AD8350 データシートの表示（PDF） - Analog Devices
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AD8350 Datasheet PDF : 16 Pages
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AD8350
20
18
AD8350-20
16
14
12
10
AD8350-15
8
6
4
2
0
0
500
1000
1500
2000
RFEXT – ⍀
Figure 11. Power Gain vs. External Feedback Resistors
for the AD8350-15 and AD8350-20 with RS = 100 Ω and
RL = 100 Ω
The power gain of any two-port network is dependent on the
source and load impedance. The effective gain will change if the
differential source and load impedance is not 200 Ω. The single-
ended input and output resistance of the AD8350 can be modeled
using the following equations:
RIN
=
RF
RF + RL
RINT
+ RL
+1+ gm
× RL
(4)
and
ROUT =
RF +
1
1
+
1
RS RINT
≈ RF + RS
1 + gm × RS
for RS ≤ 1 kΩ
1+ gm ×
1
1
+
1
(5)
RS RINT
where
RF
= RFEXT//RFINT
RFEXT = R Feedback External
RFINT = 662 Ω for the AD8350-15
= 1100 Ω for the AD8350-20
RINT = 25000 Ω
gm = 0.066 mhos for the AD8350-15
= 0.110 mhos for the AD8350-20
RS
RL
RIN
ROUT
= R Source (Single-Ended)
= R Load (Single-Ended)
= R Input (Single-Ended)
= R Output (Single-Ended)
The resultant single-ended gain can be calculated using the
following equation:
( ) GV
=
RL
RL × gm × RF − 1
+ RS + RF + RL × RS
× gm
(6)
Driving Lighter Loads
It is not necessary to load the output of the AD8350 with a
200 Ω differential load. Often it is desirable to try to achieve a
complex conjugate match between the source and load in order
to minimize reflections and conserve power. But if the AD8350
is driving a voltage responding device, such as an ADC, it is no
longer necessary to maximize power transfer. The harmonic
distortion performance will actually improve when driving
loads greater than 200 Ω. The lighter load requires less cur-
rent driving capability on the output stages of the AD8350
resulting in improved linearity. Figure 12 shows the improve-
ment in second and third harmonic distortion for increasing
differential load resistance.
–66
–68
–70
HD3
–72
–74
–76
–78
HD2
–80
–82
200 300 400
500 600 700 800
900 1000
RLOAD – ⍀
Figure 12. Second and Third Harmonic Distortion vs.
Differential Load Resistance for the AD8350-15 with
VS = 5 V, f = 70 MHz, and VOUT = 1 V p-p
REV. A
–11–
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