AD6645 Analog Devices, AD6645 Datasheet - Page 19

• Manufacturer Part Number: AD6645
• Description: 14-Bit, 80 MSPS/105 MSPS A/D Converter
• Manufacturer: Analog Devices

Specifications of AD6645

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 105MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2.2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: QFP

LAYOUT INFORMATION
The schematic of the evaluation board (see Figure 43)
represents a typical implementation of the AD6645. A multi-
layer board is recommended to achieve best results. It is highly
recommended that high quality, ceramic chip capacitors be
used to decouple each supply pin to ground directly at the
device. The pinout of the AD6645 facilitates ease of use in the
implementation of high frequency, high resolution design practices.
All of the digital outputs are segregated to two sides of the chip,
with the inputs on the opposite side for isolation purposes.
Care should be taken when routing the digital output traces. To
prevent coupling through the digital outputs into the analog
portion of the AD6645, minimal capacitive loading should be
placed on these outputs. It is recommended that a fanout of
only one gate should be used for all AD6645 digital outputs.
The layout of the encode circuit is equally critical. Any noise
received on this circuitry results in corruption in the digitization
process and lower overall performance. The encode clock must be
isolated from the digital outputs and the analog inputs.
Table 8. Twos Complement Output Coding
AIN Level
VREF + 0.55 V
VREF
VREF − 0.55 V
AIN Level
VREF − 0.55 V
VREF
VREF + 0.55 V
Output State
Positive FS
Midscale
Negative FS
Output Code
01 1111 1111 1111
00 … 0/11 … 1
10 0000 0000 0000
Rev. D | Page 19 of 24
JITTER CONSIDERATIONS
The SNR for an ADC can be predicted. When normalized to
ADC codes, the following equation accurately predicts the SNR
based on three terms: jitter, average DNL error, and thermal
noise. Each of these terms contributes to the noise within the
converter.
where:
f
t
and internal encode circuitry).
ε is the average DNL of the ADC (typically 0.41 LSB).
n is the number of bits in the ADC.
V
analog input of the ADC (typically 0.9 LSB rms).
For a 14-bit ADC, such as the AD6645, aperture jitter can
greatly affect the SNR performance as the analog frequency is
increased. Figure 42 shows a family of curves that demonstrate the
expected SNR performance of the AD6645 as jitter increases.
The chart is derived from the preceding equation.
For a complete discussion of aperture jitter, see the AN-756
application note, Sampled Systems and the Effects of Clock Phase
Noise and Jitter. The AN-756 application note can be found on
www.analog.com.
SNR
20
ANALOG
j rms
NOISE rms
log
is the rms jitter of the encode (rms sum of encode source
=
⎡
⎢
⎣
80
75
70
65
60
55
. 1
is the analog input frequency.
(
2
76
is the voltage rms thermal noise that refers to the
0
π
×
−
f
AIN = 110MHz
ANALOG
AIN = 150MHz
0.1
×
AIN = 190MHz
t
j
Figure 42. SNR vs. Jitter
rms
0.2
)
2
+
JITTER (ps)
⎛ +
⎜
⎝
1
2
0.3
n
ε
⎞
⎟
⎠
2
+
⎛
⎜
⎜
⎝
0.4
2
×
AIN = 30MHz
AIN = 70MHz
2
×
2
0.5
V
n
NOISE
AD6645
rms
0.6
⎞
⎟
⎟
⎠
2
⎤
⎥
⎥
⎦
1
2 /