Precision DACs are essential in many consumer, industrial, and military applications, but high-resolution DACs can be costly. Frequency-to-voltage converters have good nonlinearity specifications—typically, 0.002% for the AD650—and are inherently monotonic. This Design Idea shows how you can use a frequency-to-voltage converter and a DDS (direct-digital-synthesizer) chip for precise digital-to-analog conversion. The DDS chip generates a precision frequency proportional to its digital input. This frequency serves as the input to a voltage-to-frequency converter, thereby generating an 18-bit analog voltage proportional to the original digital input. Figure 1 shows how the AD650 is configured for frequency-to-voltage conversion. With R₁+R₃=20 kΩ and C_OS=620 pF, a full-scale input frequency of 100 kHz produces a full-scale output voltage of 10V. (See Analog Devices (www.analog.com) application note AN-279 for more details on using the AD650 as a frequency-to-voltage converter.)

Resolution of 18 bits requires a programmable clock source with a frequency resolution of 0.38 Hz (100 kHz/262,144). The AD9833 low-power DDS IC with on-chip 10-bit DAC is ideal for this task, because setting the clock frequency requires no external components. The device contains a 28-bit accumulator, which allows it to generate signals with 0.1-Hz resolution when you operate it with a 25-MHz master clock. Figure 2 shows a block diagram of the AD9833 DDS chip. Figure 3 shows the complete system. The most significant bit of the on-chip DAC switches to the V_OUT pin of the AD9833, thus generating the 0V-to-V_DD square wave that serves as the clock input to the AD650 voltage-to-frequency converter. Writing to frequency-control registers via a simple three-wire interface sets the clock frequency, thus programming the voltage output.

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