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 operation, 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 microamps 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 200H to full-scale
3FF
H
, the circuit output voltage VO 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 coefficients 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
ISY < 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 communicating 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.