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tl084 unity gain problem

Mustafa Gulec
Intellectual 540 points
Other Parts Discussed in Thread: TL084, SN74LVC2G17

I used a tl084 as a unity gain buffer at the output of an incremental encoder which has an output of a 5V pulse. The pulse is 0 to 5V. The signal is connected to the tl084 input through a 47k resistor. The tl084 has a single supply of 12V. I tried with a load of 10k + 47pF in series to ground at the output of the tl084. I also tried open circuit at the output. In both situations I expected the signal to be as the input but the output was 5 to 12V with a 2Vpp oscillatory signal on the top of the pulse signal. Also the output signal is square wave at low frequencies but as frequency goes up it gets smaller in amplitude and turns into some other shape like triangular. I could not comprehend what is going on. What am I missing and what should I look for? Thanks.

  • 0
    Intellectual 540 points
    When I shorted the resistance at the input, ripple over the output signal goes away, but the offset remains which about 4V or so!
  • 0
    Intellectual 540 points
    When i use all 4 opamps the problem goes away interestingly!
  • 0
    TI__Guru**** 168966 points

    Mustafa,

    The input common mode for TL084 is [VCC-] + 4V to nearly [VCC+]
    With a single supply and input less than 4V, the TL084 may not work as expected.

    The output stage is also not rail to rail so the output won't sink current to 0V.

    Op amp have output slew rate limiting as a side effect of the internal frequency compensation.
    With large signal and high frequency there will be distortion from the limited slew rate.

    For what reason do you wish to buffer the signal?

    There may be a better solution.

  • 0 in reply to Ron Michallick
    Intellectual 540 points

    The output of the encoder may be connected in parallel to different circuits which causes undesired results sometimes so I decided to isolate the signal using buffer. That was my main purpose. Also sometimes the output of the encoder is open collector type. So I added a pull-up resistor and buffered the signal before it enters the filter stage.

  • 0 in reply to Mustafa Gulec
    TI__Guru**** 168966 points
    Mustafa,

    Sorry for the delay. I'll ask our logic apps support for a Schmidt buffer device.
    Do you need 4 channels or 2?
  • 0 in reply to Ron Michallick
    TI__Guru**** 168966 points
    Mustafa,

    Logic recommended SN74LVC2G17 dual channel ST buffer.

    If the inputs can be outside the ground to VCC range then a resistor ( like 1000 ohms) can be put in series. Diodes to ground and VCC can also help.
  • 0 in reply to Ron Michallick
    Intellectual 540 points

    Hi thanks for the suggestion. I require four channels.  Input can go from 5 to 30Volts but usually is in between 5 to 12V. I actually used the opamp not to lose the original signal because I have a filter and comparator stage after the opamp. May be a small first order rc filter with the addition of clamping diode would do the trick to use this part that you offered, but i must design it first in order to decide if it fits all the wiring scenarios. I'll take your consideration into account thanks again and I also appreciate if you have any other opinion..... By the way I have a bunch of boards that I need to use with tl084 and if i use all four opamps it works but if I left some of them unconnected or tie them to the ground it does not work. Do you have any idea?

  • 0 in reply to Mustafa Gulec
    TI__Guru**** 168966 points
    Mustafa,

    The amplifiers are independent other than sharing a common current reference. I have not seen them interfere with each other.
    it is best to setup the unused amp in unity gain with a valid common mode input signal.
  • 0 in reply to Ron Michallick
    Intellectual 540 points

    When I configure unused opamps as unity gain with a 12V input there is no problem; but if I connect the input to the ground then the problem arises! So your suggestion is OK!