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PCM78P Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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PCM78P Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 16 page ® PCM78 9 FIGURE 2. Block Diagram of Distortion Test Circuit. DISCUSSION OF SPECIFICATIONS TOTAL HARMONIC DISTORTION Evaluating distortion specifications can be a difficult task, as distortion is often specified in different ways. Total Harmonic Distortion (THD) is defined as the ratio of the square root sum of the squares of the value of rms harmonics to the value of the rms fundamental and is expressed in percent or dB. Note that this measurement only includes energy present in those frequencies which would contain harmonics, and therefore is less than Total Harmonic Distortion plus Noise. The Total Harmonic Distortion plus Noise (THD+N) is defined as the ratio of the square root of the sum of the squares of the value of the rms harmonics and rms noise to the value of the rms fundamental and is expressed in percent or dB. This is the most meaningful measurement of a dynamic converter’s performance because it includes all energy present in the signal that is not fundamental. A block diagram of the test circuit used to measure the THD and THD+N of the PCM78 is shown in Figure 2. This digital system is capable of differentiating harmonic energy and noise; conventional distortion analyzers which operate on a tracking notch filter principle cannot distinguish this energy, and therefore only measure THD+N. Unfortunately, in the past, these systems were used for measuring distortion performance of converters, and the distortion was often simply specified as “THD”, when in fact it was really THD+N. For this reason, it is often confusing to compare specifications of converters unless one knows exactly what was being measured. If we assume that the error due to the test circuit of Figure 2 is negligible, then the rms value of the PCM78 error referred to the input can be shown to be where N is the number of samples, EL(i) is the linearity error at each sample, EQ(i) is the quantization error at each FIGURE 3. Block Diagram of Histogram Test. Sync Timing B&K Digital Oscillator Convert Command SHC702 PCM78 Serial-To-Parallel IEEE-488 Communication HP-330 Scientific Computer Digital Distortion Analyzer Software DataPhysics Corp 0 –20 –40 –60 –80 –100 –120 25 Frequency (kHz) 50 75 0.0 100 Sync Timing B&K Digital Oscillator Convert Command SHC702 PCM78 Serial-To-Parallel 64k Memory IEEE-488 HP-330 Scientific Computer 2 1 0 –1 –2 Codes 2048 4095 0 1024 3072 THD +N = 1 N E L (i) + E Q (i) + E N (i) []2 i =1 N ∑ E rms X 100% |
Similar Part No. - PCM78P |
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Similar Description - PCM78P |
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