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ADF5901, ADF5904 readback chip id and reserved fields


During experiments I tryed to read back ADF5901 registers using DOUT line and by setting READBACK CONTROL code in R3. There are actually correct register values when ctl codes are 0b000001..0b001100 and 0b010110 for ADC readback. But I tryed to read back for ctl codes specified as 'RESERVED' in the datasheet. Suddently I got constant value 0x59010102 for ctl=27 that obviously looks like chip identification. For ADF5904 I found the same behaviour (0x59040002) for readback ctl=23.

There is nothing about this in available datasheets. The datasheet for ADF5901 is marked with 2017 year (too old?).

Could you point me to up to dated DS or reference manual with proper SPI protocol explaination because probably there are another possibilities except undocumented chip id readback.

Another thing unclear for me: what exactly means 'Reserved' parts for some registers shown in the datasheed as some value, not all-ones and not all-zeroes. For example, reserved DB19 in R1 has value 0 (Should I write always zero here? What will happen when I write one here?), DB17 in R2 has value 1 while all others are zeroes, and so on.

Thank you!