CN-0268 Datasheet, PDF (3/6 Pages) Analog Devices – Resonant Approach to Designing a Band-Pass Filter for Narrow-Band, High IF, 16-Bit, 250 MSPS Receiver Front End

Circuit Note
AD9467 Source Impedance
The AD9467 is an ideal choice for an ADC in this circuit because it
is an IF sampling ADC optimized for high performance over
wide bandwidths and ease of use. The AD9467 has an integrated
buffer that presents a fixed input impedance to the driver amplifier.
This input structure is an advantage over ADCs that use an
unbuffered front end directly coupled to the sampling switches.
Unbuffered ADCs present time varying input sample-and-hold
impedances to the drive amplifier. The addition of the input
buffer eases the drive requirements at the expense of slightly
higher power consumption. The buffered source impedance of
the AD9467 is modeled as a fixed impedance of a 530 Ω resistance
in parallel with a 3.5 pF capacitance.
When interfacing to the ADC, it is recommended that the real
input impedance be reduced from 530 Ω to a lower value within
the 200 Ω to 400 Ω range. By lowering the input impedance of
the ADC, the kickback due to the sample-and-hold structure
settles out faster, yielding improved linearity performance. The
tradeoff is increased input power because more power is required
to drive the full scale of the ADC. In this circuit example, the
input impedance of the AD9467 was reduced to 200 Ω to match
the output impedance of the ADL5565 and also to balance the
linearity vs. input power of the ADC. The input impedance of
the AD9467 was reduced to 200 Ω by placing a 310 Ω resistor in
parallel with the ADC differential input.
Antialiasing Filter Design
An antialising filter ahead of the ADC helps reduce signal
content and noise from unwanted Nyquist zones that would
otherwise alias in band and degrade the dynamic performance.
Antialiasing filters are often designed using LC networks and
must have well defined source and load impedances to achieve
the desired stop-band and pass-band characteristics. The filter
design is accomplished using software available from Nuhertz
Technologies or Agilent Technologies Advanced Design
Systems (ADS), for example.
In the circuit in Figure 1, the ADS program was used to design
a fourth-order maximally flat (Butterworth) low-pass filter.
Figure 4 shows the low-pass filter design with a source and load
impedance of 200 Ω and a 3 dB cutoff frequency of 300 MHz.
The 200 Ω impedance was chosen because it is the common
source and load impedance of the driver amplifier and ADC.
The first elements are series inductors to ease driver requirements.
In the final optimized circuit of Figure 1, the filter source
impedance is equal to approximately 21.6 Ω; however, 200 Ω was
chosen to design the low-pass portion of the filter because the
overall filter is ultimately a resonant band-pass filter, and it is
more critical that the amplifier and ADC see the correct load and
source impedance for optimized linearity performance. The effect
of doing this is amplitude loss due to the impedance mismatch.
39nH 150nH
CN-0268
8.2pF
2pF
39nH 150nH
Figure 4. Low-Pass Filter Design
The low-pass filter design was further tuned by creating resonance
to cause peaking at the band of interest. This resulted in a
narrow-band, band-pass filter at a high IF. Placing an inductor
across the ADC differential inputs nulls the input capacitance of
the ADC and creates peaking. Figure 5 shows the calculation
used to determine the resonant inductor value. In the case of
the 3.5 pF source impedance of the AD9467, a parallel inductor
of 181 nH is necessary to null the capacitive susceptance; leaving
only the high impedance resistive portion of the RC parallel
equivalent. The resonant frequency chosen for the calculation
was 200 MHz.
Z_L
Z_R
Z_C
Figure 5. Resonant Match
Z_C
=
1
jωC
Z_L = jωL
Y_C
=
1
Z_C
Y_L
=
1
Z_L
Y_C + Y_L = 0
L
=
1
ω ²C
Measured Performance
Figure 1 shows the final circuit configuration. The outputs of the
ADL5565 were padded with 5.6 Ω on each output to improve
the stability of the driver amplifier. The recommended series
resistance is generally between a few ohms to several tens of ohms.
A larger resistor value improves on stability; however, the tradeoff is
a power loss because the series resistor forms a voltage divider
with the impedance at the ADC inputs, resulting in signal
attenuation.
Following the series resistors at the output of the ADL5565 are
1 nF dc blocking capacitors. Following that is the antialiasing
filter and then the parallel resistor of 310 Ω to reduce the input
impedance of the ADC. Finally, the 15 Ω resistors in series with
the ADC inputs isolate the internal switching transients from
the filter and the amplifier.
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