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OPA2137: OPA saturation problem

Part Number: OPA2137
I am using OPA2137 to pickup signals coming from an energy harvesting device (EHD).  
I have attached a PPT with two slides.  The first slide shows the OPA wiring (sorry for the graphics...) and the second slide shows the overall testing setup.
The problem is that the amplifier (Out A) goes immediately to saturation when connected to the batteries.  This is before connecting the EHD to +InA.  In addition, when I disconnect the ground of the PCB of the amplifier from the housing of the testing setup, the output (Out A) goes to zero-that is not saturated anymore.  (I am feeding the EHD through the +inA because the EHD has high impedance and the oscillations is in the range of 1Khz.)
In slide 2 you can see the general setup.  As you can see the OPA is inside the testing chamber.  Maybe I need to house the OPA in a separate shielded package.
I appreciate very much your feedback!saturation problem.pptx
  • User,

    When the batteries are connected and the input is unconnected, the output will be unknown because the input is very high impedance and there is no DC bias voltage. If the EHD is capacitive like shown in the drawing then a resistor is needed from input to GND to provide a DC operating point. Otherwise the output will drift to one rail to the other.

    The input bias current is low so this new resistor can be 1M , 10M ohms or more.
  • Thank you Ron,

    I just connected R1=10M resistor from the input (leg 3) to the ground as in the drawing below.  The problem remains.  Again, the signal returns to zero when I disconnect the wire that connects the OPA ground to the package (the scope ground),

    (I also connected 0.1uF capacitors from the batteries to the ground)

    Figure 2

  • User,

    If the circuit is wired as you drew it out, then removing ground connection to OPA2137 board would change amplifier gain from 11 down to 1.

    When it doubt measure it out. Check the voltage on pins 1-4, 8 to see the whole picture.
    Correct would be [pin2] = [pin1] /11; [pin3]=[pin2]; pin 8 = +9V; pin 4 = -9V
  • Thank you Ron

    Well... Going to do just what you suggested, but now connecting both batteries, it looks like that connecting a resistor to the input as you suggested did solved the problem. In my early reply I just connected the + battery and saw the same phenomenon as I described above (saturation). Now it looks ok except for too large 50Hz noise. I'll probably add a filter to the output and shield the wires inside the testing box.

    Thank you for your help!