Strange Input Current in ADA4084-1

Greetings.

I am using the ADA4084-1 in a non-inverting, follower configuration in a data acquisition application.  The input is AC coupled.  At high input power levels, the input suddenly latches to a high value, and will stay there until the input RMS voltage is brought down.

Here is the circuit.Schematic

Here is the (yellow) input at R26, for a small RMS input voltage (purple).

Good follower

When a larger input voltage is selected, there is suddenly a large offset in the input voltage.  Why?

Naughty follower

Thanks for your attention.

  • 0
    •  Analog Employees 
    on Jan 2, 2019 7:00 AM

    Hi,

    I see nothing wrong on your schematic.

    Few questions:

    What is the voltage rating of your input capacitor?

    Is you input pure AC? Why did you AC-couple it? Do you measure a large DC offset?

    Where do you measure your output voltage? Directly to the output of amplifier or after the output AC-coupling capacitor?

    Also, can you confirm that your input is a 100kHz sine wave, on your schematic it is in 800kHz. If it is indeed 800kHz this will cause a problem on your amplifier since the slew rate is not enough to compensate the input you have injected.

    Best Regards,
    Goz

  • Goz,

    Thanks for taking a look.

    What is the voltage rating of your input capacitor?

    50V.

    Is you input pure AC?

    No.

    Why did you AC-couple it?

    Because it may have a DC component.

    Do you measure a large DC offset?

    Where?

    Where do you measure your output voltage?

    I think what you mean to ask, is where is the second plot measured from?  The yellow trace is measured at the non-inverting input; the same as in the first plot.  The problem is observed at the input, not the output.

    Also, can you confirm that your input is a 100kHz sine wave

    Confirmed.  The problem appears at many frequencies.

    Regards,

    Bill

  • 0
    •  Analog Employees 
    on Jan 3, 2019 3:44 AM in reply to fingew

    Hi Bill,

    May I know where does your input signal came from? What is it's DC offset level(few mV)? Does your DC offset level vary with the amplitude of your sine wave?

    What type of capacitor are you using? Was it polarized or non-polarized?

    Can you try a different value of input and see at what level it starts to latch up? Say 1V, 2V or 4V.

    Also, can you try removing R21, isolating the input circuit from the part confirms whether the it is the parts fault or there is an error in the input circuit part. Let me know if the problem still persist upon removing.

    Best Regards,
    Goz

  • Goz,

    Thanks again for your reply.

    May I know where does your input signal came from? 

    It comes from a function generator.

    What is it's DC offset level(few mV)? 

    Whatever I want it to be.  The DC component is blocked by the capacitor.

    Does your DC offset level vary with the amplitude of your sine wave?

    To the right of the capacitor, yes.  That is what is so surprising.  Why should the input have a DC bias that varies with the input amplitude?

    Can you try a different value of input and see at what level it starts to latch up?

    At 100 kHz, the latch-up happens at around 3.5V p-p.

    Also, can you try removing R21

    R21 is only in the simulation; it doesn't exist in the real schematic.  I don't see how the C-R circuit could generate a large DC offset anyway.