Data Sheet

ADL5202

Rev. D | Page 21 of 29

An alternative narrow-band approach is presented in Figure 59.

By designing a narrow band-pass antialiasing filter between the

ADL5202 and the target ADC, the output noise of the ADL5202

outside of the intended Nyquist zone can be attenuated, helping

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

improves by several decibels (dB) when including a reasonable

order antialiasing filter. In this example, a low loss 1:3 input

transformer is used to match the 150 Ω balanced input of the

ADL5202 to a 50 Ω unbalanced source, resulting in minimum

insertion loss at the input.

Figure 59 is optimized for driving some of the Analog Devices

popular unbuffered ADCs, such as the AD9246, AD9640,

and AD6655. Table 8 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

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

frequency.

In addition, the L6 inductor shorts the ADC inputs at dc, which

introduces a zero into the transfer function. 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 8 provides initial suggestions for

prototyping purposes. Some empirical optimization may be

needed to help compensate for actual PCB parasitics.

LAYOUT CONSIDERATIONS

The ADL5202 has two output pins for each polarity, and they

are oriented in an alternating fashion. When designing the board,

care should be taken to minimize the parasitic capacitance due

to the routing that connects the corresponding outputs together.

A good practice is to avoid any ground or power plane under

this routing region and under the chokes to minimize the

parasitic capacitance.

If the common-mode load capacitance including the capaci-

tance of the trace is > 2 pF, use parasitic suppressing resistors

at the device output pins. The resistors should be placed in

the output traces just after the crossover connections. Use 5 Ω

series resistors (Size 0402) to adequately de-Q the output

system without a significant decrease in gain.

L6

50Ω

AC

1:3

75

Ω

75

Ω

1µH

5V

1µH

5V

CML

C4

4pF

5V

1nF

1nF

1nF

L1

L3

L5

L1

L3

L5

1nF

DIGITAL

INTERFACE

1/2

ADL5202

C2

4pF

AD9246

AD9640

AD6655

Figure 59. Narrow-Band IF Sampling Solution for Unbuffered ADC Applications

Table 8. Interface Filter Recommendations for Various IF Sampling Frequencies

Center Frequency

1 dB Bandwidth

L1

C2

L3

C4

L5

L6

96 MHz

27 MHz

68 nH

15 pF

220 nH

15 pF

68 nH

150 nH

140 MHz

31 MHz

47 nH

11 pF

150 nH

11 pF

47 nH

82 nH

170 MHz

25 MHz

39 nH

10 pF

120 nH

10 pF

47 nH

51 nH

211 MHz

40 MHz

30 nH

7 pF

100 nH

7.5 pF

30 nH

43 nH