AD9281ARS datasheet(13/19 Pages) AD

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AD9281

–13–

REV. E

ANALOG

CIRCUITS

DIGITAL

LOGIC

ICs

DVSS

AVSS

AVDD

DVDD

LOGIC

SUPPLY

VIN

CSTRAY

GND

= ANALOG

= DIGITAL

ADC

DIGITAL

CIRCUITS

Figure 31. Ground and Power Consideration

These characteristics result in both a reduction of electro-

magnetic interference (EMI) and an overall improvement in

performance.

It is important to design a layout that prevents noise from cou-

pling onto the input signal. Digital signals should not be run in

parallel with the input signal traces and should be routed

away from the input circuitry. Separate analog and digital

grounds should be joined together directly under the AD9281 in

a solid ground plane. The power and ground return currents

must be carefully managed. A general rule of thumb for mixed

signal layouts dictates that the return currents from digital cir-

cuitry should not pass through critical analog circuitry.

Transients between AVSS and DVSS will seriously degrade

performance of the ADC.

If the user cannot tie analog ground and digital ground together

at the ADC, he should consider the configuration in Figure 32.

Another input and ground technique is shown in Figure 32. A

separate ground plane has been split for RF or hard to manage

signals. These signals can be routed to the ADC differentially or

single ended (i.e., both can either be connected to the driver or

RF ground). The ADC will perform well with several hundred

mV of noise or signals between the RF and ADC analog ground.

DATA

ANALOG

GROUND

DIGITAL

GROUND

LOGIC

ADC

AIN

BIN

GROUND

Figure 32. RF Ground Scheme

At the receiver, the demodulation of a QAM signal back into its

separate I and Q components is essentially the modulation

process explain above but in the reverse order. A common and

traditional implementation of a QAM demodulator is shown in

Figure 30. In this example, the demodulation is performed in

the analog domain using a dual, matched ADC and a quadra-

ture demodulator to recover and digitize the I and Q baseband

signals. The quadrature demodulator is typically a single IC

containing two mixers and the appropriate circuitry to generate

the necessary 90

° phase shift between the I and Q mixers’ local

oscillators. Before being digitized by the ADCs, the mixed

down baseband I and Q signals are filtered using matched ana-

log filters. These filters, often referred to as Nyquist or Pulse-

Shaping filters, remove images-from the mixing process and any

out-of-band. The characteristics of the matching Nyquist filters

are well defined to provide optimum signal-to-noise (SNR)

performance while minimizing intersymbol interference. The

ADC’s are typically simultaneously sampling their respective

inputs at the QAM symbol rate or, most often, at a multiple of it

if a digital filter follows the ADC. Oversampling and the use of

digital filtering eases the implementation and complexity of the

analog filter. It also allows for enhanced digital processing for

both carrier and symbol recovery and tuning purposes. The use

of a dual ADC such as the AD9281 ensures excellent gain,

offset, and phase matching between the I and Q channels.

90°C

FROM

STAGE

QUADRATURE

DEMODULATOR

ADC

DSP

ASIC

CARRIER

FREQUENCY

NYQUIST

FILTERS

ADC

DUAL MATCHED

ADC

Figure 30. Typical Analog QAM Demodulator

GROUNDING AND LAYOUT RULES

As is the case for any high performance device, proper ground-

ing and layout techniques are essential in achieving optimal

performance. The analog and digital grounds on the AD9281

have been separated to optimize the management of return

currents in a system. Grounds should be connected near the

ADC. It is recommended that a printed circuit board (PCB) of

at least four layers, employing a ground plane and power planes,

be used with the AD9281. The use of ground and power planes

offers distinct advantages:

1. The minimization of the loop area encompassed by a signal

and its return path.

2. The minimization of the impedance associated with ground

and power paths.

3. The inherent distributed capacitor formed by the power plane,

PCB insulation and ground plane.

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