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AD7840SQ Arkusz danych(PDF) 8 Page - Analog Devices |
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AD7840SQ Arkusz danych(HTML) 8 Page - Analog Devices |
8 / 16 page AD7840 REV. B –8– As in the parallel mode, the LDAC signal controls the loading of data to the DAC latch. Normally, data is loaded to the DAC latch on the falling edge of LDAC. However, if LDAC is held low, then serial data is loaded to the DAC latch on the sixteenth falling edge of SCLK. If LDAC goes low during the transfer of serial data to the input latch, no DAC latch update takes place on the falling edge of LDAC. If LDAC stays low until the serial transfer is completed, then the update takes place on the six- teenth falling edge of SCLK. If LDAC returns high before the serial data transfer is completed, no DAC latch update takes place. Figure 9 shows the simplified serial input control logic for the AD7840. Figure 9. AD7840 Simplified Serial Input Control Logic AD7840 DYNAMIC SPECIFICATIONS The AD7840 is specified and 100% tested for dynamic perfor- mance specifications as well as traditional dc specifications such as integral and differential nonlinearity. These ac specifications are required for the signal processing applications such as speech synthesis, servo control and high speed modems. These applications require information on the DAC’s effect on the spectral content of the signal it is creating. Hence, the param- eters for which the AD7840 is specified include signal-to-noise ratio, harmonic distortion and peak harmonics. These terms are discussed in more detail in the following sections. Signal-to-Noise Ratio (SNR) SNR is the measured signal-to-noise ratio at the output of the DAC. The signal is the rms magnitude of the fundamental. Noise is the rms sum of all the nonfundamental signals up to half the sampling frequency (fs/2) excluding dc. SNR is depen- dent upon the number of quantization levels used in the digiti- zation process; the more levels, the smaller the quantization noise. The theoretical signal to noise ratio for a sine wave out- put is given by SNR = (6.02N + 1.76) dB (1) where N is the number of bits. Thus for an ideal 14-bit con- verter, SNR = 86 dB. Figure 10 shows a typical 2048 point Fast Fourier Transform (FFT) plot of the AD7840KN with an output frequency of 1 kHz and an update rate of 100 kHz. The SNR obtained from this graph is 81.8 dB. It should be noted that the harmonics are taken into account when calculating the SNR. Figure 10. AD7840 FFT Plot Effective Number of Bits The formula given in (1) relates the SNR to the number of bits. Rewriting the formula, as in (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. Harmonic Distortion Harmonic distortion is the ratio of the rms sum of harmonics to the fundamental. For the AD7840, total harmonic distortion (THD) is defined as THD = 20 log V 2 2 +V 3 2 +V 4 2 +V 5 2 +V 6 2 V 1 where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the 2048-point FFT plot. Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the DAC output spectrum (up to fs/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification will be determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor the peak will be a noise peak. Testing the AD7840 A simplified diagram of the method used to test the dynamic performance specifications is outlined in Figure 11. Data is loaded to the AD7840 under control of the microcontroller and associated logic at a 100 kHz update rate. The output of the AD7840 is applied to a ninth order, 50 kHz, low-pass filter. The output of the filter is in turn applied to a 16-bit accurate digi- tizer. This digitizes the signal and the microcontroller generates an FFT plot from which the dynamic performance of the AD7840 can be evaluated. |
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Podobny opis - AD7840SQ |
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