AD8045 and ADA4817 input protection

Hello,

I have seen in the Q&A discussion about limiting the input current to 5mA for input protection, but this is just a 'rule of thumb'

Is there an actual specification of the input current limit on these opamps?

AD8045 & ADA4817.

Thanks, Ken

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    •  Analog Employees 
    on May 15, 2020 4:13 AM

    Hi,

    Unfortunately, there is no specific Input Current Limit for these parts.

    And to make sure that the device will not be damaged in the event of over voltage in the input, we recommend that "rule of thumb" to limit the current. The limiting resistor (Rs) value can be achieved by:

    I can sense that you are interested to use high speed parts.

    May we know what your application is? Maybe we could check for other suitable parts.

    Thanks.

  • Hi Fdelaram,

    Thanks for the reply. That is what I suspected, but it is a shame that AD does not publish more details about the input protection. I am considering adding an external BAV99 pair to the +V and -V rails to protect the input, but this will add about 1pF, and possible non-linearity.

    The application is for measuring the saturation voltage on high power transistors (IGBT, SiC, GaN etc). Opamp requires very low noise as the saturation voltage can be a few mV, and high slew rate - say >500V/us. We currently use the AVDA4817 on our CS448 oscilloscope input and I am considering it for this application too but the input headroom is only up to +V-2.8V. I need a bit more.

    I am also looking at the LTC6228/LTC6229. A bit slower but nice low noise, and the input protection is specified at 10mA which I like.

    Any other suggestions would be appreciated.

    Thanks, Ken

  • Hi Ken,

    American engineers use rules of thumb when designing, and this is something to come to terms with. I also want to draw your attention to the fact that the maximum input current is specified for those amplifiers that have protective diodes between the inputs or between each input and the supply rails. These are my own observations.The LTC6228 datasheet clearly states this. For all the other amplifiers you have named, there are no comments about protective diodes. 

    Regards,

    Kirill

  • +1
    •  Analog Employees 
    on May 18, 2020 2:17 AM in reply to Kentxu

    Hello Ken,

    Based on your requirements which includes high SR, low noise, and high IVR, I found ADA4895.

    You may check its DS on this link.

    If you need higher Input, you can consider LT6274 since this operates with higher supplies.

    However, this has lower bandwidth.

    Both devices specify a 10mA input current.

    Thanks and regards.

  • Hello Fdelaram,

    Thanks for these useful suggestions. The ADA4895 looks interesting and good to see there is a section on input protection in the data sheet.

    Best regards, Ken

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