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negative input voltage on ADC (AD7654)

Question asked by S.R. on Oct 22, 2015
Latest reply on Oct 23, 2015 by S.R.

Hello,

 

in my Design I am using a AD7654 (16bit ADC). The analog frontend has a autozero function that uses a DAC to add a correction signal to the sensor signal. This works fine in normal operation however when the required DAC setting is determined, there may be negative voltages on the AD7654 inputs. According to the datasheet section "analog inputs" this is not a problem as it is current limited (driver delivers 10mA).

 

Upon testing the design I found the following behaviour. The DA7654's internal protection diodes clamp the input to -2,7V. When input A1 is negative and input B1 is positive then the conversion of A1 is zero and B1 is unaffected.

When input A1 is positive and input B1 is negative then the conversion of A1 is depending on how negative B1 is: starting with input B1 of about -0,8V the conversion of A1 is lower than expected. More negative voltages cause results down to zero. On the other hand, starting with input of B1 of about -1,5V, the conversion of B1 gets positive. This is of course misleading and my autozero function fails.

 

I could not find any specs that describe this behaviour. Are there any? Why are the channels different?

As applying negative voltages is described in the datasheet, I would expect that the conversion result is zero and another channel is not affected.

I will try to make the autozero function save, as everything else is working well. This would be much easier if I knew the margins. Any help is much appreciated.

 

Regards,

Stefan

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