OPA2211-HT_15 Datasheet, PDF (15/22 Pages) Texas Instruments – 1.1 nV/√Hz Noise, Low Power, Precision Operational Amplifier

OPA2211-HT
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INPUT PROTECTION
The input terminals of the OPA2211 are protected
from excessive differential voltage with back-to-back
diodes, as shown in Figure 41. 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 31 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 OPA2211, and is discussed in the
Noise Performance section of this data sheet.
Figure 41 shows an example implementing a current-
limiting feedback resistor.
R_F
-
Input
R_I
+
OPA2211
Output
Figure 41. Pulsed Operation
NOISE PERFORMANCE
Figure 42 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 OPA2211 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 42 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.
SBOS684A – AUGUST 2013 – REVISED AUGUST 2013
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
1k
R_S
100
E_O
OPA227
OPA2211
Resistor Noise
10
1
100
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (Ω)
Figure 42. Noise Performance of the OPA2211
and OPA227 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 42. 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 42 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.
Copyright © 2013, Texas Instruments Incorporated
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