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INA826: Unable to meet Input Impedance specified in Datasheet

purvi gupta
Prodigy 150 points
Part Number: INA826
Other Parts Discussed in Thread: TVS1400
Hello TI,
 
In one of our project we have used Instrumentation Amplifier i.e INA826 and we are facing some issues with it. So we need your support to solve the issue.

Issue Details:
Our Requirement is to have minimum 100Mohms impedance at the analog input. To meet this we used INA826 as the datasheet of INA826 says that it could provide input impedance of ~20Gohms. But in reality we are getting only 2Mohms on board.

Here is the Schematic of analog input. To make the debugging simpler we unmounted all the component from the board except series resistor (R4 an R7) and INA826. But still we are getting 2Mohms input impedance.



The method which we are using to measure to Impedance:

We provided 5VDC input at AIN+ via Ammeter. So the flow is like: Function Generator (+ve) -->Ammeter (+ve) --> Ammeter (-ve) --> Board (AIN+) and Board(AIN-) --> Function Generator(-ve). The current we measured in Ammeter is 4.3uAmps.
 
We request you to look into this and share us your views as early as possible.

Regards,
Purvi
  • 0
    Prodigy 150 points

    It seems the image is not visible in the earlier post. So I have attached again:

  • 0 in reply to purvi gupta
    Guru 140200 points
    Hi Purvi,

    please read section 8.3.6 of datasheet:

    "The input impedance of the INA826 is extremely high—approximately 20 GΩ. However, a path must be provided
    for the input bias current of both inputs."

    In your circuit you don't have that current path! That's very probably the reason for the measurement error.

    Kai
  • 0 in reply to purvi gupta
    TI__Mastermind 23050 points
    Hi Purvi,

    Please see this thread: e2e.ti.com/.../2578565
    This should give you a clear understanding of what you need to do to measure the input impedance.
  • 0 in reply to purvi gupta
    TI__Guru 64840 points

    Agree with Kai's comment regarding problem with floated inputs and that your 4.3uA input current has nothing to do with the INA826 input impedance. But I believe the current most likely comes from the reverse-biased leakage of SMBJ30A transient voltage suppressors (see below: 5uA at 25C).  These TVS's are NOT meant to be used on signal nodes but only on power supplies. You'll be much better off using TVS1400 with much lower leakage current at room temperature (max 16nA at 27C).

  • 0 in reply to Marek Lis
    TI__Genius 11135 points
    Purvi

    We haven't heard back from you so we assume this answered your question. If you need additional help post another reply below.

    Thanks
    Dennis