AD7394 Analog Devices, AD7394 Datasheet - Page 12

• Manufacturer Part Number: AD7394
• Description: +3/+5V Dual, Serial-Input 12-Bit DAC
• Manufacturer: Analog Devices

Specifications of AD7394

• Resolution (bits): 12bit
• Dac Update Rate: 17kSPS
• Dac Settling Time: 60µs
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Voltage Out
• Dac Input Format: Ser

AD7394
THEORY OF OPERATION
The AD7394 is a pin compatible, dual, 12-bit digital-to-analog
converter. This single-supply operation device consumes less
than 200 microamps of current while operating from power
supplies in the 2.7 V to 5.5 V range, making it ideal for battery-
operated applications. The AD7394 contains a voltage-switched,
12-bit, laser trimmed digital-to-analog converter, rail-to-rail
output op amps, two DAC registers, and a serial input shift
register. The external reference input has constant input
resistance independent of the digital code setting of the DAC.
In addition, the reference input can be tied to the same supply
voltage as V
0 to V
(SDI), clock (CLK), a chip select pin ( CS ), and two load DAC
Register pins ( LDA and LDB ). A reset ( RS ) pin is available to
reset the DAC register to zero scale or midscale, depending on
the digital level applied to the MSB pin. This function is useful
for power-on reset or system failure recovery to a known state.
Additional power savings are accomplished by activating the
SHDN pin resulting in a 1.5 mA maximum consumption sleep
mode.
DAC SECTION
The voltage switched R-2R DAC generates an output voltage
dependent on the external reference voltage connected to the
REF pin according to the following equation:
where:
D is the decimal data word loaded into the DAC register.
N is the number of bits of DAC resolution.
For the 12-bit AD7394 operating from a 5.0 V reference
Equation 1 becomes:
Using Equation 3 the AD7394 provides a nominal midscale
voltage of 2.50 V for D = 2048, and a full-scale output of 4.998 V.
The LSB step size is = 5.0 × 1/4096 = 0.0012 V.
V
V
DD
OUT
OUT
. The serial interface consists of a serial data input
=
=
DD
V
5
, resulting in a maximum output voltage span of
0 .
4096
REF
2
×
N
×
D
D
Rev. A | Page 12 of 16
(1)
(3)
AMPLIFIER SECTION
The internal DAC’s output is buffered by a low power
consumption precision amplifier. The op amp has a 60 μs
typical settling time to 0.1% of full scale. There are slight
differences in settling time for negative slewing signals vs.
positive. Also, negative transition settling time to within the
last 6 LSBs of zero volts has an extended settling time. The
rail-to-rail output stage of this amplifier has been designed
to provide precision performance while operating near either
power supply. Figure 19 shows an equivalent output schematic
of the rail-to-rail-amplifier with its N-channel pull-down FETs
that will pull an output load directly to GND. The output
sourcing current is provided by a P-channel pull-up device
that can source current to GND terminated loads.
The rail-to-rail output stage provides more than ±1 mA of
output current. The N-channel output pull-down MOSFET
shown in Figure 19 has a 35 Ω on resistance, which sets the
sink current capability near ground. In addition to resistive
load driving capability, the amplifier has also been carefully
designed and characterized for up to 100 pF capacitive load
driving capability.
REFERENCE INPUT
The reference input terminal has a constant input resistance
independent of digital code, which results in reduced glitches
on the external reference voltage source. The high 2.5 MΩ
input resistance minimizes power dissipation within the
AD7394 DAC. The V
from ground to the positive supply voltage V
simplest applications, which saves an external reference voltage
source, is connection of the V
supply. This connection results in a rail-to-rail voltage output
span maximizing the programmed range. The reference input
accepts ac signals as long as they are kept within the supply
voltage range, 0 < V
integral nonlinearity error performance are plotted in Figure 8.
The ratiometric reference feature makes the AD7394 an ideal
companion to ratiometric analog-to-digital converters such
as the AD7896.
Figure 19. Equivalent Analog Output Circuit
REF
REF
< V
input accepts input voltages ranging
DD
. The reference bandwidth and
REF
terminal to the positive V
P-CH
N-CH
DD
. One of the
V
V
AGND
DD
OUT
DD