ADL5565ACPZ-R7 datasheet(21/28 Pages) AD

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ADL5565

Rev. 0 | Page 21 of 28

By designing a narrow band-pass antialiasing filter between the

ADL5565 and the target ADC, the output noise of the ADL5565

outside of the intended Nyquist zone can be attenuated, helping

to preserve the available SNR of the ADC. In general, the SNR

improves several decibels when including a reasonable order anti-

aliasing filter. In this example, a low loss 1:1 input transformer is

used to match the ADL5565 balanced input to a 50 Ω unbalanced

source, resulting in minimum insertion loss at the input.

Figure 41 is optimized for driving some of Analog Devices popular

ADCs, such as the AD9467. Table 10 includes antialiasing filter

component recommendations for popular IF sampling 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

that the ADC input looks like a real resistance at the target center

frequency. The inductor, L5, shorts the ADC inputs at dc, which

introduces a zero into the transfer function. In addition, the ac

coupling capacitors 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 10 provides initial suggestions for proto-

typing purposes. Some empirical optimization may be needed

to help compensate for actual PCB parasitics.

105Ω

AD9467

1nF

CML

ADL5565

4Ω

Figure 41. Narrow-Band IF Sampling Solution for an ADC Application

Table 10. Interface Filter Recommendations for Various IF Sampling Frequencies

Center Frequency (MHz)

1 dB Bandwidth (MHz)

L1 (nH)

C2 (pF)

L3 (nH)

C4 (pF)

L5 (nH)

3.3

140

3.3

150

170

3.3

120

211

3.3

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