AD8676 Strange behavior

Why wont pin 1 (the output) of my AD8676 drop below 1.68V with 0V input into a non-inverting amplifer with 3 gain?

R51 and TP4 are disconnected so the circuit is open, so what you are left with is a non-inverting amplifier, gain 3, with a varying input using a 10K trimmer pot (R47). The sources varies between 0V and 5V.

Pin 7 (the output pin of the second opamp on the chip) belongs to an almost-identical circuit, different only by its slightly larger feedback resistor, and it is working fine.

  • 0
    •  Analog Employees 
    on Nov 12, 2015 10:29 PM

    Hi, Nathan.

    Are you using an evaluation board or a breadboard or vector board to test this circuit? There might be misconnection in your setup or the potentiometer is set to produce a small voltage that makes your output 1.68V. Please check pin 3 if it is really 0V.

    Let me know your if this helped.

    Regards,

    Anna

  • Hello Anna,

    Thank you for taking time to consider my problem. I am running the test on a pcb eval board. I remeasured the voltage on pin 3: it's 24mV, however, the voltage on pin 2 (the inverting input) is .558V. I believe this is what's causing the 1.68V on the output, it's about 1/3 of the voltage. This behavior is still bizarre, both inputs should be approximately the same according to all of my opamp 101 courses. Two other boards are doing the same thing but their voltage is actually higher: 2.2V and 2.4V and the inverting terminals are about .65V. Here is the schematic:

    R47 and R53 are 10k pots.

  • You are operating the OPAMP outside its input common voltage range.

    You need to stay about 2V away from the rails on this particular part.

    Looks like your negative supply is -0.232V (odd value...)

    See data sheet for details.

    Klaus

  • 0
    •  Analog Employees 
    on Nov 16, 2015 12:31 AM

    Hi, Nathan.

    In order to use the 0V input, please make sure that your negative supply must be at least -2.5V. This is what is also trying to tell.

    Just curious, same as Klaus, why are you using -0.232V for your negative supply? Is there a specific application or circuit in your system that requires this supply aside from AD8676?

    Regards,

    Anna

    BTW, thank you Klaus for helping us answer the threads.

  • Haha, yeah, that negative rail is actually an accident, it is supposed to be -5V, a wrong part was selected so I'm trying to work with it since it's already on the board. The part is a negative bias generator supposedly used to supply just enough negative potential to allow op-amp outputs to drop to 0V. KRZ, I noticed what you said before, I just didn't realize the output would be at such a value with zero input, even though it is out of the input range given those rails, the other opamp on the IC does not behave this way. Dropping the negative supply will work though, outside of this strange behavior, within the rated input characteristics the device is working fine. Thank you both for your assistance.