AD7303
–8–
REV. 0
GENERAL DESCRIPTION
D/A Section
The AD7303 is a dual 8-bit voltage output digital-to-analog
converter. The architecture consists of a reference amplifier and
a current source DAC, followed by a current-to-voltage con-
verter capable of generating rail-to-rail voltages on the output of
the DAC. Figure 20 shows a block diagram of the basic DAC
architecture.
REFERENCE
AMPLIFIER
OUTPUT
AMPLIFIER
V
O
A/B
V
DD
REF
AD7303
CURRENT
DAC
11.7kΩ
30kΩ
11.7kΩ
30kΩ
Figure 20. DAC Architecture
Both DAC A and DAC B outputs are internally buffered and
these output buffer amplifiers have rail-to-rail output character-
istics. The output amplifier is capable of driving a load of 10 kΩ
to both V
DD
and ground and 100 pF to ground. The reference
selection for the DAC can be either internally generated from
V
DD
or externally applied through the REF pin. Reference
selection is via a bit in the control register. The range on the
external reference input is from 1.0 V to V
DD
/2. The output
voltage from either DAC is given by:
V
O
A/B = 2 × V
REF
× (N/256)
where:
V
REF
is the voltage applied to the external REF pin or
V
DD
/2 when the internal reference is selected.
N is the decimal equivalent of the code loaded to the DAC
register and ranges from 0 to 255.
Reference
The AD7303 has the facility to use either an external reference
applied through the REF pin or an internal reference generated
from V
DD
. Figure 21 shows the reference input arrangement
where the internal V
DD
/2 has been selected.
30kΩ
30kΩ
REFERENCE
AMPLIFIER
AD7303
REF
0.1µF
V
DD
INT/EXT
Figure 21. Reference Input
When the internal reference is selected during the write to the
DAC, both switches are closed and V
DD
/2 is generated and
applied to the reference amplifier. This internal V
DD
/2 reference
appears at the reference pin as an output voltage for decoupling
purposes. When using the internal reference, external references
should not be connected to the REF Pin. This internal V
DD
/2
reference appears at the reference pin as an output voltage for
decoupling purposes. When using the internal reference, external
references should not be connected to the REF pin. If external ref-
erence is selected, both switches are open and the externally
applied voltage to the REF pin is applied to the reference amplifier.
Decoupling capacitors applied to the REF pin decouple both
the internal reference and external reference. In noisy environ-
ments it is recommended that a 0.1 µF capacitor be connected
to the REF pin to provide added decoupling even when the in-
ternal reference is selected.
Analog Outputs
The AD7303 contains two independent voltage output DACs
with 8-bit resolution and rail-to-rail operation. The output buffer
provides a gain of two at the output. Figures 3 to 5 show the sink
and source capabilities of the output amplifier. The slew rate of the
output amplifier is typically 8 V/µs and has a full-scale settling to 8
bits with a 100 pF capacitive load in typically 1.2 µs.
The input coding to the DAC is straight binary. Table I shows
the binary transfer function for the AD7303. Figure 22 shows
the DAC transfer function for binary coding. Any DAC output
voltage can ideally be expressed as:
V
OUT
= 2 × V
REF
(N/256)
where:
N is the decimal equivalent of the binary input code.
N ranges from 0 to 255.
V
REF
is the voltage applied to the external REF pin when
the external reference is selected and is V
DD
/2 if the
internal reference is used.
Table I. Binary Code Table for AD7303 DAC
Digital Input
MSB . . . LSB Analog Output
1111 1111 2 × 255/256 × V
REF
V
1111 1110 2 × 254/256 × V
REF
V
1000 0001 2 × 129/256 × V
REF
V
1000 0000 V
REF
V
0111 1111 2 × 127/256 × V
REF
V
0000 0001 2 × V
REF
/256 V
0000 0000 0 V
2.V
REF
V
REF
0
DAC OUTPUT VOLTAGE
00 01
DAC INPUT
CODE
FF80 81 FE7F
Figure 22. DAC Transfer Function
