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enter image description here

I have a camera core with a DVP interface which has 2.5 V data logic and the system I am using is 3.3 V level. So I have used a TI SN74AVC20T245 level shifter to shift 2.5 V to 3.3 V.

The image from the camera core has a slant noise pattern. I found out the level shifter is injecting noise to GND as the core has an analog ground which is connected to the D GND; it's picking up that noise.

This slant noise is not visible when I remove the level shifter and bypass the data lines.

Has anyone faced this issue? What are all the possible ways to remove the noise with the level shifter?

The clock frequency is 27 MHz.
The board is a 4 layer PCB and has GND on all the layer with stitching vias on all available space. There are no decoupling capacitors and termination resistors on the output side of the level shifter.

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  • \$\begingroup\$ A situational setup (like pictures) would really help. \$\endgroup\$
    – RemyHx
    Sep 5, 2022 at 5:21
  • \$\begingroup\$ In general, it’s about shared return path / lack of a proper return path like a solid ground plane. \$\endgroup\$
    – RemyHx
    Sep 5, 2022 at 5:23
  • \$\begingroup\$ @RemyHx board is 4 layer PCB and has GND on all the layer with stitching vias on all available space. \$\endgroup\$
    – Tomin Jose
    Sep 5, 2022 at 5:25
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    \$\begingroup\$ Then a picture of that pcb would be perfect. Both sides :) \$\endgroup\$
    – RemyHx
    Sep 5, 2022 at 5:26
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    \$\begingroup\$ You have a 16-bit data bus running at 27 MHz. The level shifter can drive huge amounts of current per pin to make it transition quickly, in 1-3 nanoseconds. Peaks of tens of milliamps would not surprise me, and based on your schematics, you have no source termination or even bypass caps. Or you do but are not showing them. \$\endgroup\$
    – Justme
    Sep 5, 2022 at 6:47

1 Answer 1

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This datasheet is quite old. More modern TI datasheets say:

Each VCC pin should have a good bypass capacitor to prevent power disturbance. For devices with a single supply, 0.1 μF is recommended; if there are multiple VCC pins, then 0.01 μF or 0.022 μF is recommended for each power pin. It is acceptable to parallel multiple bypass caps to reject different frequencies of noise. A 0.1 μF and a 1 μF are commonly used in parallel. The bypass capacitor should be installed as close to the power pin as possible for best results.

Also:

The high drive capability of this device creates fast edges into light loads so routing and load conditions should be considered to prevent ringing.

If the output traces are long enough for transmission line effects, consider source termination.

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