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AN-1204
APPLICATION NOTE
One Technology Wa y • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Using the ADL5562 Differential Amplifier to Drive Wide Bandwidth ADCs
for High IF AC-Coupled Applications
Rev. A | Page 1 of 3
CIRCUIT FUNCTION AND BENEFITS
This circuit provides high performance, high frequency sampling
using the ADL5562, a high performance, differential, low noise,
ultralow distortion, high output linearity, pin-strappable gain
amplifier, and high speed ADCs. The ADL5562 is optimized for
driving high frequency IF sampling ADCs. When coupled with
a high speed ADC like the AD9445, AD9246, or AD6655, it
provides exceptional SFDR performance beyond 100 MSPS at
its maximum gain.
CIRCUIT DESCRIPTION
Table 1. Devices Connected/Referenced
Product
Description
ADL5562
3.3 GHz ultralow distortion RF/IF differential
amplifier
AD9445
14-bit, 105 MSPS/125 MSPS analog-to-digital
converter
This circuit employs the ADL5562 high output linearity amplifier
to provide variable gain, isolation, and source matching to a
high speed ADC like the AD9445. Using this circuit with the
ADL5562 in a gain of 6 dB (minimum gain), an SFDR perfor-
mance of 84 dBc is achieved with an input signal of 140 MHz
sampled at 125 MSPS, as indicated in Figure 2.
The ADL5562 should be driven differentially (for optimal
performance) by a wideband 1:1 transmission line balun (or
impedance transformer) followed by two 34.8 Ω resistors in
parallel with the input impedance of the ADL5562. This
provides a wideband match to a 50 Ω source as depicted in
Figure 1. Note that the ADL5562 has different input imped-
ances for each gain setting (400 Ω, 200 Ω, and 133 Ω for gain
settings of 6 dB, 12 dB, and 15.5 dB, respectively). The 34.8 Ω
resistors provide an optimum match for a gain of 12 dB, but the
match is good enough to use the same values for a gain of 6 dB
or 15.5 dB. Figure 1 shows the ADL5562 connected for a gain of
6 dB. The outputs of the ADL5562 are ac-coupled to avoid
common-mode dc loading and to allow the amplifier to be
biased to an internally generated mid-supply level. The 33 Ω
series resistors help to improve the isolation between the
ADL5562 and any switching currents present at the analog-
to-digital sample-and-hold input circuitry.
There are several configuration options available to the designer
when using the ADL5562. Figure 1 shows a simplified wideband
interface with the ADL5562 driving an AD9445. The AD9445 is
a 14-bit, 105 MSPS/125 MSPS analog-to-digital converter with a
buffered wideband input, which presents a 2 kΩ||3 pF differential
load impedance and requires a 2 V p-p differential input swing
to reach full scale. This circuit provides variable gain, isolation,
and source matching for the AD9445. Using this circuit with the
ADL5562 in a gain of 6 dB, the wideband system response of
Figure 3 is obtained, which has a 3 dB bandwidth of approximately
700 MHz. The wideband frequency response is an advantage in
broadband 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 first Nyquist zone.
0.1µF
34.8Ω
50Ω
AC
0.1
µF
3.3V
M/A-COM
ETC1-1-13
VIN1
VIP1
VIP2
A
B
VIN2
34.8Ω
ADL5562
0.1µF
0.1
µF
33Ω
VOP
VON
33Ω
AD9445
14-BIT ADC
14
VIN+
VIN–
08397-001
Figure 1. Wideband ADC Interfacing Example Featuring the ADL5562 and the AD9445
(Simplified Schematic: Decoupling and All Connections Not Shown)
