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AD7862BR-3 датащи(PDF) 11 Page - Analog Devices |
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AD7862BR-3 датащи(HTML) 11 Page - Analog Devices |
11 / 16 page AD7862 –11– REV. 0 Figure 7 shows a histogram plot for 8192 conversions of a dc input using the AD7862 with 5 V supply. The analog input was set at the center of a code transition. It can be seen that all the codes appear in the one output bin indicating very good noise performance from the ADC. 746 756 747 748 749 750 751 752 753 754 755 9000 8000 0 4000 3000 2000 1000 6000 5000 7000 Figure 7. Histogram of 8192 Conversions of a DC Input The same data is presented in Figure 8 as in Figure 7 except that in this case the output data read for the device occurs during conversion. This has the effect of injecting noise onto the die while bit decisions are being made and this increases the noise generated by the AD7862. The histogram plot for 8192 conversions of the same dc input now shows a larger spread of codes. This effect will vary depending on where the serial clock edges appear with respect to the bit trials of the conversion process. It is possible to achieve the same level of performance when reading during conversion as when reading after conver- sion depending on the relationship of the serial clock edges to the bit trial points. The output spectrum from the ADC is evaluated by applying a sine wave signal of very low distortion to the VAX/BX input that is sampled at a 245.76 kHz sampling rate. A Fast Fourier Trans- form (FFT) plot is generated from which the SNR data can be obtained. Figure 9 shows a typical 2048 point FFT plot of the AD7862 with an input signal of 10 kHz and a sampling fre- quency of 245.76 kHz. The SNR obtained from this graph is 72.95 dB. It should be noted that the harmonics are taken into account when calculating the SNR. 745 755 746 747 748 749 750 751 752 753 754 0 4000 3000 2000 1000 6000 5000 7000 Figure 8. Histogram of the 8192 Conversions with Read During Conversion –0 –120 0 12.2k 10k 30k 50k 70k 90k –20 –40 –60 –80 –100 –10 –30 –50 –70 –90 –110 100k FSAMPLE = 245760 FIN = 10kHz SNR = –72.95dB THD = –89.99dB Figure 9. AD7862 FFT Plot Effective Number of Bits The formula given in Equation 1 relates the SNR to the number of bits. Rewriting the formula, as in Equation 2, it is possible to get a measure of performance expressed in effective number of bits (N). N = SNR −1.76 6.02 (2) The effective number of bits for a device can be calculated directly from its measured SNR. Figure 10 shows a typical plot of effective number of bits versus frequency for an AD7862BN with a sampling frequency of 245.76 kHz. The effective number of bits typically falls between 11.6 and 10.6 corresponding to SNR figures of 71.59 dB and 65.57 dB. 0 1000 200 400 600 800 10.2 11.4 11.2 11.0 10.8 11.8 11.6 12.0 10.6 10.4 FREQUENCY – kHz Figure 10. Effective Numbers of Bits vs. Frequency Total Harmonic Distortion (THD) Total Harmonic Distortion (THD) is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7862, THD is defined as THD dB () = 20 log V 2 2 +V 3 2 +V 4 2 +V 5 2 V1 where V1 is the rms amplitude of the fundamental and V2, V3, V4 and V5 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the FFT plot of the ADC output spectrum. |
Аналогичный номер детали - AD7862BR-3 |
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Аналогичное описание - AD7862BR-3 |
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