AD7684 issue (strange behavior)

Hi,

We are trying to modify the input stage of an already manufactured board by replacing AD7793 by AD7684 in order to measure up to 1KHz signals. I have attached a simplified schema of our input stage and two waveforms we have obtained from the AD7684 output.

We are only getting good performance when we connect our oscilloscope to IN+ and IN-, furthermore, we get good results when connecting just the GND of the oscilloscope wherever IN+ or IN-.

We need to measure in both scenarios IN+ > IN- and IN- > IN+, that is why we selected this AD7684 truly differential instead of AD7693

Could you provide some solutions, similar experience... any hint could help!

Best regards and thank you in advance,

attachments.zip
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  • Hi Harry,

    Thank you for your reply.

    On the "AD7684 issue.pdf" the X axis is time in miliseconds and Y axis are volts.

    On the 1 Ohm shunt we have to measure currents around mA (ie. 0.80mA or 2.4mA)...

    >>One meg is high impedance.  Try changing all four resistors to 10k-20k??

    We will try that, thank you. I will let you know.


    >>"We are only getting good performance when we connect our oscilloscope to IN+ and IN-,"

    >>Does this mean two scope probes on IN+ and IN- on your schematic, i.e. across the sense resistor?

    That is rigth, when connecting the oscilloscope probe ground to any of them IN+ or IN- the measurements are fine.

    Do you think that replacing with the AD7683 pseudo-differential we could solve this problem?

    Thank you again, regards,

Reply
  • Hi Harry,

    Thank you for your reply.

    On the "AD7684 issue.pdf" the X axis is time in miliseconds and Y axis are volts.

    On the 1 Ohm shunt we have to measure currents around mA (ie. 0.80mA or 2.4mA)...

    >>One meg is high impedance.  Try changing all four resistors to 10k-20k??

    We will try that, thank you. I will let you know.


    >>"We are only getting good performance when we connect our oscilloscope to IN+ and IN-,"

    >>Does this mean two scope probes on IN+ and IN- on your schematic, i.e. across the sense resistor?

    That is rigth, when connecting the oscilloscope probe ground to any of them IN+ or IN- the measurements are fine.

    Do you think that replacing with the AD7683 pseudo-differential we could solve this problem?

    Thank you again, regards,

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