AD5734R datasheet(16/20 Pages) AD

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AD5726

Rev. 0 | Page 16 of 20

Single +5 V Supply Operation

For operation with a 5 V supply, the reference voltage should be

set between 1.0 V and 2.5 V for optimum linearity. Figure 26

shows an AD780 used to supply a 2.5 V reference voltage. The

headroom of the reference and DAC are both sufficient to support

a 5 V supply with ±5 V tolerance. AVDD and VL should be

connected to the same supply. Separate bypassing to each pin

should be used.

10µF

0.01µF

INPUT

OUTPUT

GND

TRIM

0.2µF

AD780

AD5726

AVDD

AVSS

VREFP

VREFN

0V TO 2.5V OPERATION

SINGLE 5V SUPPLY

10kΩ

0.1µF║10µF

Figure 26. 5 V Single-Supply Operation

POWER SUPPLY BYPASSING AND GROUNDING

In any circuit where accuracy is important, careful consideration

to the power supply and ground return layout helps to ensure

the rated performance. The AD5726 has a single ground pin

that is internally connected to the digital section as the logic

reference level. The user’s first instinct may be to connect this

pin to the digital ground; however, in large systems, the digital

ground is often noisy because of the switching currents of other

digital circuitry. Any noise introduced at the ground pin could

couple into the analog output. Thus, to avoid error-causing

digital noise in the sensitive analog circuitry, the ground pin

should be connected to the system analog ground.

The ground path (circuit board trace) should be as wide as

possible to reduce any effects of parasitic inductance and ohmic

drops. A ground plane is recommended if possible. The noise

immunity of the on-board digital circuitry, typically in the

hundreds of millivolts, is well able to reject the common-mode

noise typically seen between system analog and digital grounds.

Finally, the analog and digital ground should be connected to

each other at a single point in the system to provide a common

reference. This is preferably done at the power supply

Good grounding practice is essential to maintain analog perform-

ance in the surrounding analog support circuitry as well. With

two reference inputs and four analog outputs capable of moderate

bandwidth and output current, there is a significant potential

for ground loops. Again, a ground plane is recommended as

the most effective solution to minimize errors due to noise and

ground offsets.

The AD5726 should have ample supply bypassing located as

close to the package as possible. Recommended capacitor values

are 10 μF in parallel with 0.1 μF. The 0.1 μF capacitor should

have low effective series resistance (ESR) and effective series

inductance (ESI), such as the common ceramic types, which

provide a low impedance path to ground at high frequencies to

handle transient currents due to internal logic switching.

GALVANICALLY ISOLATED INTERFACE

In many process control applications, it is necessary to provide

an isolation barrier between the controller and the unit being

controlled to protect and isolate the controlling circuitry from any

hazardous common-mode voltages that may occur. Isocouplers

provide voltage isolation in excess of 2.5 kV. The serial loading

structure of the AD5726 makes it ideal for isolated interfaces

because the number of interface lines is kept to a minimum.

Figure 27 shows a 4-channel isolated interface connected to the

AD5726 using an ADuM1400.

ENCODE

DECODE

ENCODE

DECODE

ENCODE

DECODE

ENCODE

DECODE

VIA

VIB

VIC

VID

VOA

VOB

VOC

VOD

TO SCLK

TO SDIN

TO CS

TO LDAC

ADuM1400*

SERIAL CLOCK OUT

SERIAL DATA OUT

SYNC OUT

CONTROL OUT

MICROCONTROLLER

*ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 27. Isolated Interface

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