
AD8065/AD8066
Rev. E | Page 24 of 28
V
CC
V
EE
1/2
AD8066
4.7
μ
F
0.1
μ
F
R
S1
4.7
μ
F
0.1
μ
F
V
N
2.2pF
500
R2
V
P
1/2
AD8066
4.7
μ
F
0.1
μ
F
4.7
μ
F
0.1
μ
F
AD8065
4.7
μ
F
0.1
μ
F
4.7
μ
F
0.1
μ
F
V
O
R
G
V
CC
V
EE
V
CC
V
EE
500
R4
R
S2
500
R1
500
R3
R
F
= 500
2.2pF
R
F
= 500
0
Figure 60. High Speed Instrumentation Amplifier
HIGH SPEED JFET INPUT INSTRUMENTATION
AMPLIFIER
Figure 60 shows an example of a high speed instrumentation
amplifier with high input impedance using the
AD8065/AD8066. The dc transfer function is
(
)
1
=
G
P
N
OUT
V
R
V
V
1000
For G = +1, it is recommended that the feedback resistors for
the two preamps be set to a low value (for instance 50 for 50
source impedance). The bandwidth for G = +1 will be 50
MHz. For higher gains, the bandwidth will be set by the preamp,
equaling
(
G
CR
3dB
R
f
Inamp
×
×
=
2
) (
)
F
R
Common-mode rejection of the inamp will be primarily
determined by the match of the resistor ratios R1:R2 to R3:R4. It
can be estimated
(
1
)
(
)
2
1
δ
δ
2
1
+
δ
δ
=
CM
O
V
V
The summing junction impedance for the preamps is equal to
R
F
|| 0.5(
R
G
). This is the value to be used for matching purposes.
VIDEO BUFFER
The output current capability and speed of the AD8065 make it
useful as a video buffer, shown in Figure 61.
The G = +2 configuration compensates for the voltage division
of the signal due to the signal termination. This buffer
maintains 0.1 dB flatness for signals up to 7 MHz, from low
amplitudes up to 2 V p-p (Figure 7). Differential gain and phase
have been measured to be 0.02% and 0.028° at ±5 V supplies.
+V
S
–V
S
4.7
μ
F
0.1
μ
F
2.2pF
499
249
75
499
V
I
–
AD8065
4.7
μ
F
0.1
μ
F
75
V
O
–
+
+
0
Figure 61. Video Buffer