AD7394/AD7395
–10–
REV. 0
POWER SUPPLY
The very low power consumption of the AD7394/AD7395 is a
direct result of a circuit design optimizing the use of a CBCMOS
process. By using the low power characteristics of CMOS for
the logic, and the low noise, tight matching of the complemen-
tary bipolar transistors, excellent analog accuracy is achieved.
One advantage of the rail-to-rail output amplifiers used in the
AD7394/AD7395 is the wide range of usable supply voltage.
The part is fully specified and tested for operation from +2.7 V
to +5.5 V.
POWER SUPPLY BYPASSING AND GROUNDING
Local supply bypassing consisting of a 10 µF tantalum electro-
lytic in parallel with a 0.1 µF ceramic capacitor is recommended
in all applications (Figure 21).
C
*
0.1mF
10mF
AD7394
OR
AD7395
CS
CLK
LDA, B
RS
SDI
DGND
V
OUTB
V
OUTA
*OPTIONAL EXTERNAL
REFERENCE BYPASS
REF V
DD
AGND
+2.7V TO +5.5V
Figure 21. Recommended Supply Bypassing for the
AD7394/AD7395
INPUT LOGIC LEVELS
All digital inputs are protected with a Zener-type ESD protec-
tion structure (Figure 22) that allows logic input voltages to
exceed the V
DD
supply voltage. This feature can be useful if the
user is driving one or more of the digital inputs with a 5 V CMOS
logic input-voltage level while operating the AD7394/AD7395
on a +3 V power supply. If this mode of interface is used, make
sure that the V
OL
of the 5 V CMOS meets the V
IL
input re-
quirement of the AD7394/AD7395 operating at 3 V. See Figure
12 for a graph of digital logic input threshold versus operating
V
DD
supply voltage.
V
DD
LOGIC
IN
GND
Figure 22. Equivalent Digital Input ESD Protection
In order to minimize power dissipation from input logic levels
that are near the V
IH
and V
IL
logic input voltage specifications,
a Schmitt trigger design was used that minimizes the input-
buffer current consumption compared to traditional CMOS
input stages. Figure 11 is a plot of incremental input voltage
versus supply current showing that negligible current consump-
tion takes place when logic levels are in their quiescent state.
The normal crossover current still occurs during logic transi-
tions. A secondary advantage of this Schmitt trigger is the pre-
vention of false triggers that would occur with slow moving
logic transitions when a standard CMOS logic interface or opto
isolators are used. The logic inputs SDI, CLK, CS, LDA, LDB,
RS, SHDN all contain the Schmitt trigger circuits.
DAC B REGISTER
DPR
CS
CLK
SHIFT
REGISTER
Q
DAC A REGISTER
DPR
LDA LDB RS
MSB
SDI
EN
Figure 23. Equivalent Digital Interface Logic
DIGITAL INTERFACE
The AD7394/AD7395 has a serial data input. A functional
block diagram of the digital section is shown in Figure 23, while
Table I contains the truth table for the logic control inputs.
Three pins control the serial data input register loading. Two
additional pins determine which DAC will receive the data
loaded into the input shift register. Data at the SDI is clocked
into the shift register on the rising edge of the CLK. Data is
entered in the MSB-first format. The active low chip select (CS)
pin enables loading of data into the shift register from the SDI
pin. Twelve clock pulses are required to load the 12-bit AD7390
DAC shift register. If additional bits are clocked into the shift
register, for example, when a microcontroller sends two 8-bit
bytes, the MSBs are ignored (Table IV). The lowest resolution
AD7395 is also loaded MSB-first with 10 bits of data. Again, if
additional bits are clocked into the shift register only the last 10
bits clocked in are used. When CS returns to logic high, shift-
register loading is disabled. The load pins LDA and LDB con-
trol the flow of data from the shift register to the DAC register.
After a new value is clocked into the serial-input register, it will
be transferred to the DAC register associated with its LDA or
LDB logic control line. Note, if the user wants to load both
DAC registers with the current contents of the shift register,
both control lines LDA and LDB should be strobed together.
The LDA and LDB pins are level-sensitive and should be re-
turned to logic high prior to any new data being sent to the
input shift register to avoid changing the DAC register values.
See Truth Table for complete set of conditions.
RESET (RS) PIN
Forcing the asynchronous RS pin low will set the DAC register
to all zeros, or midscale, depending on the logic level applied to
the MSB pin. When the MSB pin is set to logic high, both DAC
registers will be reset to midscale (i.e., the DAC Register’s MSB
bit will be set to Logic 1 followed by all zeros). The reset func-
tion is useful for setting the DAC outputs to zero at power-up or
after a power supply interruption. Test systems and motor
controllers are two of many applications that benefit from
powering up to a known state. The external reset pulse can be
