+

±

Device

Under Test

Audio

Analyzer

Analyzer

Input

Load

Generator

Output

R2

R1

R3

2

(Signal)

1

R

Gain

1

R

2

(Distortion)

1

3

R

Gain

1

R || R

21

OPA1622

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SBOS727B – NOVEMBER 2015 – REVISED MAY 2016

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Application Information (continued)

8.1.2 Total Harmonic Distortion Measurements

The distortion produced by OPA1622 is below the measurement limit of many commercially-available distortion

analyzers. However, a special test circuit, as shown in Figure 51, can be used to extend the measurement

capabilities.

Figure 51. Distortion Test Circuit

Consider op amp distortion an internal error source that is referred to the input. Figure 51 shows a circuit that

causes the op amp distortion to be 101 times (approximately 40 dB) 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 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. Keep the value of R3 small to

minimize its effect on the distortion measurements.

Verify the validity of this technique by duplicating measurements at high gain or high frequency where the

distortion is within the measurement capability of the test equipment.