adl5562 Analog Devices, Inc., adl5562 Datasheet - Page 17

• Manufacturer Part Number: adl5562
• Description: 3.3 Ghz Ultralow Distortion Rf/if Differential Amplifier
• Manufacturer: Analog Devices, Inc.

instrumentation applications. However, by designing for a wide
analog input frequency range, the cascaded SNR performance is
somewhat degraded due to high frequency noise aliasing into
the wanted Nyquist zone.
An alternative narrow-band approach is presented in Figure 40.
By designing a narrow band-pass antialiasing filter between the
ADL5562 and the target ADC, the output noise of the ADL5562
outside of the intended Nyquist zone can be attenuated, helping
to preserve the available SNR of the ADC. In general, the SNR
improves several decibels when including a reasonable order
antialiasing filter. In this example, a low loss 1:1 input transformer
is used to match the ADL5562 balanced input to a 50 Ω unbal-
anced source, resulting in minimum insertion loss at the input.
Table 9. Interface Filter Recommendations for Various IF Sampling Frequencies
Center Frequency
96 MHz
140 MHz
170 MHz
211 MHz
–100
–110
–120
–130
–140
–150
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
Figure 38. Measured Single-Tone Performance of the
5.25 10.50 15.75 21.00 26.25 31.50 36.75 42.00 47.25 52.50
Circuit in Figure 37 for a 100 MHz Input Signal
ADL5562 DRIVING THE AD9445 14-BIT ADC
GAIN = 6dB
INPUT = 140MHz
SNR = 66.25dBc
SFDR = 84.2dBc
NOISE FLOOR = –109.5dB
FUND = –1.081dBFS
SECOND = –84.54dBc
THIRD = –84.54dBc
FREQUENCY (MHz)
1 dB Bandwidth
28 MHz
33 MHz
32 MHz
30 MHz
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L1
5.1 nH
5.1 nH
2.2 nH
2.2 nH
Figure 40 is optimized for driving some of the Analog Devices
popular unbuffered ADCs, such as the AD9246, AD9640,
and AD6655. Table 9 includes antialiasing filter component
recommendations for popular IF sampling center frequencies.
Inductor L5 works in parallel with the on-chip ADC input
capacitance and a portion of the capacitance presented by C4 to
form a resonant tank circuit. The resonant tank helps to ensure
that the ADC input looks like a real resistance at the target
center frequency. The L5 inductor shorts the ADC inputs at dc,
which introduces a zero into the transfer function. In addition,
the ac-coupling capacitors introduce additional zeros into the
transfer function. The final overall frequency response takes on
a band-pass characteristic, helping to reject noise outside of the
intended Nyquist zone. Table 9 provides initial suggestions for
prototyping purposes. Some empirical optimization may be
needed to help compensate for actual PCB parasitics.
ADL5562
Figure 40. Narrow-Band IF Sampling Solution for an Unbuffered ADC
–10
–1
–2
–3
–4
–5
–6
–7
–8
–9
0
2.00
Figure 39. Measured Frequency Response of the Wideband
C2
56 pF
43 pF
30 pF
16 pF
1nF
1nF
FIRST POINT = –1.02dBFS
END POINT = –5.69dBFS
MID POINT = –1.09dBFS
MIN = –5.69dBFS
MAX = –0.88dBFS
81.90
161.80
ADC Interface Depicted in Figure 37
L1
L2
C2
241.70
L3
18 nH
12 nH
10 nH
10 nH
L3
L4
FREQUENCY (MHz)
C4
321.60
Application
105Ω
105Ω
401.50
481.40
CML
C4
75 pF
68 pF
75 pF
51 pF
561.30
L5
641.20
72.10
ADL5562
L5
250 nH
39 nH
22 nH
18 nH
801.00
AD9246
AD9640
AD6655