AD9281-EB Analog Devices Inc, AD9281-EB Datasheet - Page 13

• Manufacturer Part Number: AD9281-EB
• Description: BOARD EVAL FOR AD9281
• Manufacturer: Analog Devices Inc

Specifications of AD9281-EB

• Rohs Status: RoHS non-compliant

REV. F
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.
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
2. The minimization of the impedance associated with ground
3. The inherent distributed capacitor formed by the power plane,
ASIC
DSP
OR
and its return path.
and power paths.
PCB insulation and ground plane.
Figure 30. Typical Analog QAM Demodulator
DUAL MATCHED
ADC
ADC
ADC
Q
I
FREQUENCY
NYQUIST
CARRIER
FILTERS
LO
DEMODULATOR
QUADRATURE
90°C
FROM
PREVIOUS
STAGE
–13–
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.
V
IN
A
D
A
Figure 31. Ground and Power Consideration
= ANALOG
= DIGITAL
GROUND
RF
ADC
A
IC
Figure 32. RF Ground Scheme
CIRCUITS
ANALOG
AVDD
A
A
AVSS
I
A
C
C
STRAY
STRAY
GROUND
ANALOG
A
CIRCUITS
DIGITAL
DVDD
A
AIN
BIN
B
DVSS
I
D
ADC
GROUND
DATA
DIGITAL
V
AD9281
D
LOGIC
DIGITAL
SUPPLY
LOGIC
LOGIC
GND
ICs
D