3 How to Measure THD+N of an Op Amp

Op amp distortion can be considered an internal error source that can be measured by observing changes of the input offset voltage V_os and how these changes in V_os alter the signal of interest V_in. The V_os of an amplifier is the difference between the inverting and non-inverting inputs. Fluctuations in V_os are small when operating within the linear region of the amplifier. Examples that can directly or indirectly cause a change between the op amp inputs or V_os are but not limited to temperature drift, common mode and power supply voltage fluctuations, slew rate, and reduction in open loop gain or A_ol. A_ol is dependent on frequency. Ignoring the A_ol frequency dependency, Equation 2 shows the relationship between V_os, V_out and A_ol.

Equation 3 is found by rearranging Equation 2. Two observations are made from Equation 3, ${∆V_{os }}_{ }= 0 V {|}_{A_{ol} \to \infty }$ and ${∆V_{os }}_{ }= \infty {|}_{A_{ol} \to 0}$ . These observations show that A_ol is responsible for correcting amplifier errors. Parts with high open loop gain create less V_os errors. In theory a virtual short of an op amp is when the inverting and non-inverting terminals have equal voltage potentials even though there is no connection between them. The assumption ${∆V_{os }}_{ }= 0 V {|}_{A_{ol} \to \infty }$ is used in the following sections when applying the principles of superposition.

The distortion produced by TI precision op amps is often below the measurement limit of commercially available distortion analyzers. However, a special test circuit can be used to extend the measurement capabilities.
Figure 3-1 shows a circuit that amplifies the op amp distortion to be $101 \frac{V}{V}$ times (or approximately 40 dB) greater than that normally produced by the op amp. Figure 3-2 shows the THD + N of the OPA1656 is below the theoretical noise floor of the distortion analyzer. The purpose of the test circuit is to gain the THD + N of the op amp above the distortion analyzer noise floor to measure the true performance of the op amp. The addition of R_A to the otherwise standard non-inverting buffer amplifier configuration alters the feedback factor or noise gain of the circuit. The closed-loop signal gain A_CL is unchanged, but the feedback available for error correction is reduced by a factor of $101 \frac{V}{V}$ , thus extending the resolution by $101 \frac{V}{V}$ or 40 dB. For further details see Section 6. Note that the input signal and load applied to the op amp are the same as with conventional feedback without R_A. When altering the noise gain, the gain bandwidth product or GBW of the amplifier can be considered. A practical guideline can be used when designing the test circuit noise gain. If the amplifier GBW is 10 MHz or greater a noise gain of $101 \frac{V}{V}$ can be used. Amplifiers with a GBW less than 10 MHz can use a noise gain of $11 \frac{V}{V}$ . The value of R_A is kept small to minimize the effect on the distortion measurements and extraneous thermal noise.