
AD8065/AD8066
The closed-loop bandwidth is inversely proportional to the
noise gain of the op amp circuit, (
R
F
+
R
G
)/
R
G
. This simple
model is accurate for noise gains above 2. The actual bandwidth
of circuits with noise gains at or below 2 will be higher than
those predicted with this model due to the influence of other
poles in the frequency response of the real op amp.
Rev. E | Page 19 of 28
V
O
R
F
A
R
G
V
I
I
b
–
R
S
I
b
+
+V
OS
–
0
Figure 54. Voltage Feedback Amplifier DC Errors
Figure 54 shows a voltage feedback amplifier’s dc errors. For
both inverting and noninverting configurations
(
)
+
+
×
+
×
=
+
G
F
G
OS
F
b
G
F
G
S
b
O
R
R
R
V
R
I
R
R
R
R
I
error
V
The voltage error due to
I
b+
and
I
b–
is minimized if
R
S
=
R
F
||
R
G
(though with the AD8065 input currents at less than 20 pA over
temperature, this is likely not a concern). To include common-
mode and power supply rejection effects, total
V
OS
can be
modeled as
CMR
V
PSR
V
V
V
CM
S
nom
OS
OS
Δ
Δ
+
+
=
nom
OS
V
V
S
is the change in power supply from nominal conditions,
PSR
is the power supply rejection,
V
CM
is the change in
common-mode voltage from nominal conditions, and
CMR
is the common-mode rejection.
is the offset voltage specified at nominal conditions,
WIDEBAND OPERATION
Figure 42 through Figure 44 show the circuits used for
wideband characterization for gains of +1, +2, and 1. Source
impedance at the summing junction (
R
F
||
R
G
) will form a pole
in the amplifier’s loop response with the amplifier’s input
capacitance of 6.6 pF. This can cause peaking and ringing if the
time constant formed is too low. Feedback resistances of 300
to 1 k are recommended, since they will not unduly load
down the amplifier and the time constant formed will not be
too low. Peaking in the frequency response can be compensated
for with a small capacitor (C
F
) in parallel with the feedback
resistor, as illustrated in Figure 12. This shows the effect of
different feedback capacitances on the peaking and bandwidth
for a noninverting G = +2 amplifier.
For the best settling times and the best distortion, the
impedances at the AD8065/AD8066 input terminals should be
matched. This minimizes nonlinear common-mode capacitive
effects that can degrade ac performance.
Actual distortion performance depends on a number of
variables:
The closed-loop gain of the application
Whether it is inverting or noninverting
Amplifier loading
Signal frequency and amplitude
Board layout
Also see Figure 16 to Figure 20. The lowest distortion will be
obtained with the AD8065 used in low gain inverting appli-
cations, since this eliminates common-mode effects. Higher
closed-loop gains result in worse distortion performance.
INPUT PROTECTION
The inputs of the AD8065/AD8066 are protected with back-to-
back diodes between the input terminals as well as ESD diodes
to either power supply. This results in an input stage with
picoamps of input current that can withstand up to 1500 V ESD
events (human body model) with no degradation.
Excessive power dissipation through the protection devices will
destroy or degrade the performance of the amplifier. Differ-
ential voltages greater than 0.7 V will result in an input current
of approximately (|
V
+
V
| 0.7 V)/
R
I
, where
R
I
is the resistance
in series with the inputs. For input voltages beyond the positive
supply, the input current will be approximately (
V
I
V
CC
0.7)/
R
I
. Beyond the negative supply, the input current will be
about (
V
I
V
EE
+ 0.7)/
R
I
. If the inputs of the amplifier are to be
subjected to sustained differential voltages greater than 0.7 V or
to input voltages beyond the amplifier power supply, input
current should be limited to 30 mA by an appropriately sized
input resistor (
R
I
) as shown in Figure 55.
R
I
V
I
V
O
AD8065
R
I
>
(| V+– V
–
| – 0.7V)
30mA
FOR LARGE | V
+
– V
–
|
R
I
>
(V
I
– V
EE
– 0.7V)
30mA
R
I
>
(V
I
– V
EE
+ 0.7V)
30mA
FOR V
BEYOND
SUPPLY VOLTAGES
0
Figure 55. Current Limiting Resistor