ADA4522-1/ADA4522-2 huge offset voltage

Hello, 

I want to use 1 ADA4522-2 or 2 ADA4522-1 as unity gain buffer for 30V and 48V. To test the ADA4522-1 I created a minimal setup (see attached schematic). While I have 25.1491V on +IN, I get 25.1848V on -IN. That means an offset of 35.7mV. In the datasheet the ADA4522-1 promises an offset voltage of maximum 5uV. I tried two different chips and buffer setups with different input - and supply voltages but, I always get offset voltages of 35.7mV and above.  I tried to follow the application notes for the buffer setup given in the datasheet, but it didn't work out. For my design I need  small offset voltage (< 1mV). Am I missing something.

I hope that you will help me to identify the issue.

Regards David

  • Hi David,

    I would measure the current consumption in the first step.

    I would check the behavior of the input part of the amplifier in the second step. Without supplying power to the amplifier, you can check the operation of all protective diodes between the inputs and between each input and the power rail. It's just for reassurance

    I don 't understand why R1 is here.

    This amplifier has a feature when working with a signal source with high resistance that is discussed in the datasheet. In your case the source resistance is 10 kΩ, maybe this should be taken into account. If you use an additional 25 V voltage source instead of a resistive divider, you can find out if the output resistance of the divider is involved in the strange behavior of the circuit in your case.

    Regards,

    Kirill

  • +1
    •  Analog Employees 
    on Mar 6, 2020 1:10 AM

    Hi David,

    The ADA4522 doesn't like high source resistances in its inputs. I would suggest testing the part first using a simple buffer circuit without the 120 ohms resistor, then connect a 25V input directly to IN+. If the offset that you're seeing is minimized or completely removed by this action, then the voltage divider circuit that you've created could be the culprit of the offset voltage that you are seeing.

    Thanks,

    Donnie

  • Hi Donnie,

    infact there is basically no offset when connecting 25V to the input as you suggested. Unfortunately I can not avoid having a huge input resistance in my circuit later on. Can you suggest a part that does not have an issue with high input resistance. In my design I need to buffer 48V. 

    Regards 

    David

  • 0
    •  Analog Employees 
    on Mar 6, 2020 11:28 AM in reply to trodav

    Hi David,

    Are you dealing with varying source resistances? If so, then what could be the possible max source resistance that the op amp could see?

    Also, what would be your frequency of interest? A little more info on your use case would be helpful for me to recommend a much more suitable part.

    Thanks,

    Donnie