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AD7893BN-5 데이터시트(PDF) 11 Page - Analog Devices |
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AD7893BN-5 데이터시트(HTML) 11 Page - Analog Devices |
11 / 12 page AD7893 –11– REV. E Effective Number of Bits The formula for signal to (noise + distortion) ratio (see Termi- nology section) is related to the resolution or number of bits in the converter. Rewriting the formula gives a measure of perfor- mance expressed in effective number of bits (N): N = (SNR – 1.76) / 6.02 where SNR is Signal to (Noise + Distortion) Ratio. The effective number of bits for a device can be calculated from its measured signal to (noise + distortion) ratio. Figure 13 shows a typical plot of effective number of bits versus frequency for the AD7893-2 from dc to fSAMPLING/2. The sampling frequency is 102.4 kHz. The plot shows that the AD7893 converts an input sine wave of 51.2 kHz to an effective numbers of bits of 11, which equates to a signal to (noise + distortion) level of 68 dB. INPUT FREQUENCY – kHz 12.0 11.5 10.0 0 51.2 25.6 11.0 10.5 Figure 13. Effective Number of Bits vs. Frequency The same data is presented in Figure 11 as in Figure 10 except that, in this case, the output data read for the device occurs dur- ing conversion. This has the effect of injecting noise onto the die while bit decisions are being made; this increases the noise gen- erated by the AD7893. The histogram plot for 8192 conversions of the same dc input now shows a larger spread of codes with the rms noise for the AD7893-2 increasing to 210 µV. This ef- fect will vary depending on where the serial clock edges appear with respect to the bit trials of the conversion process. It is pos- sible to achieve the same level of performance when reading during conversion as when reading after conversion, depending on the relationship of the serial dock edges to the bit trial points. CODE 7500 (X–4) (X–3) 7000 6500 6000 0 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 (X–2) (X–1) (X+1) (X+2) (X+3) (X+4) X SAMPLING FREQUENCY = 102.4kHz T A = +25 °C Figure 11. Histogram of 8192 Conversions with Read Dur- ing Conversion Dynamic Performance With a combined conversion and acquisition time of 7.5 µs, the AD7893 is ideal for wide bandwidth signal processing applica- tions. These applications require information on the ADC’s effect on the spectral content of the input signal. Signal to (noise + distortion) ratio, total harmonic distortion, peak harmonic or spurious noise, and intermodulation distortion are all specified. Figure 12 shows a typical FFT plot of a 10 kHz, 0 V to +2.5 V input after being digitized by the AD7893-2, operating at a 102.4 kHz sampling rate. The signal to (noise + distortion) ratio is 71.5 dB, and the total harmonic distortion is –83 dB. FREQUENCY – kHz SNR IS SIGNAL TO (NOISE AND DISTORTION) RATIO 0 –30 –180 0 51.2 25.6 –60 –80 –120 SAMPLE RATE = 102.4kHz INPUT FREQUENCY = 10kHz SNR = 71.5dB TA = +25°C Figure 12. AD7893 FFT Plot |
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