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ADL5562 Datasheet PDF : 21 Pages
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ADL5562
Data Sheet
The single-ended gain can be determined using the following
formula. The values of RIN and RX for each gain configuration
are shown in Table 7.
AV1 =
400
× R2 × RX + RS × RL
RIN
+
RS
RS
× R2
+ R2
RS + R2
RX
10 + RL
(2)
Table 7. Values of RIN and RX for Single-Ended Gain
Gain (dB)
5.6
11.1
RIN (Ω)
200
100
RX (Ω)
R2 || 3071
R2 || 1791
14.1
66.7
R2 || 1321
1 These values based on a 50 Ω input match.
GAIN ADJUSTMENT AND INTERFACING
The effective gain of the ADL5562 can be reduced using a number
of techniques. A matched attenuator network can reduce the
effective gain; however, 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 additional series resistors at the amplifier input and the input
impedance of the ADL5562, as shown in Figure 38. A shunt
resistor is used to match to the impedance of the previous stage.
1/2 RS
AC
1/2 RS
1/2 RSHUNT
1/2 RSHUNT
0.1µF 1/2 RSERIES
0.1µF 1/2 RSERIES
VIN1
VIN2
VIP1 ADL5562
VIP2
Figure 38. Gain Adjustment Using a Series Resistor
Figure 38 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 ADL5562 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
Il(dB)
=
20 log
RIN
RSERIES +
RIN
(3)
The necessary shunt component, RSHUNT, to match to the source
impedance, RS, can be expressed as
RSHUNT = 1 −
1
1
(4)
RS RSERIES + RIN
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 ADL5562 and the ac coupling capacitors must be considered
before assuming that they make a negligible contribution.
Table 8. Gain Adjustment Using Series Resistor
Il (dB)
RIN (Ω)
RS (Ω)
RSERIES (Ω)
RSHUNT (Ω)
2
400
50
105
54.9
4
400
50
232
54.9
2
200
50
51.1
61.9
4
200
50
115
59
2
133
50
34.8
71.5
2
400
200
102
332
4
400
200
232
294
2
200
200
51.1
976
4
200
200
115
549
2
400
50
105
54.9
4
400
50
232
54.9
2
200
50
51.1
61.9
ADC INTERFACING
The ADL5562 is a high output linearity amplifier that is optimized
for ADC interfacing. There are several options available to the
designer when using the ADL5562. Figure 39 shows a simplified
wideband interface with the ADL5562 driving the AD9445. The
AD9445 is a 14-bit, 125 MSPS ADC with a buffered wideband input.
For optimum performance, drive the ADL5562 differentially
using an input balun. Figure 39 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
ADL5562) to provide a 50 Ω differential input impedance. This
provides a wideband match to a 50 Ω source. The ADL5562 is
ac-coupled from the AD9445 to avoid common-mode dc loading.
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. 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).
3.3V
ETC1-1-13
0.1µF A VIP2
VOP 0.1µF
33Ω
50Ω
34.8Ω
VIP1
VIN+
AD9445
14
AC
0.1µF B VIN1 ADL5562 0.1µF
33Ω 14-BIT ADC
VIN–
34.8Ω
VIN2
VON
Figure 39. Wideband ADC Interfacing Example Featuring the AD9445
Rev. F | Page 16 of 21
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