Hardware Reference
In-Depth Information
•
Type of output.
The output of a DAC may be in the form of voltage or current to
meet the requirement of the target application.
•
Monotonicity.
This refers to the ability of the DAC's analog output to increase
with digital code.
In this section, we use the 8-bit DAC AD7302 from Analog Devices to illustrate the use of the DAC.
The AD7302 is a dual-channel (two DACs on the same chip), 8-bit, DAC chip from Analog
Devices that has a parallel interface with the microcontroller. The AD7302 converts an 8-bit
digital value into an analog voltage. The block diagram of the AD7302 is shown in Figure 7.35.
The AD7302 is designed to be a memory-mapped device. In order to send data to the AD7302,
the
CS
signal must be pulled to low. On the rising edge of the
WR
signal, the values on pins
D7-D0 will be latched into the Input register. When the signal
LDAC
is low, the data in the Input
register is transferred to the DAC register and a new D/A
conversion is started. The AD7302
needs a reference voltage to perform the D/A conversion. The reference voltage can come from
either the external REFIN input or the internal
V
DD
. The A/B signal selects the channel (A or B)
to perform the D/A conversion. The PD pin puts the AD7302 in power-down mode and reduces
the power consumption to 1
W.
The AD7302 operates from a single 12.7- to 5.5-V supply and typically consumes 15 mW at
5 V, making it suitable for battery-powered applications. Each digital sample takes about 2
μ
μ
s to
convert. The output voltage (
V
OUT
A
or
V
OUT
B
) from either DAC is given by
V
OUT
A/B
5
2
3
V
REF
3
(
N
/256)
where
V
REF
is derived internally from the voltage applied at the REFIN pin or
V
DD
. If the voltage
applied to the REFIN pin is within 1 V of the
V
DD
,
V
DD
/2 is used as the reference voltage auto-
matically. Otherwise, the voltage applied at the REFIN pin is used as the reference voltage. The
range of
V
REF
is from 1 V to
V
DD
/2.
N
is the decimal equivalent of the code loaded to the DAC
register, ranging from 0 to 255.
AD7302
Input
register
DAC
register
I DAC A
V
OUT A
I/V
D7
D0
Input
register
DAC
register
I DAC B
V
OUT B
I/V
A/B
Control
logic
Power-on
reset
AGND
MUX
WR
CS
÷
2
V
DD
REFIN
DGND
PD
CL R
LDAC
Figure 7.35
■
Functional block diagram of the AD7302
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