CN-0227 Datasheet, PDF (2/5 Pages) Analog Devices – High Performance, 16-Bit, 250 MSPS Wideband Receiver with Antialiasing Filter

CN-0227
An input signal of 6.0 dBm produces a full-scale 2 V p-p
differential signal at the ADC input.
The antialiasing filter is a third-order Butterworth filter designed
with a standard filter design program. A Butterworth filter was
chosen because of its flat response within the pass band. A third
order filter yields an ac noise bandwidth ratio of 1.05 and can be
designed with the aid of several free filter programs such as
Nuhertz Technologies Filter Free, or the Quite Universal Circuit
Simulator (Qucs) Free Simulation.
To achieve best performance, load the ADL5562 with a net
differential load of 200 Ω. The 15 Ω series resistors isolate the
filter capacitance from the amplifier output, and the 243 Ω resistors
in parallel with the downstream impedance yield a net load
impedance of 203 Ω when added to the 30 Ω series resistance.
The 20 Ω resistors in series with the ADC inputs isolate internal
switching transients from the filter and the amplifier. The 511 Ω
resistor in parallel with the ADC serves to reduce the input
impedance of the ADC for more predictable performance.
The third-order Butterworth filter was designed with a source
impedance of 38.6 Ω, a load impedance of 269 Ω, and a 3 dB
bandwidth of 180 MHz. The calculated values from the program
are shown in Figure 1. The values chosen for the filter passive
components were the closest standard values to those generated
by the program.
19.3Ω ^23.83nH
12pF
269Ω
35.79pF
19.3Ω 23.83nH
Figure 2. Design for Third-Order Differential Butterworth Filter with
ZS = 38.6 Ω, ZL = 269 Ω, FC = 180 MHz
The internal 3.5 pF capacitance of the ADC was subtracted
from the value of the second shunt capacitor to yield a value of
32.29 pF. In the circuit, this capacitor was realized using two
62 pF capacitors connected to ground as shown in Figure 1.
This provides the same filtering effect and offers some ac
common-mode rejection.
The measured performance of the system is summarized in
Table 1, where the 3 dB bandwidth is 152 MHz. The total insertion
loss of the network is approximately 2 dB. The bandwidth response
is shown in Figure 3; the SNR and SFDR performance are
shown in Figure 4.
Circuit Note
Table 1. Measured Performance of the Circuit
Performance Specs at 2 V p-p FS
Final Results
Cutoff Frequency (−3 dB)
152 MHz
Pass-Band Flatness (6 MHz to 125 MHz) 1 dB
SNRFS at 120 MHz
72.6 dBFS
SFDR at 120 MHz
82.2 dBc
H2/H3 at 120 MHz
86.6 dBc/82.2 dBc
Overall Gain at 10 MHz
3.9 dB
Input Drive at 10 MHz
6.0 dBm
0
–2
−3dB AT 152MHz
–4
–6
–8
–10
–12
–14
–16
–18
–20
1
10
100
1000
ANALOG INPUT FREQUENCY (MHz)
Figure 3. Pass Band Flatness Performance vs. Frequency
100
95
90
85
SFDR (dBc)
80
75
SNRFS (dBFS)
70
65
60
0 20 40 60 80 100 120 140 160 180
ANALOG INPUT FREQUENCY (MHz)
Figure 4. SNR/SFDR Performance vs. Frequency
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