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TL084: Opamp's input change

SAli BEN SULEIMAN
Intellectual 780 points
Part Number: TL084
Other Parts Discussed in Thread: ADS1115, LM324, , TLV4170

Dear all,

I have a device that I measure its IV curve, this device's working voltage range is from -3 to 3 V, and in order to read the IV data I'm using the ADS1115, so I have to convert the device's input and output voltages to a unipolar signal from 0 to 3 V and to do this I'm using TL084CN , when I connect this opamp to the first terminal of the device the signal is converted successfully but when I do the same in the other terminal of the device where there is a resistor connected to measure the current, the negative side of the  input signal of the opamp disappears and I see only the positive side, is that because of the common voltage range of the opamp? could someone explain this for me, the power supply of the opamp is -5 V and 5 V. The same thing happens when I connect LM324 opamp.

The circuit schematic is attached.

Thank you,

  • 0
    TI__Guru*** 139956 points
    Suleiman,

    I think the biggest issue is that the 1k voltage dividers will pass current so that will disrupt seeing the device current on the 10k resistor.
    TL084 does require up to 4V above the negative power rail (-5V) for input minimum which will be -1V; therefore the 0V to 3V input is fine.
    LM324 should also works from the -5V, 5V supply. Is there a time when the output voltage of the op amp is not the same as it's input voltage?

    You could use a unity gain buffer at the DUT under test then do the 1k voltage divider (10k would be lower power) then buffer the divider output for the ADS1115.

    In this case, try the TLV4170 as an op amp. If has low input current and a more friendly input and output common mode range.
  • 0 in reply to Ron Michallick
    Intellectual 780 points

    Thanks Ronald for your reply and explanation.
    I connect a buffer then I connect the voltage divider and the other buffer, the issue is solved.
    Thank you