For the conversion output of the AD7908, the value at full scale input fluctuates.The AD value changes according to the voltage of the previous channel.Measured at 5 V full scale.Condition 1 VIN 1 = 5 V, VIN 2 = 5 V AD value 255Condition 2 VIN 1 = 0 V, VIN 2 = 5 V AD value 252It is the same result even if the time of VIN1 and VIN2 is sufficient.If the voltage of the previous channel is 0.5 V or 1 V, the AD value is 255.The input buffer circuit is also considered, so I think that there is no particular problem.What can be considered as a cause of this phenomenon?
There are several reason that causes the problem. It seems that it always give a 255 value. May I have your schematic including the reference and front end. And also, may I know how you configure the control register and the timing diagram of the SPI.
Thank you for reply.
So This is the front end.
Vdd isn't 5V but 5.3V. Buffer AMP is NJU7018.(Same Vdd=5.3V).
The interval between sample hold and hold is open enough time.
I will assume that your input range is 0V to 5V. I noticed that your reference voltage is 2.5V. In this case, make sure that the Control Register Bit  is clear. Second thing that I noticed is the RC filter is located before the buffer. It should be between the buffer and the ADC inputs. Last, you said that the Vdd is 5.3 but the AD7908 only accept up to 5.25V.
I will reply to the following.
/Control register  is cleared each time./The RC filter point is under assumption. Therefore, the interval of sample and hold feeling is lengthened./The same phenomenon was seen even when VDD was set to 5.1V.