I'm using an Charge Pump Inverter,MAX899R in free run mode with switching frequency of 500kHz like in the circuit attached. The problem is that I'm seeing a lot of switching noise in the output, much more than i expected as it is visible in the figure attached. Moreover the noise has a frequency of 1MHz, and not 500 KHz, is this normal ? How could i fix the problem ?

Switching Noise


Update After a suggestion of AndyAka, I checked the ground and it has the noise I'm seeing in the output, I read it would improve to use a ferrite bead after GND and after VCC but no improvements. I'm adding 3 more pictures with the noise at the ground(fig1), noise in the VCC with osciloscoppe earth probe closer to C11(fig2) and the noise in VCC with earth probe closer to C20.

Ground Signal Noise At GND

VCC with with osciloscope earth probe closer to C20 enter image description here

VCC with with osciloscope earth probe closer to C11 enter image description here

  • \$\begingroup\$ Where have you got the earth side of the scope probe? Two edges per 500 kHz will look like 1 MHz. \$\endgroup\$
    – Andy aka
    Commented Nov 10, 2016 at 17:50
  • \$\begingroup\$ The earth of my probe is connect to the GND. \$\endgroup\$ Commented Nov 10, 2016 at 17:58
  • \$\begingroup\$ But how close to C20 and, if you have the probe earth and tip grounded at the same point do you still see stuff? \$\endgroup\$
    – Andy aka
    Commented Nov 10, 2016 at 18:41
  • \$\begingroup\$ How does your layout look? C19 and C20 should have very short but wide traces to the associated pins of the IC. \$\endgroup\$
    – Grebu
    Commented Nov 10, 2016 at 20:01
  • \$\begingroup\$ @Andyaka You are right, i moved the probe earth closer to the capacitor ground and the noise got greatly removed. Now i have around 500mVp swing. I add several capacitors of 100nF at output in paralel but there was no significant improvement. Do you have any tip ? \$\endgroup\$ Commented Nov 10, 2016 at 20:07

2 Answers 2


Background: -

If you short the probe tip to probe earth and leave the probe earth still connected what do you see? - Andyaka

@Andyaka I shorted and i see the same noise I'm seeing at the output.

The basic problem here is your scope is picking up a signal in the loop formed with its ground connection. You need to use a better probe or a differential measurement. Here's apicture from TI that explains the problem: -

enter image description here

The picture was taken from Understanding, Measuring, and Reducing Output Voltage Ripple by Texas Instruments and covers the issue of good scope probing techniques.


The most likely problem here is the self resonance of the output capacitor. Try putting smaller capacitors in parallel with it. At the noise frequencies in your scope photo, the 22uF capacitor is looking like and inductor.


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