OPA211_08 Datasheet, PDF (14/17 Pages) Texas Instruments – 1.1nV/√Hz Noise, Low Power, Precision Operational Amplifier in Small DFN-8 Package

OPA211
OPA2211
SBOS377D – OCTOBER 2006 – REVISED FEBRUARY 2008
INPUT PROTECTION
The input terminals of the OPA211 are protected from
excessive differential voltage with back-to-back
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 29 of the Typical
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. See the Noise
Performance section of this data sheet for further
information on noise calculation. Figure 39 shows an
example implementing a current-limiting feedback
resistor.
R_F
-
Input
R_I
+
OPA211
Output
Figure 39. Pulsed Operation
NOISE PERFORMANCE
Figure 40 shows total circuit noise for varying source
impedances with the op amp in a unity-gain
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
2kΩ). 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 (10kΩ to 100kΩ). Above
100kΩ, 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.
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VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
1k
R_S
100
E_O
OPA227
OPA211
Resistor Noise
10
1
100
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
10M
Source Resistance, R_S (W)
Figure 40. Noise Performance of the OPA211 in
Unity-Gain Buffer Configuration
BASIC NOISE CALCULATIONS
Design of low-noise op amp circuits requires careful
consideration of a variety of possible noise
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.
The resistive portion of the source impedance
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.
Figure 40 depicts total noise for varying source
impedances with the op amp in a unity-gain
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 as the time-varying
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.
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