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AD1853 introduces spurs when given a DC offset?

Hello EZ,

I'm attempting to use the AD1853's DAC as a control signal, but I've noticed strange spurs at the output whenever I introduce a DC offset.  For instance, if I excite the inputs with a positive DC offset of a few mV, then return to a 0mV 0Hz (0mV DC signal), then at the output of the DAC I see spurs at 20kHz, 40kHz, etc. on the frequency response.

If I increase the DC offset to 10mV, the spurs seems to start at 40kHz, and repeat at 40kHz intervals.  If I decrease the DC offset to below 0mV, then I see spurs before 20kHz, and repeating at regular intervals.

Worse, these spurs modulate with my actual control signal whenever I add it to the input of the system, and I need the full 20kHz for my control signal.

Has anyone seen this?  Is there any way to mitigate/remove these spurs?  I know the AD1955 is recommended to replace the AD1853, but I cannot use either part if these spurs occur. 

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