I have used on-chip reference externaly on board and reference voltage change
its value when conversions
I use AD7887 in mode 1 (PM1=0, PM0=0), pin DIN is tied permanently low and
AIN1/Vref is buffered by opamp
before it is used on board. Is the reference voltage influenced by powering
down part via the CS# pin ?
Is it necesary tied CS# pin permanently low, when I want to use internal
reference externaly (if I want to use
mode 1 for simplicity) or is necesary to use mode 2 or mode 4?
Yes, mode2 or 4 is necessary if you want to use the internal reference of the
AD7887 asa reference source on your board. I would not recommend the use of
Mode 1, as this mode expects a falling edge of CS to power up the part.
Even though we don’t specify that the internal ref is powered down in mode 1,
we hint at it when we describe mode 4.