference. If your reference is
a zener diode with a 10 percent tolerance, it doesn’t matter how many bits of res-
olution you have, your product will have a 10 percent variation between units unless
you perform some kind of calibration as part of production.
Some microcontrollers have internal ADCs. Many of these permit you to provide
an external reference, or they let you use the supply voltage as the reference. This
typically frees the reference pin for use as another analog input. Microchip 16C7xx
parts have this feature.

Hardware Design 2 103