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LMP7701: Output clips for negative inputs

Part Number: LMP7701
Other Parts Discussed in Thread: LMP7702

Hi, I have half of an LMP7702 configured as a unity-gain buffer amp.  When the input signal gets to about -1.5 V, the output clips.  An LMP7701 in the same circuit has a 20-kohm feedback resistor, and its output clips at -0.75 V.   Supply voltages are ±6 V, and we see no dips on the  negative supply pin when the clipping occurs.  The load on each op amp is 20 kohms (with only trace capacitance).   I replaced the LMP7701 but saw the same behavior.  Any ideas?  Thanks.

  • Hi Chris,

    sounds like a wiring mistake...

    What is your input signal?

    Kai
  • Hi Kai, thanks for the reply. A wiring mistake or defect would be a likely culprit, I agree--however, if it's a defect in the PCB, it is occurring in two different locations, but having the same rather curious effect. The schematics and layout (in Altium) are correct. The input signal is basically a 200-V/s, ±2-V triangle wave produced by another LMP7701 (configured as a Howland integrator--and it is <not> clipping). --Chris
  • Hi Chris,

    The set up sounds simple enough, but can you provide us the schematic for your LMP7701/LMP7702 circuit? Please show the supplies, input signal and output load connections and conditions, etc.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • Chris,

    I believe you have a short or something close to it (~10ohm) at the output that pushes LMP7701 into a short-circuit conditions - see below.

    2844.Chris LMP7701.TSC

  • Thanks, everyone.  The cause of the clipping turned out to be a defective/damaged analog switch on the output of the op amp.  It was the last thing we checked...

  • Hi Chris,

    Glad you found the problem with your LMP7701 application circuit. Please close this E2E inquiry if the problem is resolved.

    Regards, Thomas
    Precision Amplifiers Applications Engineering