AD2S1200: SinCos Common Mode Input Voltage

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Feb 23, 2016
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I have a question about AD2S1200 sin/cos input. The datasheet says that,
voltage swing is 3.6Vpp typically, but the common mode volts is 100mV, respect
to Refout. Does it mean the common mode voltage should be Refout+/-0.1V?

But I also find another description in datasheet where it says the converter
introduce a Vref/2 offset in the sin/cos signal.
So I am confused that whether the offset should be Vref/2 or Vref, and is that
offset generated by AD2S1200 internally or we need to introduce a offset
externally in the input signal.

Also, if I find the sin/cos signal(Vpp) from resolver is smaller than the
requirement of 3.6Vpp, what should I do to amplify the input signal. May I
could try to use a large Exc signal by changing the gain of buffer circuit?

 

The Common mode Volts spec should be with respect to Refout/2. The AD2S1200
internally generates the bias that will centre the input differential signals
on refout/2. The customer can overdrive this is desired, in which case they
should ensure that the inputs are at all times between 0.2V and Vdd [see Note
1.]
If the inputs are too small the best option is to increase the gain on the
excitation buffer amp. This is the simplest and should introduce no additional
error. Another option is to introduce gain on both the Sine and Cosine
differential signals. However this is twice the effort and can introduce
amplitude mismatch between the Sine and Cosine if proper care is not taken.

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