实用模拟电路设计技术-Section+8

实用模拟电路设计技术-Section+1实用模拟电路设计技术-Section+2实用模拟电路设计技术-Section+3实用模拟电路设计技术-Section+4实用模拟电路设计技术-Section+5实用模拟电路设计技术-Section+6实用模拟电路设计技术-Section+7实用模拟电路设计技术-Section+8实用模拟电路设计技术-Section+9

SECTION 8

DISTORTION MEASUREMENTS

High Speed Op Amp DistortionHigh Frequency Two-Tone GenerationUsing Spectrum Analyzers in High FrequencyLow Distortion MeasurementsMeasuring ADC Distortion using FFTsFFT Testing

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实用模拟电路设计技术-Section+1实用模拟电路设计技术-Section+2实用模拟电路设计技术-Section+3实用模拟电路设计技术-Section+4实用模拟电路设计技术-Section+5实用模拟电路设计技术-Section+6实用模拟电路设计技术-Section+7实用模拟电路设计技术-Section+8实用模拟电路设计技术-Section+9

SECTION 8

DISTORTION MEASUREMENTS

HIGH SPEED OP AMP DISTORTIONWalt Kester

Dynamic range of an op amp may be defined in several ways. The most commonways are to specify Harmonic Distortion, Total Harmonic Distortion (THD), or TotalHarmonic Distortion Plus Noise (THD + N).

Harmonic distortion is measured by applying a spectrally pure sinewave to an opamp in a defined circuit configuration and observing the output spectrum. Theamount of distortion present in the output is usually a function of several

parameters: the small- and large-signal nonlinearity of the amplifier being tested,the amplitude and frequency of the input signal, the load applied to the output of thefundamental.

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the second and third harmonics are included. This is because the RSS process causesthe higher-order terms to have negligible effect on the THD, if they are 3 to 5 timessmaller than the largest harmonic. For example,

010.2+003.2=00109.=0104.≈01.

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实用模拟电路设计技术-Section+1实用模拟电路设计技术-Section+2实用模拟电路设计技术-Section+3实用模拟电路设计技术-Section+4实用模拟电路设计技术-Section+5实用模拟电路设计技术-Section+6实用模拟电路设计技术-Section+7实用模拟电路设计技术-Section+8实用模拟电路设计技术-Section+9

DEFINITIONS OF THD AND THD + N

Figure 8.1

It is important to note that the THD measurement does not include noise terms,源

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the fundamental is the THD+N specification.

Audio frequency amplifiers (such as the OP-275) are optimized for low noise and lowdistortion within the audio bandwidth (20Hz to 20kHz). In audio applications, totalharmonic distortion plus noise (THD+N) is usually measured with specialized

equipment, such as the Audio Precision System One. The output signal amplitude ismeasured at a given frequency (e.g., 1kHz); then the fundamental signal is removedwith a bandstop filter, and the system measures the rms value of the remainingfrequency components, which contain both harmonics and noise. The noise and

harmonics are measured over a bandwidth that will include the highest harmonics,

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实用模拟电路设计技术-Section+1实用模拟电路设计技术-Section+2实用模拟电路设计技术-Section+3实用模拟电路设计技术-Section+4实用模拟电路设计技术-Section+5实用模拟电路设计技术-Section+6实用模拟电路设计技术-Section+7实用模拟电路设计技术-Section+8实用模拟电路设计技术-Section+9

usually about 100kHz. The measurement is swept over the frequency range forvarious conditions.

THD+N results for the OP-275 are plotted in Figure 8.2 as a function of frequency.The signal level is 3V rms, and the amplifier is connected as a unity-gain follower.The data is shown for three load conditions: 600ohm, 2kohm, and 10kohm. Noticethat a THD+N value of 0.0008% corresponds to 8ppm, or –102dBc. The input voltagenoise of the OP-275 is typically 6nV/rtHz @ 1kHz, and integrated over an 80kHznoise bandwidth, yields an rms noise level of 1.7µV rms. For a 3V rms signal level,the corresponding signal-to-noise ratio is 125dB. Because the THD is considerablygreater than the noise level, the THD component is the primary contributor.Multiple plots with variable bandwidths can be used to help separate noise anddistortion.

THD + N FOR THE OP -275 OVER 100kHz BANDWIDTH

IS DOMINATED BY DISTORTION

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of the analyzer. This is to prevent overdrive distortion in the spectrum analyzer. Thefirst five harmonics were then measured and combined in an RSS fashion to get theTHD figure. The legend on the graph indicates that the measurement equipment"floor" is about 120dBc; hence at frequencies below 10kHz, the THD may be evenless.

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