AD7711A: Power Up issue

Q 

I have the AD7711A. On my board are 3 piece of the AD7711A. With the first one
I use the current source by writing the IO-Bit in the Controll register.
Sometimes the first AD7711A have strange values for the digital filter
parameters FS11-FS0.  The unused input pins are grounded und the sync-input has
an RC. The R is connected to 5V and the C against Ground and the middle ist
connected to sync. How can it occur that the values of the digital Filter are
not always default 0x146?

 

A 

The SYNC can reset the digital filter.  However, it does not control the value
loaded into the filter register.  On power up, the default values should be
loaded into the AD7711's registers.  If DVDD powers up slowly, pulling SYNC low
will reset the filter so that it starts from a known state (it does not reset
the filter register).

When the AD7711's power supplies have powered up and stabilised, the AD7711's
filter register should contain the default value.  Can the customer read back
the filter register to ensure that the correct value is loaded.  The customer
should then write to the AD7711 to enable the excitation current.  Again, I
suggest that the customer read back the register to ensure that the correct
value has been written to the I/O register.  The customer should also check
their read/write commands i.e. ensure that the correct number of SCLK pulses
are applied.  I also suggest that SCLK idle high when it is not being used. 
DIN should also be idled high when it is not being used.  This makes the serial
interface more immune to noise.  If DIN is low, spikes on the SCLK line will be
seen by the AD7711 and it will assume that a command is being written to the
AD7711.  This can cause incorrect data to be written to incorrect registers.