Datasheet

Data Sheet AD5624R/AD5644R/AD5664R
Rev. C | Page 25 of 28
APPLICATIONS INFORMATION
USING A REFERENCE AS A POWER SUPPLY FOR
THE AD5624R/AD5644R/AD5664R
Because the supply current required by the AD5624R/AD5644R/
AD5664R is extremely low, an alternative option is to use a
voltage reference to supply the required voltage to the part (see
Figure 62). This is especially useful if the power supply is quite
noisy, or if the system supply voltages are at some value other
than 5 V or 3 V, for example, 15 V. The voltage reference outputs
a steady supply voltage for the AD5624R/AD5644R/AD5664R
(see Figure 60). If the low dropout REF195 is used, it must
supply 450 µA of current to the AD5624R/AD5644R/AD5664R
with no load on the output of the DAC. When the DAC output
is loaded, the REF195 also needs to supply the current to the
load. The total current required (with a 5 kΩ load on the DAC
output) is
450 µA + (5 V/5 kΩ) = 1.45 mA
The load regulation of the REF195 is typically 2 ppm/mA,
resulting in a 2.9 ppm (14.5 µV) error for the 1.45 mA current
drawn from it. This corresponds to a 0.191 LSB error.
Figure 62. REF195 as Power Supply to the AD5624R/AD5644R/AD5664R
BIPOLAR OPERATION USING THE
AD5624R/AD5644R/AD5664R
The AD5624R/AD5644R/AD5664R have been designed for
single-supply operation, but a bipolar output range is also
possible using the circuit in Figure 63. The circuit gives an
output voltage range of ±5 V. R ail -to-rail operation at the
amplifier output is achievable using an AD820 or an OP295 as
the output amplifier.
The output voltage for any input code can be calculated as
follows:












×−






+
×






×=
R1
R2
V
R1
R2R1D
VV
DDDD
OUT
536,65
where D represents the input code in decimal (0 to 65,536).
With V
DD
= 5 V, R1 = R2 = 10 kΩ,
V5
536,65
10
−






×
=
D
V
OUT
This is an output voltage range of ±5 V, with 0x0000 corre-
sponding to a −5 V output, and 0xFFFF corresponding to a
+5 V output.
Figure 63. Bipolar Operation with the AD5624R/AD5644R/AD5664R
USING AD5624R/AD5644R/AD5664R WITH A
GALVANICALLY ISOLATED INTERFACE
In process control applications in industrial environments, it is
often necessary to use a galvanically isolated interface to protect
and isolate the controlling circuitry from any hazardous common-
mode voltages that might occur in the area where the DAC is
functioning. Isocouplers provide isolation in excess of 3 kV. The
AD5624R/AD5644R/AD5664R use a 3-wire serial logic interface,
so the ADuM130x 3-channel digital isolator provides the
required isolation (see Figure 64). The power supply to the part
also needs to be isolated, which is done by using a transformer.
On the DAC side of the transformer, a 5 V regulator provides
the 5 V supply required for the AD5624R/AD5644R/AD5664R.
Figure 64. AD5624R/AD5644R/AD5664R with a Galvanically Isolated Interface
3-WIRE
SERIAL
INTERFACE
SYNC
SCLK
DIN
15V
5V
V
OUT
= 0V TO 5V
V
DD
REF195
AD5624R/
AD5644R/
AD5664R
05856-043
3-WIRE
SERIAL
INTERFACE
R2 = 10kΩ
+5V
–5V
AD820/
OP295
+5V
AD5624R/
AD5644R/
AD5664R
V
DD
V
OUT
R1 = 10kΩ
±5V
0.1µF10µF
05856-044
0.1µF
5V
REGULATOR
GND
DIN
SYNC
SCLK
POWER
10µF
SDI
SCLK
DATA
AD5624R/
AD5644R/
AD5664R
V
OUT
V
OB
V
OA
V
OC
V
DD
V
IC
V
IB
V
IA
ADuM1300
05856-045