Datasheet

OPA653
R
T
I
BI
I
BN
e
N
e
O
R
G
=160 W
R
F
=160 W
4kTR
T
4kTR
F
4kT
R
G
V
IN+
V
IN-
V
OUT
e =
O
4kTR +(I R ) +e
T BN T N
2 2
+(I R ) +4kTR
BI F F
2
1+
R
R
F
G
1+
R
R
F
G
2
[
]
OPA653
V
LOAD
V
OUT
160 W
160 W
V
IN+
V
IN-
R
ISO
C
LOAD
R
LOAD
e =
NI
4kTR +(I R ) +e
T BN T N
+
2 2
+
4kTR
2
F
I R
2
BI F
2
OPA653
SBOS348A –DECEMBER 2008–REVISED NOVEMBER 2009 www.ti.com
OPERATING SUGGESTIONS
However, attention should be paid to the value of R
T
or other source impedance on the noninverting input.
Setting Resistor Values to Minimize Noise
High-value resistive impedance on the noninverting
The OPA653 provides a low input noise voltage.
input can add significant noise; for example, 2.4 kΩ
Figure 23 shows the op amp noise analysis model
adds a Johnson voltage noise term equal to the
with all the noise terms included. In this model, all the
amplifier itself (6.2 nV/√Hz). So while the JFET input
noise terms are taken to be noise voltage or current
of the OPA653 is ideal for high source impedance
density terms in either nV/√Hz or pA/√Hz.
applications in the noninverting configuration of
Figure 21, the overall bandwidth and noise are limited
by high source impedances.
Driving Capacitive Loads
One of the most demanding and yet very common
load conditions for an op amp is capacitive loading.
The OPA653 is very robust, but care should be taken
with light loading scenarios so output capacitance
does not lead to decreased stability, increased
frequency response peaking, overshoot, and ringing.
When the amplifier output resistance is considered,
capacitive loading introduces an additional pole in the
signal path that reduces the phase margin. Several
external solutions to this problem have been
suggested for standard op amps. Because the
OPA653 has internal gain-setting resistors, the only
real option is to use a series output resistor. This
Figure 23. Noise Analysis Circuit
option is a good solution because when the primary
considerations are frequency response flatness,
The total output spot noise voltage can be computed
pulse response fidelity, and/or distortion, a series
as the square root of the squared contributing terms
output resistor is the simplest and most effective
to the output noise voltage. This calculation adds all
technique. The idea is to isolate the capacitive load
the contributing noise powers at the output by
from the feedback loop by inserting a series isolation
superposition, then takes the square root to return to
resistor, R
ISO
, between the amplifier output and the
a spot noise voltage. Equation 1 shows the general
capacitive load as shown in Figure 24 below. In
form for this output noise voltage using the terms
effect, this configuration isolates the phase shift from
shown in Figure 23.
the loop gain of the amplifier, thus restoring the
phase margin and improving stability.
(1)
Dividing this expression by the noise gain = 1 +
R
F
/R
G
gives the equivalent input-referred spot noise
voltage at the noninverting input as shown in
Equation 2
Figure 24. Adding Series Ouput Resistance to
Isolate Capacitive Loads
(2)
Putting high resistor values into Equation 2 can
quickly dominate the total equivalent input-referred
noise. Because the gain-setting resistors, R
F
and R
G
,
are internal to the device, the user cannot change this
noise contribution, and the noise gain is equal to +2
V/V.
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