10 ______________________________________________________________________________________
MAX547
Positive Unipolar Output Voltage Range
(AGND_ = REF_/2)
For positive unipolar output operation, set AGND_ to
(REF_/2). For example, if you use Figure 4’s circuit with,
a 4.096V reference and offset AGND_ by 2.048V with
matched resistors (R1 = R2) and an op amp, it results in
a 0V to 4.0955V (nominal) unipolar output voltage,
where 1LSB = 500µV. In general, the maximum current
flowing out of any AGND_ pin is given by:
Customizing the Output Voltage Range
The AGND_ inputs can be offset by any voltage within the
supply rails if the voltage at the referring REF_ input is
higher than the voltage at the AGND_ input. Select the
reference voltage and the voltage at AGND_ so the
resulting output voltages do not come within ±0.6V of the
supply rails. Figure 4’s circuit shows one way to add posi-
tive offset to AGND_; make sure that the op amp used
has sufficient current-sink capability to take up the
remaining AGND_ current:
Another way is to digitally offset AGND_ by connecting
the output of one DAC to one or more AGND_ inputs. Do
not connect a DAC output to its own AGND_ input.
Table 5 summarizes the relationship between the refer-
ence and AGND_ potentials and the output voltage in
the different modes of operation.
Power-Supply Sequencing
The sequence in which the supply voltages come up is
not critical. However, we recommend that on power-up,
V
SS
comes up first, V
DD
next, followed by the reference
voltages. If you use other sequences, limit the current
into any reference pin to 10mA. Also, make sure that
V
SS
is never more than 300mV above ground. If there is
a risk that this can occur at power-up, connect a
Schottky diode between V
SS
and GND, as shown in
Figure 5. We recommend that you not power up the
logic input pins before establishing the supply volt-
ages. If this is not possible and the digital lines can
drive more than 10mA, you should place current-limit-
ing resistors (e.g., 470Ω) in series with the logic pins.
Reference Selection
If you want a ±2.5V full-scale output voltage swing, you
can use the MAX873 reference. It operates from a sin-
gle 5V supply and is specified to drive up to 10mA.
Therefore, it can drive all four reference inputs simulta-
neously. Because the maximum load impedance can
vary from 1.25kΩ to 12.5kΩ (four reference inputs in
parallel), the reference load current ranges from 2mA to
0.2mA (1.8mA maximum load step). The MAX873’s
I
REF_ AGND_
5k
AGND_ =
−
Ω
I
REF_ AGND_
5k
AGND_ =
−
Ω
Octal, 13-Bit Voltage-Output
DAC with Parallel Interface
OUTPUTINPUT
4095
+REF_
(
———
)
4096
1 1111 1111 1111
0V1 0000 0000 0000
1
+REF_
(
———
)
4096
1 0000 0000 0001
1
-REF_
(
———
)
4096
0 1111 1111 1111
4095
-REF_
(
———
)
4096
0 0000 0000 0001
-REF_0 0000 0000 0000
+5V
1µF
REFAB
AGNDAB
1µF
R1
REF
R2
V
DD
V
DD
DIGITAL INPUTS NOT SHOWN.
NOT ALL DACS SHOWN.
1µF
VOUTA
VOUTB
DAC B
DAC A
MAX547
-5V
V
SS
V
SS
1µF1µF
+REF
–
/21 0000 0000 0000
OUTPUT
0V0 0000 0000 0000
INPUT
8191
+REF_
(
———
)
8192
1 1111 1111 1111
Table 4. MAX547 Positive Unipolar Code Table
(AGND_ =
REF
_
)
2
Table 3. MAX547 Bipolar Code Table
(AGND_ = 0V)
Figure 4. Offsetting AGND
–
