Datasheet

AD7302
–11–
REV. 0
␣␣V
REF
is the voltage applied to the external REFIN pin when
the external reference is selected and is V
DD
/2 if the
internal reference is used.
Table I. Output Voltage for Selected Input Codes
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
00 01
DAC INPUT
CODE
FF80 81 FE7F
0
2.V
REF
V
REF
DAC OUTPUT VOLTAGE
Figure 26. DAC Transfer Function
Figure 27 shows a typical setup for the AD7302 when using its
internal reference. The internal reference is selected by tying the
REFIN pin to V
DD
. Internally in the reference section there is a
reference detect circuit that will select the internal V
DD
/2 based
on the voltage connected to the REFIN pin. If REFIN is within
a threshold voltage of a PMOS device (approximately 1 V) of
V
DD
the internal reference is selected. When the REFIN voltage
is more than 1 V below V
DD
, the externally applied voltage at
this pin is used as the reference for the DAC. The internal
reference on the AD7302 is V
DD
/2, the output current to voltage
converter within the AD7302 provides a gain of two. Thus the
output range of the DAC is from 0 V to V
DD
, based on Table I.
AD7302
V
OUT
A
V
OUT
B
10µF
0.1µF
V
DD
= 3 TO 5V
V
DD
A/B
REF IN
CLR
PD
V
DD
AGND DGND
CS WR LDAC
D7–D0
V
OUT
A
V
OUT
B
DATA BUS CONTROL INPUTS
Figure 27. Typical Configuration Selecting the Internal
Reference
Figure 28 shows a typical setup for the AD7302 when using an
external reference. The reference range for the AD7302 is from
1 V to V
DD
/2 V. Higher values of reference can be incorporated,
but will saturate the output at both the top and bottom end of
the transfer function. There is a gain of two from input to output
on the AD7302. Suitable references for 5 V operation are the
AD780 and REF192. For 3 V operation a suitable external
reference would be the AD589 a 1.23 V bandgap reference.
AD7302
V
OUT
A
V
OUT
B
10µF
0.1µF
V
DD
= 3 TO 5V
V
DD
AD780/REF192
WITH V
DD
= 5V
OR
AD589 WITH V
DD
= 3V
REF IN
GND
V
OUT
V
IN
0.1µF
EXT REF
V
DD
AGND DGND
D7–D0
V
OUT
A
V
OUT
B
DATA BUS CONTROL INPUTS
A/B
CLR
PD
CS WR LDAC
Figure 28. Typical Configuration Using An External
Reference