Datasheet

Data Sheet AD8376
Rev. B | Page 17 of 24
Figure 42 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
1/2
AD8376
301Ω
C2
A0 TO A4
C4
1µH
1µH
L1
L1
L3
L3
CML
165Ω
165Ω
L5
AD9246
AD9640
AD6655
06725-042
Figure 42. Narrow-Band IF Sampling Solution for Unbuffered ADC Applications
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