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.
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:
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.
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