AD8350 查看數據表（PDF） - Analog Devices
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AD8350 Datasheet PDF : 16 Pages
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AD8350
CAC
CAC
RS
RSHUNT
VS
8765
RL
AD8350
–
RS
RSHUNT
RL
1234
CAC
CAC
ENBL (5V)
0.1F
+VS (5V TO 10V)
Figure 8. Gain Reduction Using Shunt Resistor
CAC
RFEXT
CAC
The insertion loss and the resultant power gain for multiple
shunt resistor values is summarized in Table I. The source
resistance and input impedance need careful attention when
using Equation 1. The reactance of the input impedance of the
AD8350 and the ac-coupling capacitors need to be considered
before assuming they have negligible contribution. Figure 10
shows the effective power gain for multiple values of RSHUNT for
the AD8350-15 and AD8350-20.
Table I. Gain Adjustment Using Shunt Resistor,
RS = 100 ⍀ and RIN = 100 ⍀ Single-Ended
RSHUNT–⍀
50
100
200
300
400
IL–dB
6.02
3.52
1.94
1.34
1.02
Power Gain–dB
AD8350-15
AD8350-20
8.98
11.48
13.06
13.66
13.98
13.98
16.48
18.06
18.66
18.98
RS
8765
RL
AD8350
VS
–
RS
RL
1234
CAC
ENBL
0.1F
CAC
(5V)
+VS
(5V TO 10V)
RFEXT
Figure 9. Dynamic Gain Reduction
Figure 8 shows a typical implementation of the shunt divider
concept. The reduced input impedance that results from the
parallel combination of the shunt resistor and the input impedance
of the AD8350 adds attenuation to the input signal effectively
reducing the gain. For frequencies less than 100 MHz, the input
impedance of the AD8350 can be modeled as a real 200 Ω resis-
tance (differential). Assuming the frequency is low enough to
ignore the shunt reactance of the input, and high enough such
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:
RIN
IL
(dB)
=
20
×
Log10
(RIN + RS )
RIN ʈRSHUNT
(RIN ʈRSHUNT + RS )
where
(3)
RIN ʈRSHUNT
=
RIN
RIN
× RSHUNT
+ RSHUNT
and RIN
= 100Ω single−ended
20
18
16
AD8350-20
14
12
AD8350-15
10
8
6
4
2
0
0
100 200 300 400 500 600 700 800
RSHUNT – ⍀
Figure 10. Gain for Multiple Values of Shunt Resistance
for Circuit in Figure 8
The gain can be adjusted dynamically by employing external
feedback resistors as shown in Figure 9. The effective attenua-
tion is a result of the lowered input impedance as with the shunt
resistor method, yet there is no additional noise contribution at
the input of the device. It is necessary to use well-matched resistors
to minimize common-mode offset errors. Quality 1% tolerance
resistors should be used along with a symmetric board layout to
help guarantee balanced performance. The effective gain for mul-
tiple values of external feedback resistors is shown in Figure 11.
–10–
REV. A
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