AD7655 return's 0xffff

hi all.

I'm using ad7655 to read some optic sensor's.

for signal larger than 1v ADC return's all '1' (0xFFFF).and for smaller value it returning wrong data

for example             

0.5 v  =>   0xC546!!

0.1v   =>   0xA200!!

value of decoupling capacitors(for reference pin and supply) are exactly taken by datasheet.

for reference pin 1uf ceramic and 47u tantalum.

and for AVDD 47u tantalum and 100n ceramic and .....

so what is wrong?

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