AD7679 : Error in reading Voltage less than 1V

Hi Swathi B K,

           The ground is important as a point of reference. As you mentioned there will be the wrong result when the source ground is disconnected from the board. When doing this although the source output the voltage correctly when measured at the source output pin but when this source is connected to the ADC, the ADC won't be able to see it clearly, since the source ground and the board ground are not common. It is recommended to connect the source ground and the board ground together, so both will have a common reference point.

Regards,

Jonathan

Hi Jonathan,

Thanks for your reply

I have a small correction here, external supply a voltage given directly to board from external POWER SUPPLY in range 0 - 4V.  My application involves a measurement of voltages from series of RC Network maintained the ground of my board and RC network ground are connected.

With external power supply readings are accurate with less than 1% error, but Voltage applied from RC network is as mentioned below:

ADC input           ADC output(V)
   0 - 0.47V               0
    0.5V                     0.0016
    0.8                       0.48
 0.9 - 1V                  1.007

ADC output was constant values without any variations as mentioned for voltages less than 1V. For voltages greater than 1V ADC output is proper but with reduced accuracy.

NOTE : Voltage applied from RC network and  voltage at IN+ & IN- pins of AD7679 are same for each voltage applied in the range 0 - 4V.

Tried giving from different networks maintaining both the grounds shorted, but its working is same as giving error values for voltages below 1V.

Hi Swathi B K,

         As I understand the problem is when external supply is connected via the RC network going to the ADC. In this setup, the ADC output is constant at input voltage is less than 1V but not accurate at more than 1V. Is this correct? But with external supply only direct to the ADC, the read is accurate. 

        If my understanding is correct, the RC network may have cause the problem and also some accuracy at the external supply as well. The RC network may cause unstable on the external supply output. Can you try one of these just to verify that the RC network going to the ADC is stable. If the RC network is connected to the AD7679, I assume, the RC network has a differential input. Kindly connect the In+ and In- of the RC network together. Since it is differential the two should have zero result.

    In the above schematic, you have a  U22-x. What device is this? From the schematic above, where is the RC network connected? Was it before the U22?

Regards,

Jonathan

Hi Swathi B K,

         As I understand the problem is when external supply is connected via the RC network going to the ADC. In this setup, the ADC output is constant at input voltage is less than 1V but not accurate at more than 1V. Is this correct? But with external supply only direct to the ADC, the read is accurate. 

        If my understanding is correct, the RC network may have cause the problem and also some accuracy at the external supply as well. The RC network may cause unstable on the external supply output. Can you try one of these just to verify that the RC network going to the ADC is stable. If the RC network is connected to the AD7679, I assume, the RC network has a differential input. Kindly connect the In+ and In- of the RC network together. Since it is differential the two should have zero result.

    In the above schematic, you have a  U22-x. What device is this? From the schematic above, where is the RC network connected? Was it before the U22?

Regards,

Jonathan