AD9754 Amplitude Control for DDS

Question

Hello, 
 I plan to use AD9754 as a DDS for generating a 32KHz sine (worst case scenario 100KHz) and I need to be able to fine tune its output amplitude, so I thought about using LTC2758. The topology which was suggested to me is to use LTC2758 output as voltage reference for AD9754. But I'd like to know if the other way around would give better performane (since LTC2758 is a MDAC and AD9754 isn't), ie, using AD9754 as multiplying reference for one of LTC2758 channels. Will the 18-bit resolution be reduced to 14-bit in any of these cases? Which topology will yield the smallest gain errors/drifts? As for output amplifier, I'm considering using AD829. 
 
 Best regards, 
 Vitor.

ShineC · Accepted Answer

Hi Vitor,

For AD9754, based on the DC specifications table in the datasheet, typical gain error (without internal reference) is +/-0.5% of FSR and typical gain drift (without internal reference) is +/-50ppm of FSR/degC. These specifications will remain the same when a perfect external voltage reference is used but, of course, a perfect reference does not exist.

It is okay to use an 18-bit MDAC to modulate the output of a 14-bit DAC but it will not increase the latter's resolution. By architecture, AD9754's output will be scaled by 1/(2^14) with respect to the full scale output current determined by equation 3 in page 9 of the datasheet.

You may compare the gain error and gain drift of two DACs then use the better one as voltage reference of the other.

, please feel free to weigh in.

Regards,
Shine

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AD9754 Amplitude Control for DDS