CN-0171 Datasheet, PDF (1/3 Pages) Analog Devices – Low Noise, Low Distortion Single-Ended Input Drive Circuit for Differential Input IF Sampling ADCs

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Circuit Note
CN-0171
Devices Connected/Referenced in this Circuit Note
ADL5535/ 20 MHz to 1.0 GHz 16 dB/20 dB IF Gain
ADL5536 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
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
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.
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.
5V
470nH
100pF 22nH
50Ω
ADL5535/
ADL5536
68nH
22pF
56nH
27pF
M/A-COM
BA-007159-000000 33Ω
1
FERRITE BEAD
10Ω AT 100MHz* 66Ω
8pF
72nH
8.2pF
0.1µF
0.1µF
33Ω
FERRITE BEAD 66Ω
10Ω AT 100MHz*
1.8V 1.8V
AVDD DRVDD
VIN + A
AD9268
VIN – A
AGND VCM
0.1µF
*MURATA BLM18BA100SN1
Figure 1. Schematic of the ADL5535 Driving the AD9268 16-Bit ADC, (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0
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