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AD9629 linearity error

Question asked by crazyfrog on Dec 19, 2017
Latest reply on Jan 2, 2018 by DougI

Hello, am using on my custom design an AD9629 used in differential mode.

 

It is working, however there are error when I applied a DC voltage from a differential ADA4940. there is some error on the conversion.

 

VIN+      VIN-     (Vin+-VIN-)    ADC value therorical value    real value
0,4073    1,3615    -0,9542        95                   49
0,4331    1,3353    -0,9022        202                 153
0,4835    1,2832    -0,7997        413                  372
0,5339    1,2310    -0,6971        625                  588
0,5850    1,1790    -0,5940        838                  800
0,8782    0,8748    +0,0034        2070                2055
1,1452    0,5985    +0,5467        3192                3194
1,3273    0,4103    +0,917          3956                3972

 

The sample frequency is 5MghZ, DCO shows the correct frequency
VCM is correctly decoupled with 100nF & 1uF
The pcb is a 6 layers with a ground plane.
RBIAS is 10K
Vref is stable at 0.9925V ( internal)

 

I have no clue why there is such an error between the theoritical value and the value measured.

 

I have change the SPI register USR2 ( 0x101) to 0x8C with no success.

 

Does anyone why differential ADC has such poor linearity?

 

thanks in advance for any suggestions.

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