EMC Case Analysis | How to Easily Eliminate Circuit Noise in ToF 3D Ranging Applications?
01
What is ToF?
02
The noise problem in ToF
03
How to easily eliminate ToF power circuit noise?
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Murata Recommends
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Verifying the effectiveness of noise countermeasures
06
Summarize
01
What is ToF?
ToF is the abbreviation of Time of Flight, which is a distance measurement method that measures the distance to an object by using the time difference between the irradiated wave and the reflected wave. A distance image sensor using ToF irradiates light such as infrared light, measures the distance for each pixel, and records depth information to obtain the three-dimensional structure of the target.
ToF is often used in facial authentication in smartphones, etc. Because ToF can measure the three-dimensional shape of the face, it can prevent authentication from being circumvented by scanning photos , so smartphones are increasingly adopting ToF distance measurement methods.
In addition to ToF, there are other distance measurement methods. For example, the method using a stereo camera, the Structured Light method using patterned infrared light and observing deformation, etc. However, ToF has the characteristics of simple structure, wide range of distance support, and high resolution, so it is considered to be more suitable for smartphones.
Application scope of ToF and other optical ranging methods
In order to perform high-precision distance measurement over long distances, ToF needs to emit steep high pulses , as shown in the figure below. In circuit design, generating this high pulse waveform will inevitably involve high pulse currents, so targeted circuit noise countermeasures are also required, especially ripple noise. Next, let's take a look and give Murata's countermeasures.
Relationship between ToF pulse waveform and accuracy
02
The noise problem in ToF
The block diagram of ToF transmission and reception is shown below. On the transmission side, a steep and large pulse current is applied to the VCSEL (Vertical Resonance Surface Emitting Laser) to generate infrared pulse light, and this pulse light is irradiated to the measurement target. The receiving side receives the light returned from the measurement target and converts it into an electrical signal.
ToF transmission and reception block diagram
When a steep high output pulse is issued (below), a large current suddenly flows in the driver circuit and a large voltage spike is likely to form on the power supply line.
Noise Interference Model in ToF
The concern is that the resulting noise will interfere with the receiving side, causing the ToF sensor to lose sensitivity.
The shielding layer can prevent most of the noise emission, but in addition, it is also necessary to prevent the noise from being transmitted through the power supply line.
This article introduces an effective method of using EMI filters to suppress noise propagation.
03
How to resolve it easily
ToF power circuit noise?
Inserting ferrite beads can effectively solve the noise problem in this type of ToF.
Power line spike noise suppression using ferrite beads
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Ferrite beads are inserted into the portion where power flows in on the transmission side to suppress the propagation of noise through the power line.
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Insert a ferrite bead into the receiving-side GND connection portion to suppress noise propagation through the GND line.
Suitable for larger installation spaces
BLE18PK100SN1
BLE18PS080SN1
Note that when using ferrite beads for noise countermeasures, you need to consider the noise frequency that is likely to cause problems when selecting , and also select the most suitable model according to the installation space .
If there is enough installation space , you can choose Murata's BLE18PK100SN1 series, which supports large current (6000mA), high impedance, and excellent DC superposition characteristics; you can also choose BLE18PS080SN1 , which is thinner (0.75mm max), supports large current (8000mA), and also has excellent DC superposition characteristics.
Suitable for small installation spaces
BLM15KD200SN1
If there is not enough installation space , it is recommended to choose Murata BLM15KD200SN1 , which has a small size of 1005 and wide bandwidth support.
——— Murata e-book | Basic Noise Suppression Tutorial ———
04
Verifying the effectiveness of noise countermeasures
First, let's confirm whether inserting a ferrite bead causes a change in the noise suppression effect.
Confirming the effect of inserting ferrite beads
How to confirm the effectiveness of noise countermeasures:
Voltage① → Confirmed by Ripple Noise
Voltage② → Confirmed by transient response
Current → Confirm via Current wave form
As can be seen from the figure above, the insertion of ferrite beads not only confirmed the noise elimination effect, but also confirmed that it had no effect on the voltage and current waveforms. In the transmission side power line ①, the ripple noise did change, and the noise flowing to the power supply side was reduced; in the power supply input part ② of the VCSEL, the transient response changed, and there was no adverse effect on the voltage waveform. In addition, the change in the current waveform flowing through the VCSEL can also be confirmed.
Effect Confirmation - Ripple Noise
As shown in the figure below, by inserting the ferrite bead BLE18PK100SN1 into the power line, it can be seen that the ripple in part ① is reduced to about 1/5. The confirmation result shows that the ripple noise is greatly reduced by inserting the ferrite bead.
Ripple noise reduction effect generated by ferrite beads
Effect confirmation - transient response
On the other hand, the insertion of ferrite beads does not adversely affect the voltage supplied to the VCSEL. When checking the transient response before and after the insertion of the ferrite beads, no decrease in wave height due to the insertion of the ferrite beads was found. The insertion of the ferrite beads did not adversely affect the operation of the circuit.
Effect of Ferrite Bead Insertion on Transient Response
Effect confirmation - VCSEL current waveform
Next, we checked the waveform of the current flowing through the VCSEL. There was almost no difference in the current waveform before and after the ferrite bead was inserted. The result was that no adverse effects such as a decrease in wave height were observed even after the ferrite bead was inserted.
Effect of Ferrite Bead Insertion on VCSEL Current Waveform
05
Summarize
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A steep large pulse current flows through the ToF transmission section, and this pulse current tends to generate large ripple noise in the power supply line.
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By inserting ferrite beads, noise can be reduced without adversely affecting the voltage, current waveforms and transient response.
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From the perspective of supporting large current and small size, the noise suppression measures using Murata's ferrite beads recommended in this article are recommended.
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