A
Digital to
Analog Converter(DAC) is a device for
converting a digital
system's
binary data to an analog
signal. They use a resistor network, usually a R/2R network:
Schematic buildup of a R/2R DAC
R R R
___ ___ ___
URef --------*--___-----___-----___---------.
| | | | |
| |2R | |2R | |2R | |2R | | 2R
|_| |_| |_| |_| | |
| | | | |
| | | | --- GND
| | | |
O O O O ___
/: /: /: /: .-___--.
| : | | : | | : | | : | | R |
---: | ---: | ---: | ---: | | |
: | : | : | : | | |\ | ANALOG
: `-------*-------*-------*--*-|-\__*___OUTPUT
: : : : .-|+/
MSB : : LSB | |/
|DB3_____DB2 _DB1_____DB0| |
\ / ---
\/
DIGITAL INPUT
O |\ ___
/: digital switch -|-\_ OPAMP -___- resistor
| : | -|+/
|/
The
resistor network is called R/2R since it consists of
resistors with resistance R and resistors with double of this
resistance(2 x R, shortened to 2R). This acts as a voltage
divider, dividing the voltage for each R/2R step. The switches is
ofcourse not mechanical relays, but
transistor circuits. The
OPAMP is used as an inverting voltage-follower, This means that
the output will have the same voltage as the input, but with the
opposite polarity.
Example:
If the
reference voltage(URef)= +10V, switching on only the
MSB(Most Significant Bit) will give an output from the OPAMP of
-5V, Only DB2 will give -2.5V, DB1=- 1.25, DB0=-0.625. If we
switch on two or more bits their
values will be added,
DB3+DB0=-5.625V. This means we never get the full -URef and the
minimum
resolution is Uref/16(with four bits you can have 16
different combinations), but if we add more steps/bits we can get
nearer, and control the output more accuratly.
Some DACs can use an AC source as a reference, and is used as for
example a digital volume control. This DAC type is called Four-
quadrant multiplying DAC.
There also exists DACs that can
convert the input value to a
voltage. An example of this type is the AD557, that will convert
a 8-bit digital signal to a output with 0.01V per step, giving a
controlled area form 0V to 2.55V.
The third type of DAC is similar to the voltage-output DAC but
has a current output.
DACs are used in many modern devices like
CD players,
computer sound chipsets and
graphic adapters.