EVAL-AD5680EB AD [Analog Devices], EVAL-AD5680EB Datasheet - Page 16

• Manufacturer Part Number: EVAL-AD5680EB
• Description: 5 V 18-Bit nanoDAC in a SOT-23
• Manufacturer: AD [Analog Devices]

AD5680
USING A REFERENCE AS A POWER SUPPLY FOR
THE AD5680
Because the supply current required by the AD5680 is extremely
low, an alternative option is to use a voltage reference to supply
the required voltage to the part (see Figure 35). 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, for example, 15 V.
The voltage reference outputs a steady supply voltage for the
AD5680; see Table 6 for a suitable reference. If the low dropout
REF195 is used, it must supply 325 μA of current to the
AD5680, 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
The load regulation of the REF195 is typically 2 ppm/mA,
which results in a 2.7 ppm (13.5 μV) error for the 1.33 mA
current drawn from it. This corresponds to a 0.177 LSB error.
USING THE AD5680 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 AD5680 uses a 3-wire serial logic interface, so the
ADuM130x 3-channel digital isolator provides the required
isolation (see Figure 36). 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 AD5680.
INTERFACE
325 μA + (5 V/5 kΩ) = 1.33 mA
SERIAL
3-WIRE
Figure 35. REF195 as Power Supply to the AD5680
15V
SYNC
SCLK
DIN
REF195
5V
V
DD
AD5680
V
REF
250µA
V
OUT
= 0V TO 5V
Rev. 0 | Page 16 of 20
POWER SUPPLY BYPASSING AND GROUNDING
When accuracy is important in a circuit, it is helpful to carefully
consider the power supply and ground return layout on the
board. The printed circuit board containing the AD5680 should
have separate analog and digital sections, each having its own
area of the board. If the AD5680 is in a system where other
devices require an AGND-to-DGND connection, the connection
should be made at one point only. This ground point should be
as close as possible to the AD5680.
The power supply to the AD5680 should be bypassed with 10 μF
and 0.1 μF capacitors. The capacitors should be located as close
as possible to the device, with the 0.1 μF capacitor ideally right
up against the device. The 10 μF capacitors are the tantalum
bead type. It is important that the 0.1 μF capacitor has low
effective series resistance (ESR) and effective series inductance
(ESI), for example, common ceramic types of capacitors. This
0.1 μF capacitor provides a low impedance path to ground for
high frequencies caused by transient currents due to internal
logic switching.
The power supply line itself should have as large a trace as
possible to provide a low impedance path and to reduce glitch
effects on the supply line. Clocks and other fast switching
digital signals should be shielded from other parts of the board
by digital ground. Avoid crossover of digital and analog signals
if possible. When traces cross on opposite sides of the board,
ensure that they run at right angles to each other to reduce
feedthrough effects on the board. The best board layout
technique is the microstrip technique where the component
side of the board is dedicated to the ground plane only and the
signal traces are placed on the solder side. However, this is not
always possible with a 2-layer board.
SCLK
DATA
SDI
POWER
Figure 36. AD5680 with a Galvanically Isolated Interface
V1A
V1B
V1C
ADuM1300
VOB
VOC
VOA
REGULATOR
5V
SCLK
SYNC
DIN
AD5680
GND
V
DD
V
10µF
OUT
0.1µF