參數(shù)資料
型號： OPA211_08
廠商： Texas Instruments, Inc.
英文描述： 1.1nV/☆Hz Noise, Low Power, Precision Operational Amplifier in Small DFN-8 Package
中文描述： 1.1nV /☆赫茲噪聲，低功耗，高精度運算放大器，小DFN - 8封裝
文件頁數(shù)： 14/17頁
文件大小： 407K
代理商： OPA211_08
www.ti.com
INPUT PROTECTION
The input terminals of the OPA211 are protected from
excessive
differential
voltage
diodes, as shown in
Figure 39
. In most circuit
applications, the input protection circuitry has no
consequence. However, in low-gain or G = 1 circuits,
fast ramping input signals can forward bias these
diodes because the output of the amplifier cannot
respond rapidly enough to the input ramp. This effect
is
illustrated
in
Figure
Characteristics. If the input signal is fast enough to
create this forward bias condition, the input signal
current must be limited to 10mA or less. If the input
signal current is not inherently limited, an input series
resistor can be used to limit the signal input current.
This input series resistor degrades the low noise
performance
of
the
OPA211.
Performance
section of this data sheet for further
information on noise calculation.
Figure 39
shows an
example implementing a current-limiting feedback
resistor.
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
100k
10M
Source Resistance, R ( )
100
1k
10k
10k
1k
100
10
1
V
O
R
S
E
O
E
= e
O
n
+ (i R ) + 4kTR
S
2
2
Resistor Noise
OPA227
OPA211
BASIC NOISE CALCULATIONS
Design of low-noise op amp circuits requires careful
consideration
of
a
variety
contributors: noise from the signal source, noise
generated in the op amp, and noise from the
feedback network resistors. The total noise of the
circuit is the root-sum-square combination of all noise
components.
OPA211
Output
R
F
Input
-
+
R
I
NOISE PERFORMANCE
Figure 40
shows total circuit noise for varying source
impedances
with
the
op
configuration (no feedback resistor network, and
therefore no additional noise contributions). Two
different op amps are shown with total circuit noise
calculated. The OPA211 has very low voltage noise,
making it ideal for low source impedances (less than
2k
). A similar precision op amp, the
OPA227
, has
somewhat higher voltage noise but lower current
noise. It provides excellent noise performance at
moderate source impedance (10k
to 100k
). Above
100k
, a FET-input op amp such as the
OPA132
(very low current noise) may provide improved
performance. The equation in
Figure 40
is shown for
the calculation of the total circuit noise. Note that e
n
=
voltage noise, i
n
= current noise, R
S
= source
impedance, k = Boltzmann’s constant = 1.38
×
10
–23
J/K, and T is temperature in K. For more details on
calculating noise, see the
Basic Noise Calculations
section.
OPA211
OPA2211
SBOS377D–OCTOBER 2006–REVISED FEBRUARY 2008
with
back-to-back
29
of
the
Typical
See
the
Noise
Figure 40. Noise Performance of the OPA211 in
Unity-Gain Buffer Configuration
of
possible
noise
The
produces thermal noise proportional to the square
root of the resistance. This function is plotted in
Figure 40
. The source impedance is usually fixed;
consequently, select the op amp and the feedback
resistors to minimize the respective contributions to
the total noise.
resistive
portion
of
the
source
impedance
Figure 39. Pulsed Operation
Figure 40
depicts total noise for varying source
impedances
with
the
op
configuration (no feedback resistor network, and
therefore no additional noise contributions). The
operational amplifier itself contributes both a voltage
noise component and a current noise component.
The voltage noise is commonly modeled as a
time-varying component of the offset voltage. The
current
noise
is
modeled
component of the input bias current and reacts with
the source resistance to create a voltage component
of noise. Therefore, the lowest noise op amp for a
given application depends on the source impedance.
For low source impedance, current noise is negligible
and voltage noise generally dominates. For high
source impedance, current noise may dominate.
amp
in
a
unity-gain
amp
in
a
unity-gain
as
the
time-varying
14
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Copyright 2006–2008, Texas Instruments Incorporated
Product Folder Link(s):
OPA211 OPA2211
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