ADL5565ACPZ-R7 datasheet(20/29 Pages) AD

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ADL5565

Data Sheet

Rev. D | Page 20 of 28

ADC INTERFACING

The ADL5565 is a high output linearity amplifier that is optimized

for ADC interfacing. There are several options available to the

designer when using the ADL5565. Figure 40 uses a wideband

1:1 transmission line balun followed by two 40 Ω resistors in

parallel with the three input impedances (which change with

the gain selection of the ADL5565) to provide a 50 Ω

differential impedance and provides a wideband match to a

50 Ω source. The ADL5565 is ac-coupled from the AD9467 to

avoid common-mode dc loading. The 33 Ω resistors improve the

isolation between the ADL5565 and any switching currents

present at the analog-to-digital, sample-and-hold circuitry. The

AD9467 input presents a 530 Ω differential load impedance and

requires a 2 V to 2.5 V differential input swing to reach full scale

(VREF = 1 V to 1.25 V). This circuit provides variable gain,

isolation, and source matching for the AD9467.

Applying a full-scale, single-tone signal from the ADL5565, an

SFDR of 91.9 dBc is realized (see Figure 37). Applying two half-

scale signals from the ADL5565 in a gain of 6 dB, an SFDR of

86.4 dBc is achieved at 100 MHz (see Figure 38). The bandwidth

of the circuit in Figure 40 is shown in Figure 39.

3045607590

105

120

GAIN = 6dB

SNR = 69.42dBc

SFDR = 91.9dBc

SECOND = –95.5dBc

THIRD = –96.5dBc

NOISE FLOOR = –115.6dB

FREQUENCY (MHz)

–15

–30

–45

–60

–75

–90

–105

–120

–135

–150

Figure 37. Measured Single-Tone Performance of the

Circuit in Figure 40 for a 100 MHz Input Signal

2F2 – F1

2F1 – F2

F1 – F2

2F2 – F1

2F1 + F2

F1 + F2

3045607590

105

120

FUNDAMENTAL1 = –7.034dBFS

FUNDAMENTAL2 = –7.053dBFS

IMD (2f1 – f2) = –90.677dBc

IMD (2f2 + f1) = –92.101dBc

NOISE FLOOR = –115.2dB

FREQUENCY (MHz)

–15

–30

–45

–60

–75

–90

–105

–120

–135

–150

Figure 38. Measured Two-Tone Performance of the Circuit in Figure 40 for a

100 MHz and 102 MHz Input Signals

–

0100

200

300

400

500

FREQUENCY (MHz)

Figure 39. Measured Frequency Response of the Wideband

ADC Interface Depicted in Figure 40

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.

0.1µF

40Ω

50Ω

0.1

µF

ETC1-1-13

VIN1

VIP1

VIP2

VIN2

40Ω

ADL5565

0.1µF

33Ω

VOP

VON

33Ω

AD9467

16-BIT ADC

VIN+

VIN–

Figure 40. Wideband ADC Interfacing Example Featuring the AD9467

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