Causes of camera module noise and how to deal with it

What is camera power noise

There is a common problem of power supply noise in cameras. Power supply noise is the frequency noise on the sensor power supply that interferes with the sensor's pixel acquisition, causing horizontal, vertical, or strip-like bright and dark stripes to appear on the image. As users gradually increase their requirements for photo quality, camera power supply noise must be further suppressed. This article draws on the experience of camera sensor manufacturers in dealing with power supply noise and summarizes it into a document, aiming to provide some basic experiences that can be followed. Let’s start with the analysis from the aspects of what power supply noise is, how power supply noise is generated, and how to avoid the introduction of power supply noise at the sensor, power supply, board level, and FPC levels.

Obvious stripe noise can be seen in the camera preview, as shown in the figure below. But when I tested the ripple of the AVDD power supply, it was clean and no suspicious noise was found. By testing the spectrum of the camera AVDD with a spectrum analyzer, we found that from 10KHz to 10MHz, a noise appears every 300KHz, as shown in the figure below. The reason is that the rear camera AVDD is shared with the LCD and light sensor, causing the AVDD to be mixed with some noise that can interfere with the camera sensor, causing power supply noise problems.

Power supply noise

In order to deliberately meet the 100ohm differential impedance of the MIPI wiring, the camera FPC designed the wiring to be too thin, causing the wiring to break easily and cause a black screen preview problem.

When stacking projects, I did not pay attention to the heat dissipation problem of the camera. As a result, the sensor overheated during camera preview and vertical stripes appeared in the preview image. This was solved after adding heat dissipation measures.

Streaks caused by overheating

H noise

The H noise of the camera is generally caused by the power supply noise of the camera. The noise fluctuation of the power supply is usually manifested as horizontal stripes or stripes in the imaging. As shown in the figure below, obvious band-like horizontal stripe noise is visible.

stripe noise

The camera sensor reads pixel information line by line in units of lines. When reading each line, it first reads the value of the reset state (zero state), then reads the current signal value, and then performs differential processing. Differential processing can effectively eliminate DC Deviation associated with other noise sources. If the power supply VDD when collecting the reset state value is different from the power supply VDD when collecting the current signal value, the interpolation ΔVDD between the two VDDs will be superimposed on the differential signal. The differential signal between different pixels in the same row changes, but the superimposed ΔVDD noise remains unchanged in the entire row. If ΔVDD causes the signal value to be high, or causes the reset state signal to be low, the entire row will become brighter than the real situation; if ΔVDD noise causes the signal value to be low, or causes the reset state signal to be high, the entire row will The rows will appear darker than they actually are. The sensor reads the pixel values of the array row by row, and ΔVDD also changes row by row. For a uniform imaged object some rows will be brighter and some rows darker, resulting in row-like stripes. The power supply is more likely to produce stripe noise in darker environments because the signal value will be very low in the dark state. For example, in a very dark environment, the stripe noise is ±1, and the pixel signal value output is 4. Because the fluctuation between rows reaches ±25%, the stripe noise will be very obvious. In a very bright environment, assuming that the pixel signal value output is 100, the fluctuation between rows is only ±1%, and the stripe noise is minimal.

But it does not mean that if the power supply has noise fluctuations, there must be stripe noise. When the row scanning period is consistent with the power supply noise fluctuation period. There will be no streak noise, as shown in the figure below. When the line scanning period is inconsistent with the power supply noise fluctuation period, stripe noise will basically be generated.

Heat dissipation precautions

Due to the rapid development of camera pixels and the increasingly rich functions of today's cameras, the power consumption of cameras is also getting higher and higher. A short preview can significantly heat up the camera. If the temperature of the camera module is too high, it will seriously affect image quality and reliability. It is important to consider the heat dissipation of the camera module when designing the structural stack. For the camera module sensor, the operating temperature range of conventional CMOS camera modules is -30~70℃, and the temperature range that can ensure image quality is 0~50℃ (note that this is the temperature of the sensor, not the ambient temperature). The figure below shows the performance of the camera module at high temperatures. It is obvious that the noise problem has intensified, seriously affecting the imaging quality.