ADL5561-EVALZ AD [Analog Devices], ADL5561-EVALZ Datasheet - Page 16

• Manufacturer Part Number: ADL5561-EVALZ
• Description: 2.9 GHz Ultralow Distortion RF/IF Differential Amplifier
• Manufacturer: AD [Analog Devices]

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 combination
of an addition series resistor 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
These values are based on a 50 Ω output match.
1
1
/
/
AC
A
2
2
Il
R
R
(
V
S
S
dB
1
=
)
=
R
Figure 36. Gain Adjustment Using Series Resistor
IN
20
1
1
/
/
2
2
+
R
R
log
SHUNT
SHUNT
⎛
⎜
⎜
⎝
400
R
⎛
⎜
⎜
⎝
R
S
IN
R
S
×
+
SERIES
and R
R
R
IN
R
200
100
66.7
R
2
2
IN
0.1 µF
0.1µF
and R
IN
⎞
⎟
⎟
⎠
(Ω)
+
×
X
R
for Single-Ended Gain
AC
R
50Ω
1
1
IN
/
/
S
2
2
X
R
R
R
⎞
⎟
⎟
⎠
+
for each gain configuration
SERIES
SERIES
2
R
2
ETC1-1-13
×
Figure 37. Wideband ADC Interfacing Example Featuring the AD9445
R
X
R
VIN1
VIN2
VIP1
VIP2
+
R
R2 || 307
R2 || 179
R2 || 132
X
X
R
34.8Ω
34.8Ω
(Ω)
S
ADL5561
×
10
1
1
1
R
+
L
0.1 µF
0.1µF
R
L
A
B
Rev. B | Page 16 of 24
(2)
(3)
VIN1
VIN2
VIP2
VIP1
ADL5561
3.3V
VON
VOP
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 using Equation 3
and Equation 4. The reactance of the input impedance of the
ADL5561 and the ac-coupling capacitors must be considered
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 the 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 ADL5561) 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).
R
0.1µF
0.1µF
SHUNT
R
400
400
200
200
133
400
400
200
200
400
400
200
IN
=
S
, can be expressed as
(Ω)
33Ω
33Ω
R
1
S
−
14-BIT ADC
AD9445
R
VIN+
VIN–
SERIES
R
50
50
50
50
50
200
200
200
200
50
50
50
1
S
(Ω)
1
+
14
R
IN
R
105
232
51.1
115
34.8
102
232
51.1
115
105
232
51.1
SHUNT
SERIES
, to match to the source
(Ω)
R
54.9
54.9
61.9
59
71.5
332
294
976
549
54.9
54.9
61.9
SHUNT
(Ω)
(4)