Power Supply Rails

Document created by analog-archivist Employee on Feb 23, 2016
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I'm using the AD5522 with the following Supplies: AVDD = 13.5V, AVSS = -7V,
DVCC = 3.3V, VREF = 5V. The device works fine, except the current measure unit.
According the datasheet, the MEASOUT pin should have a center
voltage of 2.25V, but in my setup it has 1.2V. The relative change at the
MEASOUT pin, if the DUT current changes, is OK and as expected. If I set the
AVSS supply from -7V to -12V, the center voltage at the MEASOUT pin also
goes to 2.25V and is OK now. At AVSS = -12V, the current measurement
works fine.

 

On page 33 of the AD5522 datasheet (Rev. E, 05/2012), in the Choosing Power
Supply Rails section, the AVSS foot room requirement of 4V is referenced in an
equation that defines AVSS for the measure out circuit to function correctly.  
With an offset DAC code of OxA492, a minimum AVSS of approximately -15V is
recommended.

Please also note in the datasheet (Rev. E, 05/2012) that the equation for MV in
Table 11 is incorrect. The corrected transfer function is: 
MV = 0.2 x (Vdut – DutGnd) – 0.2 x (Vmin)  
Vmin =  (-7/2  x Vref x Offset_Dac_Code/216)

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