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AD7988-1 don't answer

Hello. I designed circuit based on AD-7988-1. The circuit repeates the reference design for the CS-mode (SDI pullup).
After circuit has been assembled, I checked the power supply and the reference voltage to the output circuit - everything is fine: VDD=2v5, VIO=3v3, VREF=5v0
At the In+ is applied different voltages: 0 and 5v. Pin IN- grounded.
SPI request from master according the datasheet (I mean).
In the SCK pin I see 16 clock pulses, but on the SDO pin I see 3v3 in case weak pullup (>30kOhm) or 3v3 whiсh reduced to 0 when I connect oscilloscope