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AD7091R outputting incorrect values.

Question asked by lms on May 19, 2014
Latest reply on Jul 27, 2014 by KarenNE



I have a  CN0335 ADC Board which is a +/- 10V isolated system with the AD7091ADC, i'm very new with terminology and working with ADC's so i'm a bit confused with what is happening.


i have the CN0335 board connected to a FPGA which is reading the data via SPI. when I read the the data sheet for the CN3035 it says that even low the ADC is 12 bits a 16 bit word is outputted to be compatible with processor serial ports. As i am using a FPGA and only interested in the first 12 bits I only clock SCLK for 12 faling edges then bring CS_N back high.


The problem that i am having is if i represent the data as signed decimal on the FPGA logic anayse rand i increase the input signal to the adc from 0V DC  to +10V DC i can see the the value start at 0 and increase to 2047 which is about 5.5V and then the number goes - 2047 and starts counting down as i raise the input voltage to 10 V


i have changed the representation to standard decimal and from 0V to 10V the data outputted is from 0 to around 40000 which seems plausible. however if i change the polarity so i have a -0V to -10V at -OV the data being outputted is 4000 and works is its way down to 0 as the voltage reaches -10V.


therefore the problem is an output of 1000 decimal could mean a positive number around 2-3 volts or a negative number around 6-7 V.


i hope this makes sense. any help would truley be appreciated