AD9516-1 Analog Devices, AD9516-1 Datasheet - Page 79

• Manufacturer Part Number: AD9516-1
• Description: 14-Output Clock Generator
• Manufacturer: Analog Devices

APPLICATION NOTES
USING THE AD9516 OUTPUTS FOR ADC CLOCK
APPLICATIONS
Any high speed, ADC is extremely sensitive to the quality of its
sampling clock. An ADC can be thought of as a sampling mixer,
and any noise, distortion, or timing jitter on the clock is
combined with the desired signal at the analog-to-digital
output. Clock integrity requirements scale with the analog input
frequency and resolution, with higher analog input frequency
applications at ≥14-bit resolution being the most stringent. The
theoretical SNR of an ADC is limited by the ADC resolution
and the jitter on the sampling clock. Considering an ideal ADC
of infinite resolution where the step size and quantization error
can be ignored, the available SNR can be expressed
approximately by
where:
f
t
Figure 68 shows the required sampling clock jitter as a function
of the analog frequency and effective number of bits (ENOB).
See the
Many high performance ADCs feature differential clock inputs
to simplify the task of providing the required low jitter clock on
a noisy PCB. (Distributing a single-ended clock on a noisy PCB
can result in coupled noise on the sample clock. Differential
distribution has inherent common-mode rejection that can
provide superior clock performance in a noisy environment.)
The AD9516 features both LVPECL and LVDS outputs that
provide differential clock outputs, which enable clock solutions
that maximize converter SNR performance. The input
requirements of the ADC (differential or single-ended, logic
level, termination) should be considered when selecting the best
clocking/converter solution.
A
J
is the rms jitter on the sampling clock.
is the highest analog frequency being digitized.
110
100
90
80
70
60
50
40
30
SNR
10
AN-756
(
Figure 68. SNR and ENOB vs. Analog Input Frequency
dB
)
=
application note and the
20
×
log
⎛
⎜
⎜
⎝
2
π
1
f
f
A
A
t
100
(MHz)
J
⎞
⎟
⎟
⎠
SNR = 20log
AN-501
2πf
application note.
1
A
t
J
1k
18
16
14
12
10
8
6
Rev. 0 | Page 79 of 84
LVPECL CLOCK DISTRIBUTION
The LVPECL outputs of the AD9516 provide the lowest jitter
clock signals available from the AD9516. The LVPECL outputs
(because they are open emitter) require a dc termination to bias
the output transistors. The simplified equivalent circuit in
Figure 57 shows the LVPECL output stage.
In most applications, an LVPECL far-end Thevenin termination
is recommended, as shown in Figure 69. The resistor network is
designed to match the transmission line impedance (50 Ω) and
the switching threshold (V
LVDS CLOCK DISTRIBUTION
The AD9516 provides four clock outputs (OUT6 to OUT9) that
are selectable as either CMOS or LVDS level outputs. LVDS is a
differential output option that uses a current mode output stage.
The nominal current is 3.5 mA, which yields 350 mV output
swing across a 100 Ω resistor. The LVDS output meets or
exceeds all ANSI/TIA/EIA-644 specifications.
A recommended termination circuit for the LVDS outputs is
shown in Figure 71.
See the AN-586 application note at
information on LVDS.
VS_LVPECL
VS_LVPECL
LVPECL
VS
LVDS
LVPECL
200Ω
Figure 70. LVPECL with Parallel Transmission Line
Figure 69. LVPECL Far-End Thevenin Termination
Figure 71. LVDS Output Termination
DIFFERENTIAL (COUPLED)
0.1nF
0.1nF
V
(NOT COUPLED)
200Ω
SINGLE-ENDED
T
= V
S
TRANSMISSION LINE
100Ω DIFFERENTIAL
50Ω
50Ω
– 1.3V
100Ω
S
− 1.3 V).
(COUPLED)
127Ω
83Ω
VS_LVPECL
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100Ω
100Ω
127Ω
83Ω
VS_LVPECL
LVPECL
V
LVDS
VS
AD9516-1
S
LVPECL
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