CN-0268 Datasheet, PDF (2/6 Pages) Analog Devices – Resonant Approach to Designing a Band-Pass Filter for Narrow-Band, High IF, 16-Bit, 250 MSPS Receiver Front End

CN-0268
Circuit Note
To achieve the optimal level of performance that the ADL5565
and AD9467 have to offer, it is important to properly follow the
design guidelines as specified on the respective data sheets. Some of
the important design criteria include properly matching the
input and output impedance of the ADL5565 for minimum
signal loss and optimum linearity performance, systematic
design of an antialiasing filter for improved dynamic range, and
source impedance matching to the ADC inputs.
ADL5565 Input Impedance Matching
R3
R1 _{0.1µF R4}
ETC1-1-13
VIP2
VIP1
VOP
50Ω
ADL5565
R5
R2 ^0.1µF _R6
VIN1
VON
VIN2
Figure 2. ADL5565 Input Impedance Match
Figure 2 shows the recommended input matching network for
the ADL5565. The input impedance of the ADL5565 is gain
dependent, and the differential input impedance is 200 Ω for 6 dB
gain, 100 Ω for 12 dB gain, and 67 Ω for 15.5 dB gain. To match
the 50 Ω source impedance of the signal generator to the input
impedance of the ADL5565, R1 and R2 must be chosen so that
their sum in parallel with the input impedance of the ADL5565,
ZI, is equal to 50 Ω. To maintain balance in the differential circuit,
R1 must equal R2. The following formula can be used to calculate
the necessary matching resistors.
R1 = R2
2R1 || Zl = 50 Ω
R1 = R2 = ²⁵
1 − (50/ Z_l )
Table 1 shows the calculated termination resistors and pin
configuration for the different gain settings of the ADL5565.
An alternative configuration to the one shown in Figure 2 is
to replace the 1:1 balun, ETC1-1-13, with an impedance
transformation RF transformer. This can eliminate the need
for R1 and R2. A 1:4 transformer can be used for the 6 dB gain
configuration or a 1:2 transformer for the 12 dB gain configuration.
The advantages of this alternative configuration are lower
component count and minimum signal loss. However, pay
attention to the bandwidth of the transformer. Impedance
transformation transformers have narrower bandwidths and
higher insertion loss as compared to a 1:1 balun.
Figure 2 shows a single-ended-to-differential approach to driving
the ADL5565 using a balun or transformer. This configuration
may not be a viable or desirable option in certain applications.
The ADL5565 offers flexibility in its driver interface and can be
driven single ended, as shown, or differentially with a differential
mixer, for example. Refer to the ADL5565 data sheet for details
on the different input interfaces.
ADL5565 Output Load Matching
The ADL5565 linearity performance has been optimized for a
200 Ω output load. This is a common output impedance used
to interface to ADCs and for filter design. With an optimized
output load of 200 Ω, the output IP3 of the ADL5565 at 200 MHz
is 46 dBm.
In situations where a 200 Ω output load may not fit the application,
tradeoffs can be made between the output load of the ADL5565
and its linearity performance. Figure 3 shows a plot of third-
order intermodulation (IMD3) vs. frequency for commonly
used output loads.
0
–20
–40
50Ω LOAD
–60
100Ω LOAD
200Ω LOAD
400Ω LOAD
–80
–100
–120
–140
0
50 100 150 200 250 300 350 400 450 500
FREQUENCY (MHz)
Figure 3. ADL5565 IMD3 vs. Frequency for 50 Ω, 100 Ω, 200 Ω, and 400 Ω
Output Loads, 3.3 V Supply, Gain = 6 dB
Table 1. Gain, Input Impedance, and R1, R2, R3, R4, R5, and R6 Values for ADL5565
Gain (dB)
ADL5565 Input Impedance, Zl, (Ω)
R1 (Ω) R2 (Ω)
6
200
33
33
12
100
50
50
15.5
67
Open
Open
R3 (Ω)
Open
0
0
Rev. 0 | Page 2 of 6
R4 (Ω)
0
Open
0
R5 (Ω)
0
Open
0
R6 (Ω)
Open
0
0