
REV. B
AD5308/AD5318/AD5328
–11–
FUNCTIONAL DESCRIPTION
The AD5308/AD5318/AD5328 are octal resistor-string DACs
fabricated on a CMOS process with resolutions of 8, 10, and
12 bits, respectively. Each contains eight output buffer amplifiers
and is written to via a 3-wire serial interface. They operate
from single supplies of 2.5 V to 5.5 V, and the output buffer
amplifiers provide rail-to-rail output swing with a slew rate
of 0.7 V/
μ
s. DACs A, B, C, and D share a common reference
input, V
REF
ABCD. DACs E, F, G, and H share a common
reference input, V
REF
EFGH. Each reference input may be
buffered to draw virtually no current from the reference source,
may be unbuffered to give a reference input range from 0.25 V
to V
DD
, or may come from V
DD
. The devices have a power-
down mode in which all DACs may be turned off individually
with a high impedance output.
Digital-to-Analog Section
The architecture of one DAC channel consists of a resistor-
string DAC followed by an output buffer amplifier. The voltage
at the V
REF
pin provides the reference voltage for the corre-
sponding DAC. Figure 4 shows a block diagram of the DAC
architecture. Since the input coding to the DAC is straight
binary, the ideal output voltage is given by
V
V
D
OUT
REF
N
=
×
2
where
D
= decimal equivalent of the binary code that is loaded to the
DAC register:
0–255 for AD5308 (8 bits)
0–1023 for AD5318 (10 bits)
0–4095 for AD5328 (12 bits)
N
= DAC resolution
V
OUT
A
GAIN MODE
(GAIN = 1 OR 2)
V
REF
ABCD
BUF
DAC
REGISTER
INPUT
REGISTER
RESISTOR
STRING
OUTPUT
BUFFER AMPLIFIER
REFERENCE
BUFFER
V
DD
V
DD
Figure 4. Single DAC Channel Architecture
Resistor String
The resistor-string section is shown in Figure 5. It is simply a
string of resistors, each of value R. The digital code loaded to
the DAC register determines at which node on the string the
voltage is tapped off to be fed into the output amplifier. The
voltage is tapped off by closing one of the switches connecting
the string to the amplifier. Because it is a string of resistors, it is
guaranteed monotonic.
TO OUTPUT
AMPLIFIER
R
R
R
R
R
Figure 5. Resistor String
DAC Reference Inputs
There is a reference pin for each quad of DACs. The reference
inputs can be buffered from V
DD
, or unbuffered. The advantage
with the buffered input is the high impedance it presents to the
voltage source driving it. However, if the unbuffered mode is
used, the user can have a reference voltage as low as 0.25 V and
as high as V
DD
since there is no restriction due to the headroom
and footroom of the reference amplifier.
If there is a buffered reference in the circuit (e.g., REF192), there
is no need to use the on-chip buffers of the AD5308/AD5318/
AD5328. In unbuffered mode, the input impedance is still large
at typically 45 k
per reference input for 0 V to V
REF
mode and
22 k
for 0 V to 2 V
REF
mode.
Output Amplifier
The output buffer amplifier is capable of generating output
voltages to within 1 mV of either rail. Its actual range depends
on the value of V
REF
, the gain of the output amplifier, the offset
error, and the gain error.
If a gain of 1 is selected (GAIN bit = 0), the output range is
0.001 V to V
REF
.
If a gain of 2 is selected (GAIN bit = 1), the output range is
0.001 V to 2 V
REF
. Because of clamping, however, the maxi-
mum output is limited to V
DD
– 0.001 V.
The output amplifier is capable of driving a load of 2 k
to
GND or V
DD
, in parallel with 500 pF to GND or V
DD
. The
source and sink capabilities of the output amplifier can be seen
in the plot in TPC 11.
The slew rate is 0.7 V/
μ
s with a half-scale settling time to
±
0.5 LSB
(at eight bits) of 6
μ
s.
POWER-ON RESET
The AD5308/AD5318/AD5328 are provided with a power-on
reset function so that they power up in a defined state. The
power-on state is
Normal operation
Reference inputs unbuffered
0 V to V
REF
output range
Output voltage set to 0 V
LDAC
bits set to
LDAC
high
Both input and DAC registers are filled with zeros and remain
so until a valid write sequence is made to the device. This is
particularly useful in applications where it is important to know
the state of the DAC outputs while the device is powering up.