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Offset output data for AD7705

Question asked by pifa on Dec 13, 2012
Latest reply on Oct 23, 2014 by Daniel_B

I put AD7705 in AD7705_1.pdf  (Uref-> R35) and in AD7705_2.pdf  (Uref-> MCP1525). There is no difference.

 

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CLK=1, 2,4576 MHz, 50 Hz

 

Gain=16, bipolar mode, buffer off

 

Self Calibration

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When you change the resistance get the following data:

 

Pt100       delta (AIN1+ -- AIN1-)    REF+     REF-       AIN1-         AIN1+      DATA r-r

 

0  Om                     0 mV          2,55 V       0 V       0,125 V       0,125 V      0x8000

10 Om                   10 mV          2,55 V       0 V       0,125 V       0,135 V      0x8FFA  

20 Om                   20 mV          2,55 V       0 V       0,125 V       0,145 V      0x9FFB

30 Om                   30 mV          2,55 V       0 V       0,125 V       0,155 V      0xBF17

40 Om                   40 mV          2,55 V       0 V       0,125 V       0,165 V      0xE07F

50 Om                   50 mV          2,55 V       0 V       0,125 V       0,175 V      0xF8EF

 

 

Measurements made in "AD7705_2.pdf" with respect to the GND.

 

By changing the GAIN=4 nothing changes.

 

 

I have the following question:

 

1 - why the output is non-linear (DATA r-r)

2 - why is offset when Pt100=0 (DATA=0x8000)

3 - Where I was wrong

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