Datasheet

Data Sheet ADA4817-1/ADA4817-2
Rev. B | Page 21 of 28
HIGH SPEED JFET INPUT INSTRUMENTATION
AMPLIFIER
Figure 53 shows an example of a high speed instrumentation
amplifier with a high input impedance using the ADA4817-1/
ADA4817-2. The dc transfer function is
( )








+
−=
G
F
PN
OUT
R
R
VVV
2
1
(16)
For G = 1, it is recommended that the feedback resistors for the
two preamps be set to 0 Ω and the gain resistor be open. The
system bandwidth for G = 1 is 400 MHz. For gains higher than 2,
the bandwidth is set by the preamp, and it can be approximated by
In-amp
−3 dB
= (f
CR
× R
G
)/(2 × R
F
)
Common-mode rejection of the in-amp is primarily determined by
the match of resistor ratios, R1:R2 to R3:R4. It can be estimated by
( )
( )
211
21
δδ+
δ−δ
=
CM
O
V
V
(17)
The summing junction impedance for the preamps is equal
to R
F
|| 0.5(R
G
). Keep this value relatively low to improve the
bandwidth response like in the previous example.
V
CC
V
EE
10µF0.1µF
10µF0.1µF
R
S1
V
N
R2
350Ω
V
P
ADA4817-1
V
O
R
G
V
CC
V
EE
V
CC
V
EE
R4
350Ω
R
S2
10µF0.1µF
10µF0.1µF
R1
350Ω
R3
350Ω
R
F
= 500Ω
R
F
= 500Ω
10µF0.1µF
10µF0.1µF
07756-050
ADA4817-2
U1
ADA4817-2
U2
Figure 53. High Speed Instrumentation Amplifier