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.

1nF

50Ω

1:3

AD8375

301Ω

A0 TO A4

06724-042

1µH

CML

165Ω

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