REV. A
AD7390/AD7391
–10–
UNIPOLAR OUTPUT OPERATION
This is the basic mode of operation for the AD7390. As shown
in Figure 10, the AD7390 has been designed to drive loads as
low as 5 kΩ in parallel with 100 pF. The code table for this
operation is shown in Table IV.
AD7390
0.1F
CLK
V
OUT
REF
V
DD
GND
R
10F
6
7
5
1
2
3
4
SDI
CLR
LD
2.7V TO 5.5V
R
L
5k
C
L
100pF
C
RS
EXT
REF
0.01F
Figure 10. AD7390 Unipolar Output Operation
Table IV. AD7390 Unipolar Code Table
Hexadecimal Decimal Output
Number Number Voltage (V)
in DAC Register in DAC Register V
REF
= 2.5 V
FFF 4095 2.4994
801 2049 1.2506
800 2048 1.2500
7FF 2047 1.2494
000 0 0
The circuit can be configured with an external reference plus
power supply, or powered from a single dedicated regulator or
reference, depending on the application performance requirements.
BIPOLAR OUTPUT OPERATION
Although the AD7391 has been designed for single-supply opera-
tion, the output can be easily configured for bipolar operation.
A typical circuit is shown in Figure 11. This circuit uses a clean
regulated 5 V supply for power, which also provides the circuit’s
reference voltage. Since the AD7391 output span swings from
ground to very near 5 V, it is necessary to choose an external
amplifier with a common-mode input voltage range that extends
to its positive supply rail. The micropower consumption OP196 has
been designed just for this purpose and results in only 50 micro-
amps of maximum current consumption. Connection of the equally
valued 470 kΩ resistors results in a differential amplifier mode
of operation with a voltage gain of two, which results in a circuit
output span of ten volts, that is, 25 V to 15 V. As the DAC is
programmed with zero-code 000
H
to midscale 200
H
to full-scale
3FF
H
, the circuit output voltage V
O
is set at 25 V, 0 V and 15 V
(minus 1 LSB). The output voltage V
O
is coded in offset binary
according to Equation 4.
V
D
O
=
×
512
15–
(4)
where D is the decimal code loaded in the AD7391 DAC register.
Note that the LSB step size is 10/1024 = 10 mV. This circuit has
been optimized for micropower consumption including the 470 kΩ
gain setting resistors, which should have low temperature coeffi-
cients to maintain accuracy and matching (preferably the same
material, such as metal film). If better stability is required, the power
supply could be substituted with a precision reference voltage such
as the low dropout REF195, which can easily supply the circuit’s
162 µA of current, and still provide additional power for the
load connected to V
O
. The micropower REF195 is guaranteed
to source 10 mA output drive current, but only consumes 50 µA
internally. If higher resolution is required, the AD7390 can be
used with the addition of two more bits of data inserted into the
software coding, which would result in a 2.5 mV LSB step size.
Table V shows examples of nominal output voltages V
O
provided
by the Bipolar Operation circuit application.
C
I
SY
< 162A
BIPOLAR
OUTPUT
SWING
V
O
+5V
–5V
–5V
V
OUT
AD7391
V
DD
REF
GND
+5V
< 100A
470k 470k
< 50A
OP196
DIGITAL INTERFACE CIRCUITRY OMITTED FOR CLARITY
Figure 11. Bipolar Output Operation
Table V. Bipolar Code Table
Hexadecimal Decimal Analog
Number Number Output
in DAC Register in DAC Register Voltage (V)
3FF 1023 4.9902
201 513 0.0097
200 512 0.0000
1FF 511 –0.0097
000 0 –5.0000
MICROCOMPUTER INTERFACES
The AD7390 serial data input provides an easy interface to a
variety of single-chip microcomputers (µCs). Many µCs have a
built-in serial data capability which can be used for communi-
cating with the DAC. In cases where no serial port is provided,
or it is being used for some other purpose (such as an RS-232
communications interface), the AD7390/AD7391 can easily be
addressed in software.
Twelve data bits are required to load a value into the AD7390.
If more than 12 bits are transmitted before the load LD input
goes high, the extra (i.e., the most-significant) bits are ignored.
This feature is valuable because most µCs only transmit data
in 8-bit increments. Thus, the µC sends 16 bits to the DAC
instead of 12 bits. The AD7390 will only respond to the last
12 bits clocked into the SDI input, however, so the serial-data
interface is not affected.
Ten data bits are required to load a value into the AD7391. If
more than 10 bits are transmitted before load LD returns high,
the extra bits are ignored.
