ADA4084 characteristics

Dear all,  

I probably posted a message in the wrong forum. Please find below the question related to ADA4084

Datasheet Table 3 specifies that at +/-5V, typical input impedance is [100KOhms||1.1pF]Differential and [200MOhms||2.5pF]Common mode.

Are these parameters guaranteed by design and can they be considered stable from one production batch to another?

In other words, for a voltage follower circuit, if we want to have a first order passive filter, is it possible to only add a resistor in series on the non inverting input? Can we considered the following circuits as equivalent? 

In addition to this, regarding Isy parameter specifies at 0.595mA. Does it mean that total opamp supply current is 0.595mA? Or, does it mean 0.595mA on +5V rail and 0.595mA on -5V rail implying total current of 2x0.595=1,19mA? 

Thank you in advance for your answers. 

Best regards, 

Benoît, 

  • Benoit,

    The 100k is calculated for a bipolar diff pair.  The capacitances are measured.

    For your second schematic, you have the 2.5 pF common mode, but you also have 1.1 pF to AC ground (the output pin)

    For duals, National Semi would give the total package supply current, the idea being you couldn't power only one.

    Analog Devices gives the supply current per amplifier.

    For a buffer with no load, whatever current goes in the top, has to come out the bottom, so total current is 1.05 mA max over temp,

    and PD would be 30V * 1.05 mA.

    It looks like you are not using the latest datasheet.  (rev. I)

    What Fc for your LPF??   I would not go into production relying on a typical input C, or use smaller R and larger external C.

    Harry

  • Hello, 

    Thank you for your answer. Unfortunately, I can't decrease resistor value to much in order to limit the current in case of faulty op amp (worst condition identified as to have a supply rail on op amp inputs). 

    LPF is used to prevent RFI op amp rectification but I need to have signals up to 100kHz not affected. Op amp CM and DM capacitance seem to already play a role at 100kHz. 

    In your message you wrote " but you also have 1.1 pF to AC ground" could you please reformulate? From my understanding this is how CM and DM capacitors should be interpreted. am I correct? 

         

    Thank you. Best regards, 

    Benoît, 

  • Yes.  A perfect op amp has zero ohms output impedance, so it is AC ground.

    However, as the op amp responds, the 1.1 pF is bootstrapped.  But you still have

    100k and 2.5pF which is 632 kHz, so you will have 5.7 deg phase shift at 63 kHz.

    You can't non-destructively protect against all faults, so I would drop the resistor,

    and once every 10 years when the op amp blows up, replace it.

    Harry