Datasheet

AD8375 Data Sheet
Rev. A | Page 16 of 24
Figure 41 is optimized for driving some of Analog Devices
popular unbuffered ADCs, such as the AD9246, AD9640,
and AD6655. Table 5 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
the ADC input looks like a real resistance at the target center
frequency. Additionally, the L5 inductor shorts the ADC inputs
at dc, which introduces a zero into the transfer function. In
addition, the ac coupling capacitors and the bias chokes
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 5 provides initial suggestions for prototyping purposes.
Some empirical optimization may be needed to help compensate
for actual PCB parasitics.
5
1nF
1nF
1nF
1nF
50Ω
1:3
AD8375
301Ω
C2
A0 TO A4
06724-042
C4
1µH
1µH
L1
L1
L3
L3
CML
165Ω
165Ω
L5
AD9246
AD9640
AD6655
Figure 41. Narrow-Band IF Sampling Solution for Unbuffered ADC Application
Table 5. Interface Filter Recommendations for Various IF Sampling Frequencies
Center Frequency 1 dB Bandwidth L1 C2 L3 C4 L5
96 MHz 27 MHz 390 nH 5.6 pF 390 nH 25 pF 100 nH
140 MHz 30 MHz 330 nH 3.3 pF 330 nH 20 pF 56 nH
170 MHz 32 MHz 270 nH 2.7 pF 270 nH 20 pF 39 nH
211 MHz 32 MHz 220 nH 2.2 pF 220 nH 18 pF 27 nH