AD8350AR20 Analog Devices Inc, AD8350AR20 Datasheet - Page 10

• Manufacturer Part Number: AD8350AR20
• Description: Buffer Amplifier IC
• Manufacturer: Analog Devices Inc

Specifications of AD8350AR20

• Bandwidth: 900MHz
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC
• Rohs Status: RoHS non-compliant
• Current - Supply: 28mA
• Frequency: 0Hz ~ 1GHz
• Gain: 20dB
• Noise Figure: 5.9dB
• Rf Type: CATV
• Test Frequency: 250MHz
• Voltage - Supply: 5 V ~ 10 V
• P1db: -

AD8350
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:
IL dB
where
R
IN
(
R
SHUNT
)
V
=
R
R
S
V
S
S
R
R
S
20
S
S
=
×
R
R
Log
R
R
SHUNT
SHUNT
C
C
AC
AC
IN
IN
10
×
+






R
R
(
ENBL (5V)
R
SHUNT
SHUNT
C
C
IN
AC
AC
R
(
ENBL
R
(5V TO 10V)
IN
(5V)
R
R
IN
FEXT
SHUNT
8
1
R
and
R
8
1
IN
AD8350
+
SHUNT
R
AD8350
+V
–
7
FEXT
2
+V
R
–
7
2
S
R
S
S
(5V TO 10V)
+
IN
)
6
3
0.1 F
R
6
3
S
=
5
)
4
100
5
4
0.1 F






Ω sin e e
C
C
C
C
AC
AC
AC
AC
R
R
gl
R
R
L
L
L
L
−
nded
(3)
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 R
the AD8350-15 and AD8350-20.
R
50
100
200
300
400
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.
SHUNT
–
20
18
16
14
12
10
Table I. Gain Adjustment Using Shunt Resistor,
R
8
6
4
2
0
S
0
= 100
100
IL–dB
6.02
3.52
1.94
1.34
1.02
and R
200
IN
300
AD8350-20
= 100
AD8350-15
R
SHUNT
AD8350-15
8.98
11.48
13.06
13.66
13.98
400
–
Single-Ended
Power Gain–dB
500
600
700
AD8350-20
13.98
16.48
18.06
18.66
18.98
SHUNT
800
for