Instructions
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 tuning words given a reference clock and desired output frequencies and/or phases. Third, it shows the tuning word and other configuring bits encoded as a sequence of hex codes for use in programming the AD9834 via its serial interface.
Addressing each of these individually:
The AD9834 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 AD9834's compliance voltage.
Tuning words are calculated by simply entering the desired MCLK and output frequency. Because a tuning word is limited to 28 bits, there is typically a small deviation between the desired and actual output frequencies. The actual output frequency (shown to the right of the desired frequency, surrounded by a gray field) is what is encoded in the bottom 14 bits of each codeword. Tuning words greater than 8000000 H (4000(LSB) 6000) exceed the Nyquist frequency and may cause error messages to appear. Phase is quantized to 12 bits and selected in a manner similar to output frequency. The closest available phase setting appears in a field at right and in the last 12 bits of the corresponding hex code.
Individual register hex codes can be selected from a pulldown menu labeled "Codewords" with the codes displayed in fields to the immediate right. The control register word is shown separate from the data word(s), but all words must be sent in order to program the register. The hex code fields can also be modified so that a programmed frequency and phase can be retrieved from the hex. A complete codeword sequence to initialize all registers appears immediately below this, in a field labeled "Init. sequence". It only updates when "Update" is clicked and modifying the values displayed here won't update the form fields above. Below the init codes is a "History" field which shows only those registers touched since the last time "Update" was clicked.
Note: Some parametric limits have been omitted/simplified in the implementation of this calculator. Refer to datasheet for actual limits and specifications.
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