AD9243 Analog Devices, AD9243 Datasheet - Page 19

• Manufacturer Part Number: AD9243
• Description: Complete 14-Bit, 3 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9243

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 3MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,2 V p-p,5V p-p,Uni (Vref) x 2,Uni 2.0V,Uni 5.0V
• Adc Architecture: Pipelined
• Pkg Type: QFP

REV. A
GROUNDING AND DECOUPLING
Analog and Digital Grounding
Proper grounding is essential in any high speed, high resolution
system. Multilayer printed circuit boards (PCBs) are recom-
mended to provide optimal grounding and power schemes. 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
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 coupling
onto the input signal. Digital signals should not be run in paral-
lel with input signal traces and should be routed away from the
input circuitry. While the AD9243 features separate analog and
digital ground pins, it should be treated as an analog compo-
nent. The AVSS, DVSS and DRVSS pins must be joined together
directly under the AD9243. A solid ground plane under the A/D is
acceptable if the power and ground return currents are managed
carefully. Alternatively, the ground plane under the A/D may
contain serrations to steer currents in predictable directions
where cross-coupling between analog and digital would other-
wise be unavoidable. The AD9243/EB ground layout, shown in
Figure 54, depicts the serrated type of arrangement. The analog
and digital grounds are connected by a jumper below the A/D.
Analog and Digital Supply Decoupling
The AD9243 features separate analog and digital supply and
ground pins, helping to minimize digital corruption of sensitive
analog signals.
Figure 45 shows the power supply rejection ratio vs. frequency
for a 200 mV p-p ripple applied to both AVDD and DVDD.
In general, AVDD, the analog supply, should be decoupled to
AVSS, the analog common, as close to the chip as physically
possible. Figure 46 shows the recommended decoupling for the
analog supplies; 0.1 F ceramic chip capacitors should provide
and its return path.
and power paths.
plane, PCB insulation, and ground plane.
120
100
80
60
40
Figure 45. AD9243 PSSR vs. Frequency
1
10
FREQUENCY – kHz
DVDD
AVDD
100
1000
–19–
adequately low impedance over a wide frequency range. Note
that the AVDD and AVSS pins are co-located on the AD9243
to simplify the layout of the decoupling capacitors and provide
the shortest possible PCB trace lengths. The AD9243/EB power
plane layout, shown in Figure 55 depicts a typical arrangement
using a multilayer PCB.
The CML is an internal analog bias point used internally by the
AD9243. This pin must be decoupled with at least a 0.1 F
capacitor as shown in Figure 47. The dc level of CML is ap-
proximately AVDD/2. This voltage should be buffered if it is to
be used for any external biasing.
The digital activity on the AD9243 chip falls into two general
categories: correction logic, and output drivers. The internal
correction logic draws relatively small surges of current, mainly
during the clock transitions. The output drivers draw large
current impulses while the output bits are changing. The size
and duration of these currents are a function of the load on the
output bits: large capacitive loads are to be avoided. Note that
the internal correction logic of the AD9243 is referenced DVDD
while the output drivers are referenced to DRVDD.
The decoupling shown in Figure 48, a 0.1 F ceramic chip
capacitor, is appropriate for a reasonable capacitive load on the
digital outputs (typically 20 pF on each pin). Applications
involving greater digital loads should consider increasing the
digital decoupling proportionally, and/or using external buffers/
latches.
A complete decoupling scheme will also include large tantalum
or electrolytic capacitors on the PCB to reduce low-frequency
ripple to negligible levels. Refer to the AD9243/EB schematic
and layouts in Figures 51–55 for more information regarding the
placement of decoupling capacitors.
0.1 F
Figure 46. Analog Supply Decoupling
Figure 48. Digital Supply Decoupling
Figure 47. CML Decoupling
0.1 F
0.1 F
0.1 F
DVDD
DVSS
AD9243
CML
DRVDD
DRVSS
AVDD
AVDD
AVSS
AVSS
AD9243
AD9243
AD9243
0.1 F