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TLV3702: input and output noise

Zhenyu
Prodigy 140 points

Part Number: TLV3702

Hi,

I replaced one comparator (U5 in the schematic) on my PCB with the TLV3702. Voltages of the marked red points A, B and C have abnormal glitches as shown in the attached waveforms. Green points voltages are normal and noise free. During the test, C18 voltage is held zero. Before the replacement, there was no such issue.

The voltage waveforms are measured using Yokogawa 700924 differential probe. I notice that if the GND_zone1 is connected to the ground clip of a passive probe(the other end of the passive probe cable is disconnected from the oscilloscope, so the probe cable is floating), then all glitches are gone. I tried multimeter cables and a piece of 20-inch wire(all floating). They can all remove the glitches. Tried one Lecroy CP031 differential probe. The result is the same as the Yokogawa probe. Frequency of the glitches is the same as the 15V switching mode power supply. Pin 5 of U5B is lifted to 15V during the test. No jumper wires are connected to the board. Waveforms are probed one at a time. 

Can you shed some light on this? I am not sure if the noises are actually there or introduced by my probe. 

Thanks.

Zhenyu

  • 0
    TI__Prodigy 600 points

    Hi Zhenyu,

    If the measuring points A, B, C are referencing to ground (GND_zone1), why wouldn't you use a passive probe instead of using a differential probe?

    A few things to try:

    1. add a pair of bypass caps (0.1uF and 3.3uF) as close as possible across pin 8 and pin 4 of U5A
    2. Add a metal container over the board possibly grounded to GND_zone1 and share the fining
    3. Use a passive probe and ground it to GND_zone1 and share the finding

    4. If you could access a battery-powered oscilloscope and use it to see if the glitches are gone. Please share the finding

    All these suggestions should help to identify the source and nature of these glitches, and to help to formulate a solution.


    Please reply to this forum. If this answers your question, please kindly marked it as Answered.

    Best regards.

    -Jian Zhang