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ADC count in AD7606 saturates beyond 2.7V

Question asked by anil.spatil on Mar 12, 2012
Latest reply on Mar 13, 2012 by clairec

Hi,

We are evaluating AD7606 ADC for our project. We have evaluation kit of ADC EVAL_AD7606 with us and we have interfaced this ADC with blackfin ADSP. We have written the software code for interfacing and reading the ADC counts.

While reading the datasheet of AD7606, we have come across following formula to calculate the ADC count –

Please refer Page 26 of the datasheet of AD7606 - “ADC TRANSFER FUNCTION” section -

 

Also note following excerpt from the very next page in the section “INTERNAL/EXTERNAL REFERENCE”

“An externally applied reference of 2.5 V is also gained up to 4.5 V, using the internal buffer. This 4.5 V buffered reference is the reference used by the SAR ADC.”

 

So REF in the above formula should be considered as 4.5V.

Now we calculate the ADC code for a VIN of 5V = (5/5)*32768*(4.5/2.5) = 58982.4

But it is not possible as the ADC has a saturation value of +-32768 counts. This implies that maximum input voltage which can be given to input channels is 2.7V. If we give input voltages greater than 2.7V, the ADC counts will be saturate.

We verified these results by actually observing the ADC counts at various input voltages. It was confirmed that ADC counts saturate at around 2.7V.

 

Contrary to our observations, the datasheet mentions that ADC can take “True bipolar analog input voltages of +-5V.” This means that ADC counts should vary between -32768 to +32767 for the input voltages of -5V to +5V.

Can you please help us understand why our observation are not matching with datasheet specifications?

 

Other settings –

External reference of 2.5V is used.

VDRIVE voltage = 3.3V

AVCC = 5V

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