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On my AD2S1210-EP board the measurement at the outputs EXC and NEXC shows a
distorted signal of the sinewave.

The output sine wave is overlayed with a distortion voltage (2.5MHz and 400mV).

The supply voltage is filtered according to datasheet:  see "filter.png"

The output is buffered for EXC and NEXC (similar) as shown in the picture
below. The output of the OP's goes to an excitation circuit.

CLKIN is 10MHz.

Please let me know if a similar behaviour is known and where the
problem could be.


The 'noise' that you are seeing is normal. It is coming from the DDS which is
generating the excitation outputs. Because the noise is on the excitation
frequency and is therefore common to both the Sine and Cosine inputs it is
taken out by the operation of the tracking loop. In addition the resolver
itself acts as a natural filter. This means that the 'noise' which you are
seeing will have no impact on the performance of the AD2S1210 in a resolver
application. If you are concerned with the appearance of the noise I would
suggest a filter stage in the excitation buffer. This is done on the AD2S1210
with a capacitor in parallel with the feedback resistor on our buffer circuit.