I am trying to use LMH6642MAX op amplifier in the non-inverting closed-loop configuration (gain 3.5) for a periodic square input signal 20 kHz (2 Volts High, 0 Volts Low).

The problem comes when I measure the voltage at the output, the signal swings between 6 an 7 volts. I would expect to oscillate between 0 and 7V (3.5 * 2). Any clue why this is happening? It is a rail to rail op amp so it should be able to output 0V.

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As some of you have clearly mentioned the pin numbers in the schematics are not correct. The package that I have used is the SOIC, so the non-inverting pin is the 3 and the inverting is the pin 2 and this configuration is what I have in the PCB.

Yesterday I finally realized that IC was malfunctioning so I have replaced it. Now it works as expected but when the output is high the op-amp oscillates as shown in the image:

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However, after reading this document (https://www.ti.com/lit/an/snoa367c/snoa367c.pdf?ts=1635793735182) I realized that small capacitance in the output could make the op-amp oscillate. So I thought that the capacitance of the oscilloscope probe could be sufficient to make the op-amp oscillate. So I attached a 1k Ohm resistor and the reading as displayed in the image is correct.

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I am not sure if this was the root cause or just was a coincidence because I have tried to emulate the circuit using LTspice but I do not achieve the same result. With such small capacitance, I am not able to get the same results

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  • 2
    \$\begingroup\$ A warm welcome to the site. I imagine you've checked this but...are the nets and PCB tracks on pins 3 and 5 actually connected? \$\endgroup\$
    – TonyM
    Nov 2, 2021 at 12:09
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    \$\begingroup\$ What on earth does "2.5k8" mean? And, what is "1_k3" all about? \$\endgroup\$
    – Andy aka
    Nov 2, 2021 at 12:27
  • 1
    \$\begingroup\$ The last digit is just the resistor enumeration that I have used, maybe not very clear. Just to clarify, it is just a 2.5K ohms resistor and 1K Ohm resistor. \$\endgroup\$
    – Jonw3
    Nov 2, 2021 at 13:08
  • 1
    \$\begingroup\$ To be honest, I cant read much on those blurry scope shots.. Could you use two probes and show input voltage and output in the same shot ? that way one would at least know which output level is correct and which one isn't \$\endgroup\$
    – tobalt
    Nov 2, 2021 at 13:33
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    \$\begingroup\$ The connections look suspect. Some have a dot, and others do not. Measure the voltages on pin5 to supply ground and then pin 2 to supply 12V. Same for the capacitors. Why does pin 3 and 1 have a dot and not pin 4? Post a PCB drawing to clarify. \$\endgroup\$
    – Aaron
    Nov 2, 2021 at 13:35

2 Answers 2


If the physical device you using is the DIP8 package version of the LMH6642, then the pinout in your schematic does not at all match the DIP8 pinout. Perhaps you are using a prototyping board with a DIP package, but have overlooked that the Kicad pin numbering is for the 5-Pin SOT-23 version?

Your schematic certainly looks like those pin numbers are for the SOT-23 package.

The datasheet for this device shows all the package pinouts.


Most likely this is due to lack 0V on V-. (Supply not input Vin- !)

Or faulty connection to a low R pullup... tisk.. or ESD failed IC from finger trouble.

I noticed you indicated the design is a gain of 3.5. Actually it is 1+ Rf/Rin =3.

2.5k8 (or 2.58k) / 1k3 = 2+1 = 3


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