OPA2211-EP Datasheet, PDF (16/32 Pages) Texas Instruments – OPA2211-EP 1.1-nV/√Hz Noise, Low-Power, Precision Operational Amplifier

OPA2211-EP
SBOS761 – NOVEMBER 2015
www.ti.com
Feature Description (continued)
Figure 39 depicts total noise for varying source impedances with the operational amplifier 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 operational amplifier 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.
Figure 41 shows both inverting and noninverting operational amplifier circuit configurations with gain. In circuit
configurations with gain, the feedback network resistors also contribute noise. The current noise of the
operational amplifier 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.
7.3.4 Total Harmonic Distortion Measurements
OPA2211-EP series operational amplifiers have excellent distortion characteristics. THD + Noise is below
0.0002% (G = +1, V_OUT = 3 V_RMS) throughout the audio frequency range, 20 Hz to 20 kHz, with a 600-Ω load.
The distortion produced by OPA2211-EP series operational amplifiers is below the measurement limit of many
commercially available distortion analyzers. However, a special test circuit illustrated in Figure 42 can be used to
extend the measurement capabilities.
Operational amplifier distortion can be considered an internal error source that can be referred to the input.
Figure 42 shows a circuit that causes the operational amplifier distortion to be 101 times greater than that
normally produced by the operational amplifier. 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 101, thus extending the resolution by 101.
Note that the input signal and load applied to the operational amplifier 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.
7.4 Device Functional Modes
The OPAx211 has a single functional mode and is operational when the power-supply voltage is greater than 4.5
V (±2.25 V). The maximum power supply voltage for the OPAx211 is 36 V (±18 V).
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