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AD7781CRZ-REEL

20-Bit, Pin-Programmable, Low Power Sigma-Delta ADC

Analog Devices 

GROUNDING AND LAYOUT

Because the analog input and reference input of the ADC are

differential, most of the voltages in the analog modulator are

common-mode voltages. The excellent common-mode reject-

tion of the part removes common-mode noise on these inputs.

The digital filter provides rejection of broadband noise on the

power supply, except at integer multiples of the modulator

sampling frequency. The digital filter also removes noise from

the analog and reference inputs, provided that these noise sources

do not saturate the analog modulator. As a result, the AD7781

is more immune to noise interference than conventional high

resolution converters. However, because the resolution of the

AD7781 is so high and the noise levels from the AD7781 are so

low, care must be taken with regard to grounding and layout.

The printed circuit board (PCB) that houses the AD7781 should

be designed so that the analog and digital sections are separated

and confined to certain areas of the board. A minimum etch

technique is generally best for ground planes because it gives

the best shielding.

It is recommended that the GND pin of the AD7781 be tied

to the AGND plane of the system. In any layout, pay attention

to the flow of currents in the system and ensure that the return

paths for all currents are as close as possible to the paths that the

currents took to reach their destinations. Avoid forcing digital

currents to flow through the AGND sections of the layout.

AD7781

The ground plane of the AD7781 should be allowed to run under

the AD7781 to prevent noise coupling. The power supply lines

to the AD7781 should use as wide a trace as possible to provide

low impedance paths and reduce the effects of glitches on the

power supply line. Fast switching signals such as clocks should

be shielded with digital ground to avoid radiating noise to other

sections of the board, and clock signals should never be run near

the analog inputs. Avoid crossover of digital and analog signals.

Traces on opposite sides of the board should run at right angles

to each other. This reduces the effects of feedthrough through

the board. A microstrip technique is by far the best, but it is not

always possible with a double-sided board. In this technique,

the component side of the board is dedicated to ground planes,

and the signals are placed on the solder side.

Good decoupling is important when using high resolution ADCs.

AVDD should be decoupled with 10 μF tantalum capacitors in

parallel with 0.1 μF capacitors to GND. DVDD should be decoupled

with 10 μF tantalum capacitors in parallel with 0.1 μF capac-

itors to GND, with the system’s AGND to DGND connection

being close to the AD7781. To achieve the best results from

these decoupling components, place them as close as possible

to the device, ideally right up against the device. All logic chips

should be decoupled with 0.1 μF ceramic capacitors to DGND.

Rev. 0 | Page 15 of 16

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