adl5561acpz-wp Analog Devices, Inc., adl5561acpz-wp Datasheet - Page 16

• Manufacturer Part Number: adl5561acpz-wp
• Description: 2.9 Ghz Ultralow Distortion Rf/if Differential Amplifier
• Manufacturer: Analog Devices, Inc.

ADL5561
The single-ended gain can be determined using the following
formula. The values of R
are shown in Table 7.
Table 7. Values of R
Gain (dB)
5.6
11.1
14.1
1
GAIN ADJUSTMENT AND INTERFACING
The effective gain of the ADL5561 can be reduced using a number
of techniques. A matched attenuator network can reduce the
effective gain, but this requires the addition of a separate
component that can be prohibitive in size and cost. Instead, a
simple voltage divider can be implemented using the combin-
ation of an addition series resistors at the amplifier input and
the input impedance of the ADL5561, as shown in Figure 36.
A shunt resistor is used to match to the impedance of the
previous stage.
Figure 36 shows a typical implementation of the divider concept
that effectively reduces the gain by adding attenuation at the
input. For frequencies less than 100 MHz, the input impedance
of the ADL5561 can be modeled as a real 133 Ω, 200 Ω, or
400 Ω resistance (differential) for maximum, middle, and
minimum gains, respectively. Assuming that the frequency is
low enough to ignore the shunt reactance of the input and high
enough so that the reactance of moderately sized ac-coupling
capacitors can be considered negligible, the insertion loss, Il,
due to the shunt divider can be expressed as
The necessary shunt component, R
impedance, R
The insertion loss and the resultant power gain for multiple
shunt resistor values are summarized in Table 8. The source
resistance and input impedance need careful attention when
These values are based on a 50 Ω output match.
1
1
/
/
AC
A
2
2
Il
R
V
R
R
(
SHUNT
1
S
S
dB
=
)
R
Figure 36. Gain Adjustment Using Series Resistor
=
N
=
1
1
S
+
20
, can be expressed as
/
/
2
2
⎛
⎜
⎜
⎝
R
R
R
400
1
R
R
log
SHUNT
SHUNT
s
S
S
×
+
−
⎛
⎜
⎜
⎝
R
IN
R
R
2
R
2
and R
SERIES
⎞
⎟
⎟
⎠
SERIES
IN
R
200
100
66.7
×
1
IN
0.1 µF
0.1µF
R
and R
R
(Ω)
IN
S
1
R
+
X
+
+
2
for Single-Ended Gain
R
R
1
1
/
/
2
R
2
2
IN
X
×
R
R
IN
for each gain configuration
SERIES
SERIES
⎞
⎟
⎟
⎠
R
SHUNT
X
R
+
X
R
, to match to the source
S
×
VIN1
VIN2
VIP1
VIP2
10
R
R2 || 307
R2 || 179
R2 || 132
X
R
+
L
(Ω)
R
ADL5561
L
1
1
1
Rev. 0 | Page 16 of 24
(2)
(3)
(4)
using Equation 3 and Equation 4. The reactance of the input
impedance of the ADL5561 and the ac-coupling capacitors
must be con-sidered before assuming they make a negligible
contribution.
Table 8. Gain Adjustment Using Series Resistors
Il (dB)
2
4
2
4
2
2
4
2
4
2
4
2
ADC INTERFACING
The ADL5561 is a high output linearity amplifier that is
optimized for ADC interfacing. There are several options
available to the designer when using the ADL5561. Figure 37
shows a simplified wideband interface with the ADL5561
driving a AD9445. The AD9445 is a 14-bit, 125 MSPS ADC
with a buffered wideband input.
For optimum performance, the ADL5561 should be driven
differentially using an input balun. Figure 37 uses a wideband
1:1 transmission line balun followed by two 34.8 Ω resistors in
parallel with the three input impedances (which change with
the gain selection of the AD5561) to provide a 50 Ω differential
input impedance. This provides a wideband match to a 50 Ω
source. The ADL5561 is ac coupled from the AD9445 to avoid
common-mode dc loading. The 33 Ω series resistors help to
improve the isolation between the ADL5561 and any switching
currents present at the analog-to-digital sample and hold input
circuitry. The AD9445 input presents a 2 kΩ differential load
impedance and requires a 2 V p-p differential input swing to
reach full scale (VREF = 1 V).
AC
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 indicated in
Figure 38 and Figure 39.
50
Ω
Figure 37. Wideband ADC Interfacing Example Featuring the AD9445
ET
C1-1-13
R
400
400
200
200
133
400
400
200
200
400
400
200
34.8Ω
34.8Ω
IN
(Ω)
0.1 µF
0.1µF
R
50
50
50
50
50
200
200
200
200
50
50
50
A
B
S
(Ω)
VIP2
VIP1
VIN1
VIN2
ADL5561
3.3V
R
105
232
51.1
115
34.8
102
232
51.1
115
105
232
51.1
VOP
VON
SERIES
(Ω)
0.1µF
0.1µF
33Ω
33Ω
R
54.9
54.9
61.9
59
71.5
332
294
976
549
54.9
54.9
61.9
SHUNT
14-BIT ADC
AD9445
VIN+
VIN–
(Ω)
14