we used AD2S83 before, now he hope our AD2Sxx have incremental encoder output, so he select AD2S90. But the frequency of his resolver's output signal is only 50 Hz. And AD2S90's input signal's frequency is 3 KHz minimum. So it seems AD2S90 isn't the correct choice, isn't it? Then which ADI's part can meet his requirement, that is, 50 Hz input signal, incremental encoder output?
You are right. The frequency low limit of AD2S90 device is 3 kHz which means the AD2S90 won’t work with the excitation frequency selected to 50 Hz. However if customer is looking for incremental encoder output with low excitation frequency signal we may suggest him simple solution based on the AD2S83 which he was using in the past. Please open link below: http://www.analog.com/static/imported-files/application_notes/5033918389419AN-26 5.pdf Chapter 5 of this PDF document explain how to modify the AD2S83 circuitry to create incremental encoder outputs. Basically what they need is one XOR gate, one inverter and two D type flip flops. Connection diagram is presented in the figure 6.