OPA211-EP_15 Datasheet, PDF (17/26 Pages) Texas Instruments – 1.1nV/√Hz NOISE, LOW POWER, PRECISION OPERATIONAL AMPLIFIER

OPA211-EP
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BASIC NOISE CALCULATIONS (continued)
Figure 48 illustrates both inverting and noninverting
op amp circuit configurations with gain. In circuit
configurations with gain, the feedback network
resistors also contribute noise. The current noise of
the op amp reacts with the feedback resistors to
create additional noise components. The feedback
resistor values can generally be chosen to make
these noise sources negligible. The equations for
total noise are shown for both configurations.
8.5 TOTAL HARMONIC DISTORTION
MEASUREMENTS
OPA211 series op amps have excellent distortion
characteristics. THD + Noise is below 0.0002% (G =
+1, V_OUT = 3V_RMS) throughout the audio frequency
range, 20Hz to 20kHz, with a 600Ω load.
The distortion produced by OPA211 series op amps
is below the measurement limit of many commercially
available distortion analyzers. However, a special test
circuit illustrated in Figure 49 can be used to extend
the measurement capabilities.
Op amp distortion can be considered an internal error
source that can be referred to the input. Figure 49
shows a circuit that causes the op amp distortion to
be 101 times greater than that normally produced by
the op amp. The addition of R₃ to the otherwise
standard noninverting amplifier configuration alters
the feedback factor or noise gain of the circuit. The
closed-loop gain is unchanged, but the feedback
available for error correction is reduced by a factor of
SBOS638 – JUNE 2012
TOTAL HARMONIC DISTORTION
MEASUREMENTS (continued)
101, thus extending the resolution by 101. Note that
the input signal and load applied to the op amp are
the same as with conventional feedback without R₃.
The value of R₃ should be kept small to minimize its
effect on the distortion measurements.
Validity of this technique can be verified by
duplicating measurements at high gain and/or high
frequency where the distortion is within the
measurement capability of the test equipment.
Measurements for this data sheet were made with an
Audio Precision System Two distortion/noise
analyzer, which greatly simplifies such repetitive
measurements. The measurement technique can,
however, be performed with manual distortion
measurement instruments.
8.6 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 42 through Figure 44). When disabled, the
output assumes a high-impedance state.
Copyright © 2012, Texas Instruments Incorporated
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