ADL5561

Rev. B | Page 17 of 24

This circuit provides variable gain, isolation, and source matching

for the AD9445. Using this circuit with the ADL5561 in a gain

of 6 dB, an SFDR performance of 87 dBc is achieved at 140 MHz

and a −3 dB bandwidth of 760 MHz, as shown in Figure 38

and Figure 39.

0

6.25 12.50 18.75 25.00 31.25 37.50 43.75 50.00 56.25 62.50

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

–140

–150

FREQUENCY (MHz)

ADL5561 DRIVING THE AD9445 14-BIT ADC

GAIN = 6dB

INPUT = 140MHz

SNR = 64.69dBc

SFDR = 87.44dBc

NOISE FLOOR = –107.9dB

FUND = –1.096dBFS

SECOND = –89.64dBc

THIRD = –87.52dBc

Figure 38. Measured Single-Tone Performance of the

Circuit in Figure 37 for a 140 MHz Input Signal

FREQUENCY (MHz)

FIRST POINT = –1.12dBFS

END POINT = –4.38dBFS

MID POINT = –0.81dBFS

MIN = –4.38dBFS

MAX = –0.70dBFS

0

–1

–2

–3

–4

–5

–6

–7

–8

–9

–10

2.00

81.90

241.70

401.50

561.30

721.10

161.80

321.60

481.40

641.20

801.00

Figure 39. Measured Frequency Response of the Wideband

The wideband frequency response is an advantage in broad-

band applications, such as predistortion receiver designs and

instrumentation applications. However, by designing for a wide

analog input frequency range, the cascaded SNR performance

is somewhat degraded due to high frequency noise aliasing into

the wanted Nyquist zone.

An alternative narrow-band approach is presented in Figure 40.

By designing a narrow band-pass antialiasing filter between the

ADL5561 and the target ADC, the output noise of the ADL5561

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 ADL5561 balanced input to a 50 Ω unbalanced

source, resulting in minimum insertion loss at the input.

Figure 40 is optimized for driving some of the Analog Devices

popular unbuffered ADCs, such as the AD9246, AD9640,

and AD6655. Table 9 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. The L5 inductor 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 9 provides initial suggestions for prototyping

purposes. Some empirical optimization may be needed to help

compensate for actual PCB parasitic.

105Ω

L5

105Ω

AD9246

AD9640

AD6655

1nF

L1

C2

L3

1nF

L1

L3

C4

CML

ADL5561

4Ω

4Ω

Figure 40. Narrow-Band IF Sampling Solution for an Unbuffered ADC Application

Table 9. 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)

96

30

3.3

47

27

75

100

140

33

3.3

47

27

33

120

170

32

3.3

56

27

22

110

211

33

3.3

47

27

18

56