Datasheet

AD8065/AD8066
Rev. J | Page 25 of 28
Table 5. RMS Noise Contributions of Photodiode Preamp
Contributor Expression RMS Noise with R
F
= 50 kΩ, C
S
= 15 pF, C
F
= 2 pF
R
F
(×2)
57142 .fRkT
2
F
××××
64.5 μV
Amp to f
1
1
fVEN ×
2.4 μV
Amp (f
2
– f
1
)
12
F
DFM
S
ff
C
CCCC
VEN −×
+++
×
2
31 μV
Amp to (past f
2
)
57.1
2
××
+++
×
3
F
FDM
S
f
C
CCCC
VEN
260 μV
270 μV (Total)
V
CC
V
EE
1
/
2
AD8066
4.7μF
0.1μF
R
S1
4.7μF
0.1μF
V
N
2.2pF
500Ω
R2
V
P
1
/
2
AD8066
4.7μF
0.1μ F
4.7μF
0.1μ F
AD8065
4.7μF
0.1μF
4.7μ F
0.1μ F
V
O
R
G
V
CC
V
EE
V
CC
V
EE
500Ω
R4
R
S2
500Ω
R1
500Ω
R3
R
F
= 500Ω
2.2pF
R
F
= 500Ω
02916-E-060
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
()
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+−=
G
PN
OUT
R
VVV
1000
1
For G = +1, it is recommended that the feedback resistors for
the two preamps be set to a low value (for instance 50 Ω for
50 Ω source impedance). The bandwidth for G = +1 is 50 MHz.
For higher gains, the bandwidth is set by the preamp, equaling
(
)
(
)
F
GCR
3dB
RRfInamp ××=
−
2/
Common-mode rejection of the in-amp is primarily
determined by the match of the resistor ratios R1:R2 to R3:R4.
It can be estimated
()
()
211
21
δδ+
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.