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AD766 Datasheet(PDF) 6 Page - Analog Devices |
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AD766 Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page AD766 REV. A –6– system where only a single positive and a single negative supply are available. In this case, the positive logic and positive analog supplies may both be connected to the single positive supply. The negative logic and negative analog supplies may both be connected to the single negative supply. Performance would benefit from a measure of isolation between the supplies intro- duced by using simple low-pass filters in the individual power supply leads. Figure 6. Alternate Recommended Schematic Figure 7. Power Dissipation vs. Clock Frequency As with most linear circuits, changes in the power supplies will affect the output of the DAC. Analog Devices recommends that well regulated power supplies with less than 1% ripple be incor- porated into the design of any system using these device. MEASUREMENT OF TOTAL HARMONIC DISTORTION The THD specification of a DSP DAC represents the amount of undesirable signal produced during reconstruction of a digital waveform. To account for the variety of operating conditions Figure 8. Distortion Test Circuit in signal processing applications, the DAC is tested at two output frequencies and at three signal levels over the full oper- ating temperature ranges. A block diagram of the test setup is shown in Figure 8. In this test setup, a digital data stream, representing a 0 dB, –20 dB or –60 dB sine wave is sent to the device under test. The frequen- cies used are 1037 Hz and 49.07 kHz. Input data is latched into the AD766 at 500 kSPS. The AD766 under test produces an analog output signal using the on-board op amp for 1 kHz and an external op amp for 50 kHz. The automatic test equipment digitizes the output test wave- form, and then an FFT to 250 kHz is performed on the results of the test. Based on the first 9 harmonics of the fundamental 1037 Hz and the first 3 harmonics of the 49.07 kHz output waves, the total harmonic distortion of the device is calculated. Neither a deglitcher nor an MSB trim is used during the THD test. The circuit design, layout and manufacturing techniques em- ployed in the production of the AD766 result in excellent THD performance. Figure 9 shows the typical unadjusted THD per- formance of the AD766 for various amplitudes of 1 kHz and 50 kHz sine waves. As can be seen, the AD766 offers excellent performance even at amplitudes as low as 60 dB. Figure 10 illustrates the typical THD versus frequency performance from the internal amplifier for a filtered AD766 output. At frequen- cies greater than approximately 30 kHz, depending on the low- pass filter used, an improvement in THD of 3–4 dB over the performance shown in the figure can be achieved. Figure 11 illustrates the consistent THD performance of the AD766 over temperature. Figure 9. Typical Unadjusted THD |
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