Datasheet

INPUT PROTECTION NOISE PERFORMANCE
OPA211
Output
R
F
Input
-
+
R
I
VOLTAGENOISESPECTRALDENSITY
vsSOURCERESISTANCE
100k 1M
SourceResistance,R ( )W
S
100
1k 10k
10k
1k
100
10
1
VotlageNoiseSpectralDensity,E
O
R
S
E
O
E =e
O n S
+(i R ) +4kTR
n S
2 2 2
ResistorNoise
OPA227
OPA211
SHUTDOWN
BASIC NOISE CALCULATIONS
OPA211
OPA2211
www.ti.com ...................................................................................................................................................... SBOS377G – OCTOBER 2006 – REVISED MAY 2009
The input terminals of the OPA211 are protected from Figure 46 shows total circuit noise for varying source
excessive differential voltage with back-to-back impedances with the op amp in a unity-gain
diodes, as shown in Figure 45 . In most circuit configuration (no feedback resistor network, and
applications, the input protection circuitry has no therefore no additional noise contributions). Two
consequence. However, in low-gain or G = 1 circuits, different op amps are shown with total circuit noise
fast ramping input signals can forward bias these calculated. The OPA211 has very low voltage noise,
diodes because the output of the amplifier cannot making it ideal for low source impedances (less than
respond rapidly enough to the input ramp. This effect 2k Ω ). A similar precision op amp, the OPA227 , has
is illustrated in Figure 32 of the Typical somewhat higher voltage noise but lower current
Characteristics. If the input signal is fast enough to noise. It provides excellent noise performance at
create this forward bias condition, the input signal moderate source impedance (10k Ω to 100k Ω ). Above
current must be limited to 10mA or less. If the input 100k Ω , a FET-input op amp such as the OPA132
signal current is not inherently limited, an input series (very low current noise) may provide improved
resistor can be used to limit the signal input current. performance. The equation in Figure 46 is shown for
This input series resistor degrades the low-noise the calculation of the total circuit noise. Note that e
n
=
performance of the OPA211, and is discussed in the voltage noise, I
n
= current noise, R
S
= source
Noise Performance section of this data sheet. impedance, k = Boltzmann ’ s constant = 1.38 × 10
– 23
Figure 45 shows an example implementing a J/K, and T is temperature in degrees Kelvin.
current-limiting feedback resistor.
Figure 45. Pulsed Operation
The shutdown (enable) function of the OPA211 is
Figure 46. Noise Performance of the OPA211 and
referenced to the positive supply voltage of the OPA227 in Unity-Gain Buffer Configuration
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
Design of low-noise op amp circuits requires careful
pin. For example, with V
CC
at ± 15V, the device is
consideration of a variety of possible noise
enabled at or below 12V. The device is disabled at or
contributors: noise from the signal source, noise
above 14.65V. If dual or split power supplies are
generated in the op amp, and noise from the
used, care should be taken to ensure the valid high
feedback network resistors. The total noise of the
or valid low input signals are properly referred to the
circuit is the root-sum-square combination of all noise
positive supply voltage. This pin must be connected
components.
to a valid high or low voltage or driven, and not left
open-circuit. The enable and disable times are The resistive portion of the source impedance
provided in the Typical Characteristics section (see produces thermal noise proportional to the square
Figure 41 through Figure 43 ). When disabled, the root of the resistance. This function is plotted in
output assumes a high-impedance state. 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