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DAC38RF80: NCO operation of RF Sampling DAC

Mark Halpern
Prodigy 20 points
Part Number: DAC38RF80

Hello,

I am trying to understand the operation of the NCO in a DAC38RF80 as shown in Figure 66 on document SLASEA3C,

1) What is the mapping between the 48 bit accumulator output and the 16 bits used to index into the lookup table, ie are the least significant (LS) or the most significant (MS) 16 accumulator bits used?

I have seen another TI DAC with a 32 bit accumulator. What is the advantage of 48 bits over 32?

2) The lookup table outputs are 16 bits wide, but it is a 14 bit DAC. Are the extra bits used to increase the internal accuracy when combining sin and cos to make the output, with the 14 MS bits of that calculation used at the output?

3) Changing the interpolation rate (using the GUI on the EVM) changes the output frequency. What is the mechanism there? Fig 66 does not indicate any interpolation effects.

  • 0
    TI__Guru 70889 points

    Hello

    Mark Halpern said:
    1) What is the mapping between the 48 bit accumulator output and the 16 bits used to index into the lookup table, ie are the least significant (LS) or the most significant (MS) 16 accumulator bits used?

    We cannot disclose this as this is TI's intellectual property (IP)

    You can have 48bits to increase the frequency resolution. The frequency resolution is Fdac/2^bit. If you have 48-bits, then frequency resolution increases

    Mark Halpern said:
    2) The lookup table outputs are 16 bits wide, but it is a 14 bit DAC. Are the extra bits used to increase the internal accuracy when combining sin and cos to make the output, with the 14 MS bits of that calculation used at the output?

    There are internal truncation when translating from 16-bit wide to 14-bit wide. SNR wise, it is best to use the highest number of bits to perform DSP algorithms.

    Mark Halpern said:
    3) Changing the interpolation rate (using the GUI on the EVM) changes the output frequency. What is the mechanism there? Fig 66 does not indicate any interpolation effects.

    Changing interpolation only changes interface rate. It should not change the output frequency of the NCO. The NCO is determined by the final DAC output frequency.