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LTC2326-16 8C93 - Only +/-5V Input

Dear,

My LTC2326-16 only display +/-5V when reading using arduino
I tried to read many topic but i did not find any related one.

First I connect the LTC 2326 correctly with internal reference, I have measured REFIN 2.048V and REFBUF 4.096V -> correct

Secondly, the chip is soldered correctly,

Thirdly, I use the Arduino library to communicate with LTC2326-16  

//===================== int16 ==================== 15bit with 1 bit signed to read from LTC2326  ======

int16_t LTC2326_16::read()
{
int16_t val;

digitalWriteFast(_cnv, LOW); // Reset CNV for another conversion later on
SPIFIFO.write16(0);
val = SPIFIFO.read();

return val;
}

//===================== int16 ====================

NOTE that :  int16_t from -32768 : 32767

By applying  IN+ and IN- I have  some value 

(IN+ - IN-)  = 2.09V  ::  VAL = 13406

(IN+ - IN-)  = -2.09V  ::  VAL = -13408

(IN+ - IN-)  = 5.09V  ::  VAL = 32574

(IN+ - IN-)  = -5.09V  ::  VAL = -32294

(IN+ - IN-)  = 6.09V  ::  VAL = 26542   ->> ???? WHY 

(IN+ - IN-)  = -6.09V  ::  VAL = 27650     ->> ???? WHY 

-=========

According to the setup, REF internal is used, (IN+ - IN-) should have +/-10.24V range.
However, when the input voltage is higher than 5.1V, the ADC value is reduced (see 6.09V case)???

As I understand, The output data is in 2’s complement format, does it affect my reading

Sorry for my bad English,

could you please help me??
thank you so much