AD8500/AD8539/ADA4051 Input resistance


The some datasheet that it have describe the input resistance, like ADA4051, but the others datasheet didn't describe the input resistance, like AD8539 and AD8500

How to know input resistance?    especially no describe the input resistance???

Does the input resistance  have common value for no describe datatsheet ? 

  • 0
    •  Analog Employees 
    on Sep 18, 2018 2:41 AM


    AD8500 is a CMOS amplifier just like the ADA4051 and normally have a very high input impedance. Its Ibias spec of 1pA should give you a hint that it has very high input impedance. It is safe to assume a value in the high Mohms or Gohms range. AD8539 Ibias is in pA range also, its input resistance would probably in the Mohms and Gohms range also.

    By the way, ADA4051 input resistance is typically 8Mohms. It could be a good reference for AD8500.

    For further confirmation, I will communicate with the engineer of the parts you are asking to try and see whether I can provide you a typical value of the input resistance. I'll get back to you on this.

  • Hello,

    I also wonder the ADA4051 and AD820 input resistance, especial the input Differential Mode and input Common Mode.

    The input differnal mode resistance of ADA4051 is only 8MΩ, but the input differnal  mode resistance of AD820 has 100TΩ.

    does it have any difference for input stage structure ?    They should be the same FET structure.

    Could you explain what is input differnal mode resistance  and input common mode resistance?

    By the way, the ADA4505 only mention the input resistance is 220GΩ, What does this input resistance refer to input differnal mode or common resistance?

  • 0
    •  Analog Employees 
    on Sep 19, 2018 2:55 AM in reply to kimwin107

    Hi Kimwin,

    Some amplifiers are specified according to their lead application and at times a certain parameter is considered not necessary to give typical numbers.

    Input common mode resistance is the resistance between the input terminals when either the inverting or the non-inverting is grounded while Input differential mode resistance is the resistance between the inputs. For more information on opamp input impedance, you can check on this tutorial.

    ADA4505 220Gohm is a common mode resistance.