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TL082: The output does not drive to 0V when used as a difference Amplifier

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Part Number: TL082

I am using a TL082 JFET input dual op amp as two difference amplifiers with a single supply of +24V. The first amp has a gain of 1 and an offset of 1.5V and it works fine. The input voltage to this amp comes from across a sense resistor fitted in a 4 - 20mA loop circuit . The output of the first diff. amp feeds into one input of the second diff. amp, and the other input of second amp connects to a the voltage divider setup that is adjustable by a pot. The gain of the second amp is 2.5 and its offset voltage is 0V. I have no trouble getting the full-scale output when 20mA flows through the loop circuit . The problem I have is that for input current of 4mA, when I adjust the pot to calibrate the amp to go to zero, I cannot produce a 0V at the output. The lowest output voltage that I get is about 1.5V. I tried simulating the circuit using the TI's op amp model for TL082 and I got the same result. Do I need an offset to the 2nd amp in order to drive it to 0V?

Thanks, Mike.

 

2 Replies

  • Hi Mike,

    you might need an OPAmp that is specified to drive the output down to 0V. The TL082 cannot do this.

    Kai
  • Mike,

    Are you sure the first amp is working well? For a robust design, the input pin voltages should be greater than 4V. I'm not sure you have that now.

    The output is emitter follower as seen in section 8.2 "Functional block diagram" in the TL082 datasheet. Therefore the output won't go to VCC- , your ground. An output pull down resistor could help.

    TL3472 output will get closer to ground. If low input current is needed then TLV2171 or TLV2172 will work.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

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