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Question about AD8436

Kazu
on Jan 26, 2021

Hi,

I'm  a DFAE in Japanese disty.

Our customer is using a part of AD8436 as  attached inverting amplifier.

<Q1>: Is it no problem to use it as an operation amplifier?

If  the answer to Q1 is "no problem" , please also take care Q2 below.

The attached schematic amplifies input voltage to about -10.7 times. 300mV input is amplified to -3.2V.

However, some AD8346 have a bit big Vout fluctuation as rising room temperature. Normally Vout fluctuates about 2%, -3.2V at Ta=25 degC, -3.14V at Ta=60 degC

Some AD8346 have about 5%, they output -3.05V at Ta=60 degC.

<Q2>: As for the schematic, is there any way to lower such Vout fluctuation caused Ta rising

PDF

Regards,

Kazu

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  • 0
    •  Analog Employees 
    on Jan 28, 2021 4:13 AM 2 months ago

    Hi Kazu,

    Good day!
    Thank you for considering the AD8436 in your application.
    For your inquiries, the output buffer may be used as an operational amplifier. The datasheet mentioned that this precision op amp may be configured with gain and also as a Sallen-key low pass filter, and it seems you're already using it as an operational amplifier with gain.

    For Q2, Are you using discrete resistors? I would think that the rise in temperature is also affecting the resistor matching and may contribute to the additional errors. This article explains this resistor matching https://www.analog.com/en/analog-dialogue/raqs/raq-issue-27.html.
    You can check some of our Precision Resistor Network to ensure resistor matching across temp. Though this is still on top of the AD8436's temp drift, I'm going to have to check on this further. But let me know your system's accuracy spec in order for us to help you further.

    Another thing on the schematic diagram, you can also add a compensating resistor at non inverting pin of the amplifier to balance out the bias current effect on the output.

    Hope this helps.
    Thanks Kazu.

    Best Regards,
    Dann 

  • 0 on Feb 1, 2021 5:42 AM 2 months ago in reply to DBautista

    Hi Dann,

    Thank you for supporting this case.

    Regarding resistors for Q2, I use SMD resistors, RR0816P (+/-25ppm/C) , shown the following website.

    https://www.susumu.co.jp/english/product/category.php?cid=5?cid=5

    I also added a compensating 24k ohm resistor between the non inverting pin and A.G (GND), but the problem was not improved. 

    Another problem happened when I did the electromagnetic radiation test  (JIS B402: Power Standard) to AD8346.

    - Test condition

       Emit 450MHz, 5W of electromagnetic waves 20cm distance from the AD8346.

    - Test result

       The output from the OBUFOUT pin became 0V even though OBUFIN- was applied 300mV.

    I also did the same test to EVAL-AD8346 and the test result was also same. The OBUFOUT pin of EVAL became 0V.

    I switched the DC BUFFER of the AD8346 to general OPAMPs (NJRC, T.I in my lab) and did the same test.

    The test result was not same. These general amps were working correctly.

    <Q3>: What is suspicious cause of this emission test result? (BUFOUT became 0V and general amps were different, no    

                problem)

    <Q4>: Do you have following test results?  Temp test, Noise test and Electromagnetic wave emission test.

     

    Regards,

    Kazu

Reply
  • 0 on Feb 1, 2021 5:42 AM 2 months ago in reply to DBautista

    Hi Dann,

    Thank you for supporting this case.

    Regarding resistors for Q2, I use SMD resistors, RR0816P (+/-25ppm/C) , shown the following website.

    https://www.susumu.co.jp/english/product/category.php?cid=5?cid=5

    I also added a compensating 24k ohm resistor between the non inverting pin and A.G (GND), but the problem was not improved. 

    Another problem happened when I did the electromagnetic radiation test  (JIS B402: Power Standard) to AD8346.

    - Test condition

       Emit 450MHz, 5W of electromagnetic waves 20cm distance from the AD8346.

    - Test result

       The output from the OBUFOUT pin became 0V even though OBUFIN- was applied 300mV.

    I also did the same test to EVAL-AD8346 and the test result was also same. The OBUFOUT pin of EVAL became 0V.

    I switched the DC BUFFER of the AD8346 to general OPAMPs (NJRC, T.I in my lab) and did the same test.

    The test result was not same. These general amps were working correctly.

    <Q3>: What is suspicious cause of this emission test result? (BUFOUT became 0V and general amps were different, no    

                problem)

    <Q4>: Do you have following test results?  Temp test, Noise test and Electromagnetic wave emission test.

     

    Regards,

    Kazu

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