Circuit Note

CN-0171

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced in this Circuit Note

ADL5535/

ADL5536

20 MHz to 1.0 GHz 16 dB/20 dB IF Gain

Block

AD9268

16-Bit, 80 MSPS/105 MSPS/125 MSPS,

1.8 V Dual Analog-to-Digital Converter

Low Noise, Low Distortion Single-Ended Input Drive Circuit for Differential Input IF

Sampling ADCs

Rev. 0

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CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 uses the ADL5535/ADL5536

single-ended IF low noise 50 Ω gain block to drive the AD9268

16-bit differential input analog-to-digital converter (ADC).

The circuit includes an interstage bandpass filter for noise

reduction and anti-aliasing. The use of a single-ended IF gain

stage followed by a transformer to perform the single-ended-

to-differential conversion is an optimum solution for this

application where both low noise and low distortion are required.

The ADL5535/ADL5536 is a high linearity (third order output

intercept, OIP3 = +45 dBm at 190 MHz), single-ended, fixed gain

amplifier that can be used as a driver for high performance IF

sampling of analog-to-digital converters. The ADL5535 has a gain

of 16 dB and provides a simple approach to raise the signal from

approximately 400 mV p-p to the 2 V p-p full-scale level required

by the ADC. The ADL5535 low noise figure (3.2 dB at 190 MHz)

and low distortion ensure that the ADC performance is not

compromised. The ADL5536 can be used where a gain of 20 dB is

required.

CIRCUIT DESCRIPTION

Figure 1 shows the schematic of the ADL5535/ADL5536

driving the AD9268 16-bit ADC clocked at a sample rate of

122.88 MSPS. The ADL5535 has a single-ended input and

output impedance of 50 Ω. A 1:1 impedance transformer

(M/A-COM BA-007159-000000, 4.5 MHz to 3000 MHz), along

with termination resistors and series ferrite beads, is used to

present a 50 Ω load for the anti-aliasing filter interface. The

filter interface between the ADL5535 and the AD9268 is a

sixth-order Butterworth low-pass filter designed using a

standard filter program. The interface provides a 50 MHz, 1 dB

bandwidth centered around 175 MHz. Following the sixth-

order filter, a shunt LC tank circuit (72 nH, 8.2 pF) was inserted

to further reduce the low frequency response of the filter, giving

more of a band-pass response to the filter. The normalized

wideband response is shown in Figure 2.

AD9268

ADL5535/

ADL5536

50

Ω

470nH

22pF

22nH

56nH

8pF

27pF

100pF

5V

68nH

8.2pF

72nH

0.1µF

0.1µF

0.1µF

33

Ω

33

Ω

M/A-COM

BA-007159-000000

1

FERRITE BEAD

10

Ω AT 100MHz*

FERRITE BEAD

10

Ω AT 100MHz*

1.8V

1.8V

66

Ω

66

Ω

*MURATA BLM18BA100SN1

AVDD

AGND

VCM

DRVDD

VIN – A

VIN + A

Figure 1. Schematic of the ADL5535 Driving the AD9268 16-Bit ADC, (Simplified Schematic: All Connections and Decoupling Not Shown)