OPA211AI datasheet(14/17 Pages) TI

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INPUT PROTECTION

VOLTAGENOISESPECTRALDENSITY

vsSOURCERESISTANCE

100k

10M

SourceResistance,R ( )

100

10k

100

+(i R ) +4kTR

ResistorNoise

OPA227

OPA211

BASIC NOISE CALCULATIONS

OPA211

Output

Input

NOISE PERFORMANCE

OPA211

OPA2211

SBOS377D – OCTOBER 2006 – REVISED FEBRUARY 2008

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

illustrated

Figure

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

the

OPA211.

See

the

Noise

Performance section of this data sheet for further

Figure 40. Noise Performance of the OPA211 in

Unity-Gain Buffer Configuration

information on noise calculation. Figure 39 shows an

example implementing a current-limiting feedback

resistor.

Design of low-noise op amp circuits requires careful

consideration

variety

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;

Figure 39. Pulsed Operation

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

Figure 40 shows total circuit noise for varying source

impedances

with

the

amp

unity-gain

impedances

with

the

amp

unity-gain

configuration (no feedback resistor network, and

therefore no additional noise contributions). The

therefore no additional noise contributions). Two

operational amplifier itself contributes both a voltage

different op amps are shown with total circuit noise

noise component and a current noise component.

calculated. The OPA211 has very low voltage noise,

The voltage noise is commonly modeled as a

making it ideal for low source impedances (less than

time-varying component of the offset voltage. The

Ω). A similar precision op amp, the OPA227, has

current

noise

modeled

the

time-varying

somewhat higher voltage noise but lower current

component of the input bias current and reacts with

noise. It provides excellent noise performance at

the source resistance to create a voltage component

moderate source impedance (10k

Ω to 100kΩ). Above

of noise. Therefore, the lowest noise op amp for a

100k

Ω, a FET-input op amp such as the OPA132

given application depends on the source impedance.

(very low current noise) may provide improved

For low source impedance, current noise is negligible

performance. The equation in Figure 40 is shown for

and voltage noise generally dominates. For high

the calculation of the total circuit noise. Note that en =

source impedance, current noise may dominate.

voltage noise, in = current noise, RS = 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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