OPA211AIDRGR Datasheet, PDF (15/37 Pages) Texas Instruments – 1.1nV/√Hz Noise, Low Power, Precision Operational Amplifier in Small DFN-8 Package

OPA211
OPA2211
www.ti.com...................................................................................................................................................... SBOS377G – OCTOBER 2006 – REVISED MAY 2009
INPUT PROTECTION
The input terminals of the OPA211 are protected from
excessive differential voltage with back-to-back
diodes, as shown in Figure 45. 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 32 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, and is discussed in the
Noise Performance section of this data sheet.
Figure 45 shows an example implementing a
current-limiting feedback resistor.
R_F
NOISE PERFORMANCE
Figure 46 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 46 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 degrees Kelvin.
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
-
Input
R_I
+
OPA211
Output
Figure 45. Pulsed Operation
SHUTDOWN
The shutdown (enable) function of the OPA211 is
referenced to the positive supply voltage of the
operational amplifier. A valid high disables the op
amp. A valid high is defined as (V+) – 0.35V of the
positive supply applied to the shutdown pin. A valid
low is defined as (V+) – 3V below the positive supply
pin. For example, with V_CC at ±15V, the device is
enabled at or below 12V. The device is disabled at or
above 14.65V. If dual or split power supplies are
used, care should be taken to ensure the valid high
or valid low input signals are properly referred to the
positive supply voltage. This pin must be connected
to a valid high or low voltage or driven, and not left
open-circuit. The enable and disable times are
provided in the Typical Characteristics section (see
Figure 41 through Figure 43). When disabled, the
output assumes a high-impedance state.
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
1M
Source Resistance, R_S (W)
Figure 46. Noise Performance of the OPA211 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 46. The source impedance is usually fixed;
consequently, select the op amp and the feedback
resistors to minimize the respective contributions to
the total noise.
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Product Folder Link(s): OPA211 OPA2211