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AD5680

Data Sheet

Rev. C | Page 16 of 20

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 36). 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

325 μA + (5 V/5 kΩ) = 1.33 mA

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.

AD5680

SYNC

SCLK

DIN

15V

VOUT = 0V TO 5V

VREF

VDD

REF195

250µA

3-WIRE

SERIAL

INTERFACE

Figure 36. REF195 as Power Supply to the AD5680

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 ADuM130D

3-channel digital isolator provides the required isolation (see

Figure 37). 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.

0.1µF

REGULATOR

GND

DIN

SYNC

SCLK

POWER

10µF

SCLK

DIN

AD5680

SYNC

VOUT

VOB

VOA

VOC

VDD

VIC

VIB

VIA

ADuM130D

Figure 37. AD5680 with a Galvanically Isolated Interface

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 should be the tanta-

lum 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 tech-

nique 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.

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