AD8375

Rev. 0 | 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

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