ad7394an Analog Devices, Inc., ad7394an Datasheet - Page 11

• Manufacturer Part Number: ad7394an
• Description: +3 V, Dual, Serial Input 12-/10-bit Dacs
• Manufacturer: Analog Devices, Inc.

REV. 0
generated by the microprocessor’s power-on RESET signal, by
an output from the microprocessor, or by an external resistor
and capacitor. RESET has a Schmitt trigger input which results
in a clean reset function when using external resistor/capacitor
generated pulses. See the Control-Logic Truth Table I.
POWER SHUTDOWN (SHDN)
Maximum power savings can be achieved by using the power
shutdown control function. This hardware activated feature is
controlled by the active low input SHDN pin. This pin has a
Schmitt trigger input which helps to desensitize it to slowly
changing inputs. By placing a logic low on this pin the internal
consumption of the device is reduced to nano amp levels, guar-
anteed to 1.5 A maximum over the operating temperature
range. When the AD7394/AD7395 has been programmed into
the power shutdown state, the present DAC register data is
maintained as long as V
wake-up command SHDN = 1 is given, the DAC voltage out-
puts will return to their previous values. It typically takes
80 microseconds for the output voltage to fully stabilize. In the
shutdown state the DAC output amplifier exhibits an open-
circuit with a nominal output resistance of 500 k to ground. If
the power shutdown feature is not needed, then the user should
tie the SHDN pin to the V
function.
UNIPOLAR OUTPUT OPERATION
This is the basic mode of operation for the AD7394. As shown
in Figure 24, the AD7394 has been designed to drive loads as
low as 5 k in parallel with 100 pF. The code table for this
operation is shown in Table V.
D11–D0: 12-bit AD7394 DAC data; D9–D0: 10-bit AD7395 DAC data; X = Don’t Care; The MSB of byte 1 is the first bit that is loaded into the SDI input.
MSB
B15
X
X
EXT
REF
0.01 F
Figure 24. AD7394 Unipolar Output Operation
B14
X
X
C
B13
X
X
5
R
V
DIGITAL
REF
DD
DGND
+2.7V TO +5.5V
B12
X
X
DD
remains greater than 2.7 V. Once a
BYTE 1
V
voltage thereby disabling this
DD
DAC A
DAC B
AGND
B11
D11
X
Table IV. Typical Microcontroller Interface Formats
DIGITAL INTERFACE
CIRCUITRY OMITTED
FOR CLARITY.
75k
75k
B10
D10
X
100pF
100pF
0.1 F
B9
D9
D9
V
V
10 F
OUTA
OUTB
LSB
B8
D8
D8
–11–
B7
MSB
D7
D7
Hexadecimal
Number
in DAC Register
FFF
801
800
7FF
000
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 AD7395 has been designed for single-supply op-
eration, the output can easily be configured for bipolar opera-
tion. A typical circuit is shown in Figure 25. This circuit uses a
clean regulated +5 V supply for power, which also provides the
circuit’s reference voltage. Since the AD7395 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. Connec-
tion of the equally valued 470 k resistors results in a differen-
tial amplifier mode of operation with a voltage gain of two,
which produces a circuit output span of ten volts, that is, –5 V to
+5 V. As the DAC is programmed from zero code 000
scale 200
set at –5 V, 0 V and +5 V (–1 LSB). The output voltage V
coded in offset binary according to Equation 4.
where D is the decimal code loaded in the AD7395 DAC regis-
ter. Note that the LSB step size is 10/1024 = 10 mV. This cir-
cuit has been optimized for micropower consumption including
the 470 k gain setting resistors, which should have low tem-
perature coefficients to maintain accuracy and matching (prefer-
ably the same resistor 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 262 microamps of current,
and still provide additional power for the load connected to
V
OUT
V
. The micropower REF195 is guaranteed to source 10 mA
OUT
B6
D6
D6
H
to full-scale 3FF
512
D
B5
D5
D5
Table V. Unipolar Code Table
–1
Decimal
Number
in DAC Register
4095
2049
2048
2047
0
B4
D4
D4
BYTE 0
5
H
, the circuit output voltage V
B3
D3
D3
AD7394/AD7395
B2
D2
D2
Output
Voltage (V)
[V
2.4994
1.2506
1.2500
1.2494
0
REF
B1
D1
D1
= 2.5 V]
H
to mid-
O
O
LSB
B0
D0
D0
is
is
(4)