This calculator has several distinct functions. First, it's a tool for selecting a value for RSET and checking the output level remains within limits for a given load. Second, it's an assistant for selecting a 32-bit tuning word given a reference clock and desired output frequency. Third, it shows the tuning word and other configuring bits encoded as a sequence of hex codes for use in programming the AD9851 via its parallel or serial interface. Lastly, output harmonics are shown for the selected reference clock and output frequency after an external reconstruction filter has been applied. Addressing these one at a time:
The AD9851 has complementary current outputs with limits on the current and voltage that can be supplied and still meet other datasheet specifications. The output current level, IOUT, is set by a single external resistor, RSET, and the two are related by an equation. Changing one of these fields in the calculator updates the other automatically. If too high a current is selected, an error is noted. The IOUT current develops a voltage into the selected RLOAD, shown on the schematic, and is checked against the AD9851's compliance voltage.
A tuning word is selected by simply entering the desired REFCLK, multiplier and output frequency. The internal reference clock is the product of the external REFCLK and its multiplier and has a maximum frequency that depends on supply voltage, selected at top. Because the tuning word is limited to 32 bits there is typically a small deviation between the desired and actual output frequencies, which is shown in a field at right. The actual output frequency is what is encoded as the tuning word and this comprises the last four bytes of the parallel hex codes and the first four of the serial codes. Tuning words greater than 7FFFFFFF H exceed the Nyquist frequency and will cause error messages to appear.
The AD9851 has 5 bits of programmable phase which is selected in a manner similar to the desired output frequency. The closest available phase setting appears in the field at right and in the corresponding hex code with the power down and REFCLK multiplier bits. The hex code fields are bidirectional and a known set of hex codes can be entered to retrieve the programmed frequency and phase.
If this browser is Java-enabled, a harmonic images calculator appears at bottom which graphs the first four harmonics of the output frequency (in red, sin(x)/x envelope in light red) and lists the first 16 in a table which can be cut and pasted into a spreadsheet. Changing the sampling (reference clock) or output frequencies at top automatically updates the images calculator, but the reverse is not true. Spurs (blue) are are also shown as folded 2nd or 3rd-order harmonics of each image with a uniform default SFDR of 43 dB.
Suppression of images and spurs (waveform reconstruction) can be simulated by selecting corner frequency, filter order and type of a simple analog filter (last line of images calculator; magnitude rolloff shown in green). A region corresponding to 10 bits of quantization noise is shaded at the bottom of the graph for reference. This calculator is based on a highly simplified model of the AD9851: check datasheet for parameters appropriate to your application. For more information, see our DAC Images calculator.
Note: Some parametric limits have been omitted/simplified in the implementation of this calculator. Refer to datasheet for actual limits and specifications.