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AD8220 Output Voltage Swing

Hi

We use AD623 to amplify a 350ohm full bridge strain gauge. Recently due to the global shortage, we switch to AD8220 which was available at the time. Both bridge and AD8220 are running off a single supply of 0-5V.  The problem I have is that for the first 1000 identical PCBs we made, the output on 25% of them is limited to 1 to 3V. The AD8220 is set up to take differential inputs and according to the datasheet, the diamond shape output limitation should be valid on common voltage mode only. Otherwise, the output swing mentioned in table 2 which is 0.2 to 4.80 is very misleading.

Being connected to a full-bridge strain-gauge, both input voltages are very close to 2.5V across the operational range which is within the limited input range of the chip.

Please help me understand why the chip is behaving differently from the datasheet and why its characteristics are changing randomly. After many tests, I am pretty confident it's down to the chip itself.

I can mitigate this in the new boards by reducing the gain and compensating that in the next stage but we have a lot of rejected devices due to this issue already. I have used LT6370 instead of AD623 without seeing this problem but AD8220 seem unpredictable.

Please see the attached schematic.

Thanks

  • Hi Hamedhbb,

    Thank you for reaching out to us regarding this AD8220 issue.
    I have some clarifications with regards to the issues you're having in order for us to see the big picture and help you out effectively. 
    When you mentioned about the 1000 PCBs built, is this specific for the AD623? since, AD8220 is not a direct pin compatible how did you swap it on your PCB boards.

    Also, when you mentioned the 25% failing units, does it mean you have 250 boards that failed? or on a lower scale like 3 out of 10 boards? How about the boards that didn't fail? Can you give us some data for both the passing boards and failing boards? A simple input vs output scope shot on the AD8220 will help, please probe it as close as possible to the input and output pins. 

    It will also be helpful on the debug if you could Isolate the AD8220 from the circuit and apply replicate the input configuration of your bridge circuit (Common-mode and differential voltage) in order to see if the AD8220 is really busted.

    Thanks and I hope I explained it clearly Hamedhbb.

    Best Regards,
    Dann

  • Hi Dann,

     

    Thanks for getting back to me. 1000 is the number of PCBs that are slightly modified to accommodate different pinout of AD8220 compared to AD623. I have verified the PCB changes using the 10 samples I received. It was a simple change though.

    and yes 25% means we have 250 boards that are failed because the output from AD8220 is limited to the common-mode restrictions. The differential inputs are both around 2.470 V and they change around +/-15mV over the full span of loading on the bridge. On those that failed, I have noticed the output swing on AD8220 is limited to about 1 to 3 V ( I use single supply of 5V) and it varies from one to another. Changing the voltage on the Ref pin will not help to go beyond those limits.

    I try to use scope and send some screenshots. However, I have tried to take a failed board, replace the AD8220 with AD623 using some wires and a header board and considering everything else was the same, the board started to work as it should.

    I will get back to you with the scope shots that you mentioned.

     

    Best Regards

    Hamed

  • Thanks Hamed.
    I will wait on those screenshots.
    Sorry I didn't captured, does it mean that there are 750 boards with AD8220 that are working as expected? or all the working boards uses the AD623?

    Best Regards,
    Dann

  • We make tens of thousands a year with AD623 without a probelm. out of 1000 made using AD8220 on a slighly different PCB (because of difference in pins configuration), 750 are working fine and 250 are not.

    Thanks