ADL5201

Data Sheet

Rev. C | Page 20 of 26

An alternative narrow-band approach is presented in Figure 58.

By designing a narrow band-pass antialiasing filter between the

ADL5201 and the target ADC, the output noise of the ADL5201

outside 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 a reasonable order

antialiasing filter is included. In this example, a low loss 1:3

input transformer is used to match the 150 Ω balanced input

of the ADL5201 to a 50 Ω unbalanced source, resulting in

minimum insertion loss at the input.

Figure 58 shows the ADL5201 optimized for driving some of

the popular unbuffered Analog Devices ADCs: 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 looks 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 fre-

quency 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 ADL5201 amplifier 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. To minimize the parasitic capacitance, a good

practice is to avoid any ground or power planes under this

routing region and under the chokes.

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

75Ω

75Ω

1µH

5V

1µH

5V

CML

C4

5V

1nF

1nF

1nF

L1

L3

L5

L1

L3

L5

1nF

DIGITAL

INTERFACE

ADL5201

C2

AD9246

AD9640

AD6655

Figure 58. Narrow-Band IF Sampling Solution for Unbuffered ADC Applications

Table 8. Interface Filter Recommendations for Various IF Sampling Center Frequencies

Center Frequency

(MHz)

1 dB Bandwidth

(MHz)

L1 (nH)

C2 (pF)

L3 (nH)

C4 (pF)

L5 (nH)

L6 (nH)

96

27

68

15

220

15

68

150

140

31

47

11

150

11

47

82

170

25

39

10

120

10

47

51

211

40

30

7

100

7.5

30

43