SONY AN5163K Service Manual
AN498
Si114X DESIGNER ’S GUIDE
1. Introduction
This application note provides an outline for using the Si114x proximity detector and ambient-light sensor. General considerations of electrical and optical component selection, programming, and power consumption are explained so as to cover the majority of situations. Specific topics are discussed elsewhere:
AN521: IRLED Selection Guide for Si114x Proximity Applications
AN522: Using the Si1141 for Touchless Lavatory Appliances
AN523: Overlay Considerations for the Si114x Sensor
AN540: Hair Immune Cheek Detection in SmartPhones Using the Si1141 Infrared Sensor
AN541: Smoke Detection using the C8051F990 and Si1141
AN580: Infrared Gesture Sensing
2. Optical Considerations toward Mechanical Design
This section focuses on mechanical and industrial design considerations.
2.1. Topology
Figure 1 highlights and defines the various system topologies.
Si114 irLED
No Overlay
a) No Port
Sensor and emitter underneath a single compartment, single overlay
b) Single Port
Overlay |
Overlay |
Si114 |
irLED |
Emitter and sensor are in separate compartments, each compartment has a separate overlay.
c) Dual Port
Overlay |
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Si114 |
irLED |
d) Hybrid
Emitter and sensor are underneath one overlay, but with an optical block in-between. The Hybrid is essentially a Single-port with optical blocking
Figure 1. System Topologies
Rev. 0.6 10/11 |
Copyright © 2011 by Silicon Laboratories |
AN498 |
AN498
The purpose of the system topologies discussed is to provide a sense of the level of optical leakage or cross-talk expected in a proximity system. One common misconception is that a system without an overlay and one with a transparent overlay are “the same”. Although they might appear the same to the human eye, it is important to examine this from the perspective of the device.
2.2. Optical Leakage
Also refer to "4. Proximity Measurements" on page 26 for information on how the Si114x makes Proximity Sense (PS) measurements.
Even if the overlay is clear, these systems are NOT equivalent
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irLED |
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Figure 2. Common Misconception
In a Single-Port system, there exists a reflection from the overlay. The magnitude of the reflection is a function of the index of refraction and the incident angle relative to the overlay surface normal. There is a set of equations called 'Fresnel's Equations' that provides a prediction as to the amount of light reflected back to the sensor.
In a No-Port system, the optical leakage from the overlay is not present. However, this does not mean that optical leakage does not exist. There may be optical paths causing the optical leakage other than the overlay.
The most common topology used in cell-phones and hand-held devices is actually a hybrid between the SinglePort and the Dual-Port. The Dual-Port distinguishes itself from the Hybrid in that irLED and the sensors are fully compartmentalized, even to the point that two separate overlays are used.
In the Hybrid topology, the overlay itself becomes a medium for optical leakage. Light transfers from one compartment to the other through internal overlay reflections. The Hybrid topology can approach the optical performance of the Dual-port system topology. For systems requiring the highest ADC sensitivity, the host may need to choose a higher PS_ADC_GAIN setting. To be able to use the settings with high PS_ADC_GAIN settings, the optical leakage must be carefully controlled.
Overlay Overlay
Si114x |
irLED |
Dual Port
Overlay |
Hybrid |
•Single overlay in hybrid is a medium for secondary sources of light leakage from irLED compartment to sensor compartment.
•Dual Port is recommended for long range targets where
PS_ADC_GAIN needs to be increased.
Figure 3. Dual Port vs. Hybrid Topology
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AN498
Optical leakage is also sometimes called “crosstalk”. A proximity detector aims to measure the increase in light levels caused by turning on the irLED. Ideally, only light reflected from the target will reach the sensor. Figure 4 illustrates two light rays, each hitting the Si114x photodiode. In Figure 4, note that there are three light rays originating from the irLED. One of these light rays is shown to hit the Target. The target is assumed to be a diffuse surface and radiates in all directions. When one of those rays falls onto the Si114x photodiode, it increases the ADC reading proportional to the level of reflectance emitted by the target.
Figure 4 also shows an additional two light rays originating from the irLED. These two rays do not hit the target, but instead hit the overlay. One light ray hits the top of the overlay surface (refraction is illustrated); the other hits the bottom of the overlay. A specular reflection is essentially a mirror-surface where the light ray is reflected at a very specific angle. In a specular reflection, the incident angle equals the reflected angle.
In Single-Port topologies, it is not unusual for the optical leakage to exceed the reflectance from the target. The optical leakage eats into the ADC Dynamic Range and may force the use of a less-sensitive ADC setting. A consequence of using a less-sensitive ADC setting is that it may require the use of a higher irLED current to detect the target object at the cost of higher power consumption.
Target
Target
Reflectance
Overlay
Optical
Leakage
Si114x irLED
Figure 4. Optical Leakage
The greatest source of optical leakage is the specular reflection from the overlay. For this light ray, the radiant intensity of the irLED given the radiation angle, the reflection coefficient (function of the overlay's index of refraction and incident angle), and the travel distance (inverse square relationship) all combine to form the dominant leakage path. The overlay contains two surfaces, resulting in two rays hitting the photodiode. These two rays are roughly equal in magnitude; it is, therefore, important to block both of these rays.
Reducing the leakage from these two primary paths can also be achieved by placing the irLED close to the overlay. This way, only light rays with very steep angles relative to the axial direction are allowed to originate from the irLED. The radiant intensity is typically a function of the radiation angle, and a higher radiation angle generally has a lower radiant intensity.
In addition to the primary leakage paths, there are other sources of optical leakage to consider. Unlike the primary leakage paths, where there is only a single reflection point prior to directing to the sensor, the rays of the secondary leakage paths generally take multiple bounces to get to the sensor.
For example, a light ray can bounce off the overlay and strike the PCB. The PCB can then reflect the ray as a diffuse surface, causing a small portion of that reflection to reach the Si114x photodiode through yet another specular reflection. Of course, the radiant power decreases with each bounce.
Rev. 0.6 |
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AN498
The PCB has a high transmittance to IR, but it is not obvious to the human eye since the PCB looks opaque. Use copper ground fill to make the PCB opaque to IR.
The irLED choice is also an important consideration. When an irLED has a wide radiation pattern, there is less light focused at the angles close to the axial direction. What this means is that there is more light radiated outwards at steep angles. The total light power becomes directed towards optical leakage. With more light energy in these steep angles, the more light there is to feed the secondary leakage paths. Choosing an irLED with a narrow halfangle leads generally leads to lower system optical leakage in addition to better overall proximity detection distance.
Overlay
Si114x irLED
PCB
•Both inner and outer overlay surfaces reflect incident light
•The reflection coefficient is described by Fresnel’s Equations. It is a function of the incident angle and the overlay’s index of refraction.
•PCB has high IR transmittance. Copper (e.g. ground fill) is IR-opaque.
•Minimize the distance from top of irLED to bottom of overlay.
•irLED radiation pattern (radiant intensity vs angle) is an important
consideration.
Figure 5. Optical Leakage Summary
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AN498
2.3. Optical/Mechanical Components
2.3.1. Optical Blocking Material
An Si114x proximity system uses the “near infrared” wavelengths. As such, many objects that look black are candidates for optical blocking material.
Natural Rubber is a common material known to be opaque to visible light and the near-infrared band. Another property of Natural Rubber that makes it suitable for optical blocking is its elasticity. Commercially-available rubber sheets can be cut to size for optical blocking. For example:
www.rubbersheetroll.com/rubber-sheets.htm
Nitrile Rubber or “Buna-N” O-rings are used for optical blocking in Silicon Labs' evaluation platform and can be found at the following web site:
www.mcmaster.com/#4061t111/=9ujxqp
If an adhesive thin-sheet back device is desired, a polyurethane foam material from 3M (Bumpon™) can be used as an optical blocking material:
search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=bumpon+roll
There are more IR-opaque materials available than are discussed in this document. However, since many visibleopaque materials are not necessarily IR-opaque, it is best that materials be characterized by IR opacity. The easiest method of checking the infrared opacity of a material is with a television remote control:
1.Find a common TV remote control.
2.Verify that the TV remote control is able to control the television.
3.Note the maximum distance that the TV remote control operates.
4.Locate the location of the IR sensor on the TV.
5.Verify that this is the IR sensor correct by placing the TV remote control directly on top of the IR sensor.
6.Cover the material in front of the TV remote control.
7.Attempt to control the TV.
8.If the material is opaque to the near infrared band, then, you should not be able to operate the TV even if the remote control is positioned right up to the IR sensor window.
It is possible to determine infrared opacity by calculating the maximum transmittance of the material using this procedure. For example, if the TV remote control is able to operate at a distance of 10 meters, and the optical blocking material made it impossible for the TV remote control to operate even when it is only 5 cm away, then the transmittance of the material is, at most, .000025. This indicates that the material is opaque to the near infrared spectrum.
10meters
Make sure remotecontrol works
Transmittance < 0.052
5 cm
102
Remotecontrol does not work |
Optical Block Material underTest |
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Figure 6. Infrared Opacity Test
Rev. 0.6 |
5 |
AN498
2.3.2. Infrared Properties of Printed Circuit Boards
Many materials that appear visibly opaque to the human eye can have a high transmittance to infrared wavelengths. Printed Circuit Boards, for example, have high transmittance to near infrared light. Therefore, it is important to also consider Printed Circuit Boards as optical components.
The copper layers of printed circuit boards are opaque to infrared light. Maximizing the amount of copper underneath the irLED and the Si114x is an effective method of reducing the amount of optical leakage through the PCB material.
Overlay
Si114x |
irLED |
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PCB |
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Use copper ground fill to block leakage
Figure 7. PCB has High IR Transmittance
2.3.3. Choosing an Overlay
The Si114x does not require any optical filter for proper operation.
Electrical engineers are familiar with the terms “High Pass Filter” and “Low Pass Filter”. In optics, there are commonly-used analogous terms for describing overlays.
“Long Pass Filter” refers to a material that allows long wavelengths to pass through while disallowing short wavelengths.
“Short Pass Filter” refers to a material that allows short wavelengths to pass through while disallowing longer wavelengths.
As a reference, purple light is at 400 nm; green light is 550 nm; red light is 700 nm, and the infrared light typically chosen for proximity is 850 nm.
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AN498
550 850 550 850
Shortpass Filter |
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Longpass Filter |
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Figure 8. Shortpass Filter vs. Longpass Filter
A “Long Pass Filter” with a corner wavelength of 700 nm blocks visible light while allowing the infrared spectrum to pass through. The opposite of a long pass filter is the short pass filter. So, a short pass filter with a 700 nm corner wavelength allows visible light while blocking the infrared spectrum.
In general, there is a desire for most products to hide internal electronics from visible view. Common materials and inks generally have a higher transmittance to infrared light compared to visible light. For the most part, this means that there is a tendency to having the overlay act more as a “long pass filter” than a “short pass filter”, since a design goal of many products is to obstruct visible light from the view of human users.
Let us stop for a moment and consider “what is the signal” being measured in a proximity sensing application and an ambient light sensor.
In the case of a proximity sensing application, the “signal” is 850 nm infrared light emitted from the irLED. In a Proximity Sense (PS) only application, if the overlay blocks everything except 850 nm ±30 nm, the system will operate.
Rev. 0.6 |
7 |
AN498
In an ambient light application usage where there is a desire to measure the visible ambient light, the “signal” are the wavelengths between 400 nm to 700 nm. From the perspective of an ambient light sensing application, any spectral reading above 700 nm and any spectral reading below 400 nm are “noise”. A “bandpass filter” that is shaped in the form of a human eye response is called a “photopic” filter.
Hiding the internal electronics from view somewhat conflicts with the concept of “maximizing the signal” for ALS usage.
2.3.3.1. Overlay Considerations
In proximity-only applications, the “signal” is the light of the wavelength emitted from the irLED (e.g. 850 nm). Obviously, if the transmittance at the irLED wavelength is low, then much of the light from the irLED is absorbed by the overlay, leading to lower performance.
Since there is often a “target object distance” consideration, a significant optical overlay loss can translate to a loss in sensitivity with target object distance unless the overlay loss is compensated for with a different irLED or higher irLED current.
For a proximity-only application, aside from aesthetic reasons or industrial design constraints, maximizing the 850 nm transmittance should be the design goal. For applications requiring the highest performance under direct sunlight, a bandpass overlay allowing a narrow band of wavelengths around 850 nm provides the best performance.
The following common overlay approaches are possible with the Si114x. They are listed in order of preference by performance.
Clear plastic or glass overlay
Clear overlay with ink applied through a silkscreen process
Colored plastic or glass overlay
The choice of the overlay is often an industrial design decision.
Many applications opt to start with a clear overlay material and screen-print the desired pattern using special inks. Refer to "3.7. Selecting RLED" on page 25.
Note that even if an overlay has a relatively low IR transmittance, this does not necessarily mean that the Si114x will not work with such an overlay. The overlay transmittance is merely one of many system factors that come into play.
For example, if the system must operate under low IR transmittance overlays, then the following system-level tradeoffs include:
Reducing target object distance
Increase irLED current
Higher efficiency irLED
Narrower irLED half-angle
Typically, the irLED choice can compensate for overlay transmittance loss.
A simple procedure using a common TV remote control can be used to determine the transmittance of the overlay material. Although the transmittance of the TV remote control measures the transmittance at 940 nm, the estimate is often sufficiently close to the transmittance at 850 nm.
1.Find a common TV remote control.
2.Verify that the TV remote control is able to control the television.
3.Note the maximum distance at which the TV remote control operates but begins to fail to control the TV (the boundary of operation).
4.Locate the IR sensor on the TV.
5.Verify that this is the IR sensor by placing the TV remote control directly on top of the IR sensor.
6.Cover the TV remote control with the overlay material.
7.Attempt to control the TV, noting the boundary of operation.
8.The ratio of the distance squared is the IR transmittance.
8 |
Rev. 0.6 |
AN498
10 meters
Determinedistance where remotecontrol begins tofail
Transmittance = 92
9 meters
102
Determine distance where remote control begins to fail
Overlay Material under Test
Figure 9. Estimating Overlay Transmittance
Once the IR transmittance is estimated, the estimate can be considered when choosing an irLED and its current. It is generally possible, with the correct choice of irLED and irLED current, for the target object to be detected despite the overlay transmittance. Refer to “AN521: irLED Selection Guide for Si114x Proximity Applications”.
In an application that also includes ALS, wavelengths in the visible light spectrum (400 nm to 700 nm) are now part of the “signal”. Better performance is achieved in any system by maximizing the signal.
Due to the typical system goal of obscuring electronics from view, there is a tendency to use an overlay that has a low transmittance of visible light. Since visible light is also part of the signal to be measured, it is no longer possible to simply use a visible-light-opaque material.
A balance must be maintained between the goals of obscuring electronics from visible sight versus allowing in sufficient light to measure the visible ambient light. In general, an overlay that allows 5% to 10% of visible light is recommended. A 5% to 10% transmittance should allow the electronics to be virtually invisible under most conditions, while still allowing sufficient light for proper ALS operation.
“AN523: Overlay Considerations for the Si114x Sensor” provides additional information in choosing an overlay. AN523 also provides the lux calculation coefficients needed for some overlays.
2.3.3.2. Clear Acrylic and Polycarbonate Sheets
Clear acrylic material is generally available. A common trade name for acrylic sheet is “Plexiglass” and a Google search of that term typically yields the most hits. Many web-based plastics companies offer acrylic sheets cut to custom sizes. In the U.S., the following web site has low-cost samples of clear acrylic sheets. Some polycarbonate samples are available through this web site as well:
www.eplastics.com/Plastic/
Samples;jsessionid=hv2TMsxLG1JTL9x1TGf2wJhk1TqJ6mRJgsLDXFkXyxD34yxG56nwThb5lT4LxbT9fvQbyW2
8gLGGBvQTFgznztlprMWT51fWTQyhDVQtJZKhJ8wybm8phTntW7TJSwlw!328655193
Acrylic sheets are generally thicker than Polycarbonate sheets. If sheet thickness is an important consideration, polycarbonate is a better choice.
The two main sources of polycarbonate resins are Sabic (Lexan™) and Bayer (Makrolon®).
Rev. 0.6 |
9 |
AN498
Information on Lexan™ polycarbonate can be found here: www.sabic-ip.com/sfs/SFS/en/Product/ProductLevel1/lexansolidsheet.html Distributors of Lexan™ polycarbonate sheets can be found at the following web site:
www.sabic-ip.com/sfs/SFS/en/ContactUs/ContactUs/contact_us_specialtyfilmsheet.html The following web site is a good starting point for obtaining Makrolon®: http://plastics.bayer.com/plastics/emea/en/product/makrolon/Product_description.html
2.3.3.3. Silk Screen Inks
The following inks can be used for infrared applications. Teikoku
MRX-HF IR Transmittable Black: www.teikokuink.com/en/product/techreport/146_tech.html
Teikokuink’s GLS-HF 10415 SIL IR BLACK mix is especially recommended for tempered glass overlays for highperformance ALS and proximity applications using the Si114x.
Seiko Advance Ltd. IR Black Series:
www.seikoadvance.co.jp/en/products/category/category05.php Nazdar
Nazdar 6002050584 Special 84 IR www.nazdar.com/pdf/6002050584-Special-IR-84-Transmitting-Black_Rev-1-00.pdf
2.3.3.4. Colored Overlays
Clear overlays with screen-printed ink are generally superior to colored overlay materials.
Most companies focus effort in offering different colored materials based on appearance factors to the human eye. Many of the color materials have not been characterized for their IR transmittance.
With sample “color chips”, it is generally possible to estimate the IR transmittance. There is, however, an important consideration. Most “color chip” samples come in specific thicknesses. The optical properties are a function of the thickness.
In general, the opacity and the thickness of materials have an exponential relationship. For example, if 1 mm of material has 70% transmittance, then 2 mm of material would have a transmittance of (70%)^2= 49%. 3 mm will lead to (70%)3 = 34.3% transmittance.
Although it is convenient to lump together “transmittance” in a single number, actual transmittance vs. spectral curve is far from linear. In a colored overlay, the transmittance in the IR region is typically higher than that of the visible region.
Given the exponential relationship of transmittance vs. thickness, the end spectral response looks quite different even on the same material. This is due to the exponential nature of transmittance vs. thickness. In Figure 9, the same material is shown to have a much higher variation with varying thickness while the transmittance in the IR region appears almost the same. Table 1 illustrates how the numbers are affected by an exponential relationship.
Table 1. Transmittance vs Thickness Illustration
Unit Thickness |
2 x Unit Thickness |
Transmittance |
Transmittance |
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Rev. 0.6 |
AN498
Figure 10. Transmittance vs. Thickness Spectral Graph
Coefficients calculated for a given colored overlay thickness may not apply for the same material of a different thickness. It is strongly advised that lux calculation coefficients be characterized only when a colored overlay of the proper thickness is available.
The other consequence is that high relative transmittance between the visible and IR portions of the spectrum can result in high ALS variability. If ALS variation is an important system consideration, it may be advantageous to choose an overlay with a lower infrared transmittance so that the infrared transmittance more closely matches that of the visible light transmittance. Doing so will allow lower ALS variation across different light sources. Choosing an overlay with a significant spectral difference in visible and IR generally leads to higher ALS variance once the overlay thickness tolerance has been considered.
Acrylic colored overlay samples:
www.eplastics.com/Plastic/
Samples;jsessionid=hv2TMsxLG1JTL9x1TGf2wJhk1TqJ6mRJgsLDXFkXyxD34yxG56nwThb5lT4LxbT9fvQbyW2
8gLGGBvQTFgznztlprMWT51fWTQyhDVQtJZKhJ8wybm8phTntW7TJSwlw!328655193 Makrolon color chip samples (requires registration) https://www.competenceincolor.com/ChipRequest?channel_id=27
Lexan color chip samples (requires registration) https://www.sabic-ip.com/cxp/ColorXPress
Rev. 0.6 |
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AN498
2.4. Photodiode Locations, Apertures, and View Angles
The photodiodes locations are shown in Figure 11. It is generally accepted that these photodiodes are treated as a single strip for layout and mechanical design considerations.
Pin 1
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Figure 11. Si114x Photodiode Centers
An aperture is an opening through which light enters. The aperture is transparent or translucent and is surrounded by an opaque material. The distance from the aperture and the size of the aperture define the Si114x field of view. When a light source is within the field of view, the angle formed relative to the photodiode normal vector is called the “Angle of View”.
Light S ourc e
An gle of V iew
Ape rture
F ield of V ie w
S i1 14x
Figure 12. Aperture, Field of View and View Angle
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The radiant power that falls on the photodiode is a function of the angle of view. The number of ADC codes reported by the Si114x is influenced by the angle of view through a cosine relationship. All things being equal, a light source at a larger view angle results in a lower ADC count.
Figure 13. ADC Reading of Equidistant Point Light Source vs. Angle of View
When the light source is an infinite distributed surface, the relationship of the field of view versus the total available reading is shown on Table 2.
Table 2. Relative ADC Reading vs. Field of View (Large Surface Light Source)
Field of View |
ADC Codes |
180 |
100% |
170 |
100% |
160 |
98% |
150 |
97% |
140 |
94% |
130 |
91% |
120 |
87% |
110 |
82% |
100 |
77% |
90 |
71% |
80 |
64% |
70 |
57% |
60 |
50% |
50 |
42% |
40 |
34% |
30 |
26% |
20 |
17% |
10 |
9% |
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If the target object is small (smaller than the field of view), the ADC codes reported by the Si114x are influenced by the angle of view. The field of view only needs to be as big as the expected location of the target. An example of such an application is a 50 cm range where the angle subtended by the target object is small compared to the field of view.
If the target object is large (larger than the field of view), the amount of light received by the Si114x is influenced by the field of view. An example application is the cell-phone cheek detector; the cheek represents a large object due to its location relative to the sensor. For this case, maximizing the field of view is important. For these applications, a field of view of 120 ° is recommended. This means that, for best performance, the aperture either needs to be large or near the Si114x. By increasing the aperture, the greatest amount of light can enter the Si114x, and less light needs to be thrown at the target object, leading to a more efficient design.
2.5. Close Range Application with Single-Port Topology
This section applies only to Single-Port topology when the target is close. For systems that employ optical blocking or a Dual-Port topology, the optical leakage is controlled through the optical blocking material. In a Single-Port topology, geometry is the primary method of limiting the optical leakage.
2.5.1. IrLED Choice
The irLED chosen for this must have a half-angle of 22° or less. As described in "2.3. Optical/Mechanical Components" on page 5, light power that does not exit the system generally ends up fueling optical leakage through secondary leakage paths. Choosing a low half-angle irLED causes much of the light power to be directed outside the system, resulting in lower optical leakage.
Another important consideration of the irLED is that it must be as tall as the product’s construction will allow. By choosing a tall irLED, the irLED will be nearer the overlay. Having the irLED near the overlay causes most of the light to go outside the system rather than being reflected back in and causing higher levels of optical leakage.
The Si114x Evaluation Platforms use the Osram SFH 4056. Many of these recommendations can also apply to other irLEDs as long as the radiation pattern is narrower than 22°.
2.5.2. Si1141 Orientation
It is best to orient the Si1141 so that the distance between the irLED and the photodiodes is maximized. Pin 1, Pin 10, and Pin 9 should face the irLED. The worst orientation is facing Pins 4, 5, and 6 towards the irLED. Using this orientation allows the furthest distance between the irLED and the Si1141 photodiodes leading to the least optical leakage.
irLED
photodiodes
Si1141
Figure 14. Si1141 Orientation
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2.5.3. Single-Port Design Dimensions
From the perspective of minimizing optical leakage, the overlay transmittance is not a factor. A high transmittance overlay is preferred for PS because this generally leads to lower system power as less light needs to be directed to the target to allow it to be detected. It is generally feasible to compensate for high transmittance by throwing more light out, but this becomes a power consumption consideration.
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Figure 15. Si1141 Single Port Reference Drawing
Table 3. Single-Port Dimensions (Cheek Detector Application)
PCB Surface to Overlay |
Center-to-Center |
Center-to-Center Maximum (B) |
Bottom (A) |
Minimum (B) |
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Notes:
1. The Si1141 should be oriented so that pins 4 and pin 6 are farthest away from the irLED. 2. The irLED height is assumed to be 0.8 mm. When possible, minimize the gap from the
top of the irLED and the bottom of the overlay.
3. No optical isolation between the Si1141 and irLED is assumed. 4. The target object is a cheek (5 cm) or black hair (2.5 cm).
5. The overlay thickness is 0.5 mm.
6. The irLED Power Rating is 1/16 W.
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2.5.4. Partial Optical Block
If the dimensions in Table 3 cannot be met, some optical blocking is necessary. For applications where the target object can be pressed against the overlay (e.g. cheek detection) the optical blocking should be partial only. Otherwise, there would not be a reading when a cheek is pressed against the overlay.
The key concept in designing an optical block is that the primary reflection path from the irLED to the Si1141 be traced, and these rays should be blocked.
Midpoint between irLED center and photodiode center on overlay form reflection points. Trace lines to determine minimum optical block height and width.
Small gap needed for cheek detection applications where object is against the overlay.
Overlay
block rays before they get to the overlay
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Figure 16. Partial Optical Blocking |
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2.6. Long Range Applications
If the target is small or far away, the Si1141 may need to operate at a higher ADC sensitivity setting. This is accomplished through increasing the PS_ADC_GAIN setting. When increasing the PS_ADC_GAIN setting, both reflectance and optical leakage are magnified. To allow operation at the highest possible ADC sensitivity, the optical leakage should be kept low so as possible.
The limitation to how high the PS_ADC_GAIN setting can be set is a function of:
Ambient IR
Optical Leakage
The IR ambient can be controlled through the following methods:
1.Limiting field of view by using a smaller Aperture (see "2.4. Photodiode Locations, Apertures, and View Angles" on page 12)
2.Limiting field of view by using Lenses
3.Using special overlays, such as a Visible Light Blocking Overlay (Longpass Filters) when CFL/Fluorescent lighting is the predominant lighting condition.
In general, the Dual Port topology provides the lowest leakage. The Hybrid approach can be used as long as proper optical blocking is used.
Midpoint between irLED center and photodiode center on overlay form reflection points. Trace lines to determine minimum optical block height and width.
Tight Seal between the
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Minimize overlay |
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Overlay
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Figure 17. Long Range Application using Hybrid Topology (Optical Blocking)
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