The OPA166x series op amps have excellent distortion characteristics. THD + noise is below 0.0006% (G = +1, V_O = 3V_RMS, BW = 80kHz) throughout the audio frequency range, 20Hz to 20kHz, with a 2kΩ load (see Figure 5-7 for characteristic performance).

The distortion produced by the OPA166x series op amps is below the measurement limit of many commercially available distortion analyzers. However, a special test circuit (such as Figure 6-5 shows) can be used to extend the measurement capabilities.

Op amp distortion is considered an internal error source that is referred to the input. Figure 6-5 shows a circuit that causes the op amp distortion to be gained up (refer to the table in Figure 6-5 for the distortion gain factor for various signal gains). The addition of R₃ to the otherwise standard non-inverting 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 the distortion gain factor, thus extending the resolution by the same amount. 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 must be kept small to minimize the effect on the distortion measurements.

The 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.