SBOS377L October 2006 – January 2020 OPA211
OPA211 series operational amplifiers have excellent distortion characteristics. THD + noise is below 0.0001% (G = 1, VO = 3 VRMS) throughout the audio frequency range, 20 Hz to 20 kHz, with a 600-Ω load.
The distortion produced by OPAx211 series operational amplifiers is below the measurement limit of many commercially available distortion analyzers. However, a special test circuit shown in Figure 43 can extend the measurement capabilities.
Operational amplifier distortion can be considered an internal error source that can be referred to the input. Figure 43 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.
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.