74HC4051 use problem

Engineer asked Apr 27,2022
0

Using 74HC4051, the eight inputs are connected together to a sine signal (only for testing so the inputs are connected together), and the sine signal measured at the output is as shown in the figure, the waveform is very wide, about 200mv. And when measured with an oscilloscope pen, it sometimes turns into a normal waveform by itself as shown in the figure below

The above phenomenon feels like a chance event, which happens occasionally.

And the experiment before obviously did not have this phenomenon, it was always a normal waveform. I added a bypass capacitor coupling capacitor to the op-amp in other parts of the circuit, and then it became like this.

Does anyone know what is the reason for this?

1.png2.png


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    Possibly due to the fact that the 4051 analog switch has a "break before make" characteristic - depends on the cuircuit. Also looks like a typical measurement error where HF spikes are coupling into the probes GND lead. Need to see the circuit and your measurement setup to figure that out.

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  • 0

    Looks like you are oscillating, can you zoom in on the waveform? Do you have decoupling caps on VCC and VEE? What is the layout of the board? https://www.ti.com/lit/ds/symlink/cd74hc4051-ep.pdf?ts=1599568122024

    Engineer 2022-04-28 08:27:29Reply

    Thank you for your help, I figured out that the op-amp has high-frequency self-excited oscillations, caused by improper bypass capacitor coupling capacitance.Therefore, the curve appears.

    Petr Suchomel 2022-04-29 04:50:44Reply

    Good caps close to power inputs, sometimes using a slower IC also helps if you do not need the speed.

    Reply
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