Sine_wave_generator_with_digital_control

Published:2009/7/25 5:23:00 Author:Jessie | From:SeekIC

Figure 9-29 shows a DAC1020 connected to provide a variable-frequency, sine-wave generator. This circuit is capable of producing signals at frequencies up to 30 kHz under digital control. The linearity of the output frequency to the digital-code input is within 0.1% for each of the 1024 discrete output frequencies. To adjust the circuit, set all DAC digital inputs high and trim the 25-kΩ pot for a 30-kHz output (using a frequency counter). Then connect a distortion analyzer to the circuit output and adjust the 5-kΩ and 75-kΩ pots for minimum distortion. Finally, set the 1-MΩ output control for the desired output. The circuit provides rapid switching of the output frequency, as shown in Fig. 9-30. Notice that the output frequency shifts immediately (actually with no undesired delay) by more than an order of magnitude in response to digital commands (top line of Fig. 9-30). If operation over temperature is required, the absolute change in resistance in the DAC internal ladder might cause unacceptable errors. This can be corrected by reversing the A2 inputs and inserting an amplifier (dashed lines in Fig. 9-29) between the DAC and A1. Because this amplifier uses the DAC internal feedback resistor (Fig. 9-28), the temperature error in the ladder is cancelled. This results in more stable operation. NATIONAL SEMICONDUCTOR, APPLICATION NOTE 269, 1994, P. 660.



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