The "VDD off" while and "VLED on" situation is not specified in the data sheet, although it is allowed and there is no supply sequencing issues.  Given the above, note that If VLED is on while VDD is off, There is a risk of current leakage through the LED1/2/3 pins. This leakage is not specified or tested for. 

The 47k pull ups can move to VLED ONLY if you can guarantee that VLED is always on first and VLED exceeds or matches the VDD voltage.  This is important  because the LED1/2/3 pins are sampled when VDD comes up to determine the correct power up mode.  

The Si1153 is usually used with IR LEDs that have a Vf of about 1.6 Volts.  If that is the case and VDD >= 2.5 V you can usually use the same supply for VLED and VDD.   

I can't get into the web site above but if it is your typical white 0.1 m thick Teflon plumbers tape, it will do the job.  If you are just developing and experimenting then walk into a hardware/plumbing store and buy this in a small roll.

Thin Teflon tape is devilishly hard to cut into nice pieces and attach (I use a scalpel), so If you are planning production then buy sheets of laser cut Teflon squares or circles with adhesive pre-applied in a ring in one side.  See the suppliers in the AN968 app note. 

If you glue it, do not put glue in the optical path.  Put it in a ring around that circle.

Note that Teflon requires special glues and/or pre-treatments of the Teflon surface for efficacy .   

Regards,

The packaging of the Si114x family of sensors are not tested for bio compatibility and are not intended to be pressed against skin.

The dimensional tolerance is due to the high volume production molding operation and is not something that can be bypassed.  Also keep in mind there will be some tolerance in the amount of solder paste you use and the part may ride high or low on the PCB.

Putting the part on a tiny PCB with a flexible interconnect and using foam to press the PCB+Sensor against a clear plastic/glass overlay is the most practical way to get close despite the tolerances. 

A better alternative is to design the optics so that the sensor can be 0.1 or 0.2 mm away from the overlay. This is  usually done with a light blocking flexible foam gasket around the sensor. 

Let us know if you have more questions.

Regards

There is a perceived need to measure the UV-INDEX exposure to people in a manner analogous to the dosimeter worn by X-Ray technicians, but two basic problems interfere:  The formal UV-Index measurement definition is not an “exposure” measurement and UV radiation is easily blocked from the dosimeter.  

The formal UV-Index specification created by the UN's World Health Organization and World Meteorological Organization in 1994 provides a measurement of the UV hazard to skin in a geographical site.  It evaluates the hazard to a person wondering around in this site with no attention to what task the person is doing.  e.g.:  walking, lying, swimming.   It also designed to measure the hazard in a fixed manner, requiring a cosine response with respect to the vertical. 

This UV index definition was not intended to be implemented by a wearable or any device not aimed vertically, and one can’t claim that such devices are measuring UV index unless at the very least it is aimed at zenith when it makes the measurement and has an unobstructed view of the sky.

What can be salvaged from the UV-Index definition and used by a “UV Dosimeters”, is the shape and sensitivity of the erythema curve. A UV-Index measuring device. such as the Si1133, can be worn on the wrist and be changing directions constantly, always accumulating exposure readings. The wearable’s MCU can be programmed to use a suitable integration algorithm to give UVIH (UV-Index Hours) readings which are weighted so that despite the location of the sensor (the wrist), a user in a UV-Index environment of 5 for one hour would get a reading of aproximately 5 UVIH.

There are some obvious statistical difficulties to overcome with this concept, but they are not insurmountable.

One problem with this approach in that there are many cases where most of the time the sensor is aimed in a constant direction.  The wearer could be walking down a beach with the sun at 45 degrees and the device aimed away from the sun almost the entire time. This is overcome by putting a sensor on both the wearable face and the opposing band allowing normal hand movements to briefly expose the sensor. The sensor in this case would operate in a peak detection mode over short intervals of about a minute.   

The details of the material is proprietary, but if you go to https://www.silabs.com/about-us/corporate-responsibility/environmental-data-search  and search on the part number you can find information including (under material test results) an MSDS data sheet for the "liquid epoxy" used and for the curing agent.  

The nominal answer regarding the nominal size and tolerance is that we cannot be modify the part by customization or redesign or recommend (or know) of a useful procedure by the customer. 

The previous answer recommended that you align to the part top rather than to the PCB if you want very tight tolerances to the top of the part.   That is likely to be cheaper than trying to shim the top of the part. 

Regards,

Morrie

The Si1133 and Si115x Sensor parts share a common die layout with the same photodiode array. The article points out where the various photodiodes and photodiode combinations are located and selected.   This die is centered in the 2x2 mm clear QFN package that the parts come in.  If the module is used the visible and IR photodiode array is centered in beneath the module circular opening.

Figure 1    Photo Diode Locations on the die

The figure below shows that at each level there are 12 individual squares that are connected as one named photodiode for each two squares.  The colors shown here are strictly for illustration purposes. They do not imply a filter or sensitivity. The photodiodes are named the same way except with a b suffix on the bottom. Thus, at the top there are: D1a pair, D3a pair …  D6a pair while at the bottom there are D1b pair, D3b pair …  D6b pair.  

The black corner photodiodes are covered in metal and do not measure light.   They are used to automatically compensate for photodiode current leakage. 

Figure 2  Light Travel in the Stacked Photodiodes

The following #define table shows all the photodiode selections available.  The figures below illustrate some examples with different groups of PDs selected.

#define ADCCONFIG_ADC_MUX_D1a_D4a_minus_DARK               0x0D   // (D1a+D4a) - (D5a+D6a)  

#define ADCCONFIG_ADC_MUX_D1a_minus_DARK                          0x0B   // (D1a - D5a)   

#define ADCCONFIG_ADC_MUX_L_IR                                                  0x02   // (D1b + D2b + D3b + D4b) - 2*(D5b + D6b)

#define ADCCONFIG_ADC_MUX_M_IR                                                0x01   // (D1b + D2b) - (D5b + D6b)

#define ADCCONFIG_ADC_MUX_S_IR                                                  0x00   // D1b(w) - D5

#define ADCCONFIG_ADC_MUX_UV_SHALLOW_minus_DARK       0x18   // UV shallow -  UV shallow-dark

Table 1   Si1133/53 photodiode selections:  ADCMUX[4:0] field of ADCCONFIGx register

Figure 3   The RED area shown in this figure is the one active with the ADC mux field set to 0x00

Figure 4    The RED area shown in this figure is the one active with the ADC mux field set to 0x02

Figure 5   The RED area shown in this figure is the one active with the ADC mux field set to 0x01

Photodiodes used in the Si1133/4x/5x sensors are arranged in a 3D stack, one set shallow and one set deep. The figure below shows that at each of the two levels level there are 12 individual squares that are connected as one named photodiode for each two squares.  The colors shown here are strictly for illustration purposes. They do not imply a filter or sensitivity. The photodiodes are named the same way except with a “b” suffix on the bottom instead of “a”. Thus, at the top there are: D1a pair, D3a pair …  D6a pair while at the bottom there are D1b pair, D3b pair …  D6b pair.  

The black corner photodiodes are covered in metal and do not measure light.   They are used to compensate for photodiode current leakage. 

Figure 1   Light Travel in the Stacked Photodiodes

As a result of this arrangement, the spectral response of the shallow and deep photodiodes is different. The shallow photodiodes are about 4X less sensitive in the green but about 20 times less sensitive in the IR which is can be an advantage in some applications (e.g. Ambient light sensing or ALS that suffer from IR interference. 

The resultant response curves for both shallow and deep photodiodes are shown in the figure above.  The deep one is much more sensitive with a peak at about 800 nm while the shallow one is less sensitive with a peak in the blue.

The advantage of the less sensitive one is the relative response of IR region where it suppresses relative IR response by a large factor this can be seen in the figure below that show that comparing 550 nm (green) response to the 900 nm IR response the shallow PDs suppress 900 nm IR by a factor of 7.5 while the deep PDs accentuate the 900 nm IR by a factor of  2.

The shallow PD is more effective for visible light sensing (ALS) since IR response is a major problem.   Th reduced overall sensitivity is usually acceptable especially with si1133/5x parts since they have good dark current compensation and can reach ~0.01 lux with the deep PD and ~0.1 lux with the shallow PDs.  Note that the older Si114x part does not have the dark current compensation and is limited to higher light levels.   

The packaging for the Si1143-A11 parts (and all other current Silicon Labs sensors) is not specified for bio-compatibility.  You will need to add your own clear bio-compatible layer if  that is needed.  

Attention does need to be paid to the height specification which is +/- 100 microns.  It is not possible to reduce it since that would demand  completely new more expensive assembly techniques.  The height statistics are not available. 

When trying to mate the top of a package such as this one to a surface it is wise to use the top of the package as a reference rather than the location of the PCB that the part is mounted on.  i.e.  Press the top of the sensor package against a mechanical stop (or that bio-compatible layer mentioned above) rather than the PCB.   

The packaging for the Si1143-A11 parts (and all other sensor) are not specified for bio-compatibility.  You will have to add your own clear bio-compatible layer if  that is needed.  

Attention does need to be paid to the height specification which +/- 100 microns.  It is not possible to reduce it since that would demand  completely new more expensive assembly techniques.  The height statistics are not available. 

When trying to mate the top of a package such as this one to a surface it is wise to use the top of the package as a reference rather than the PCB that the part is mounted on.  i.e.  Press the top of the package against a mechanical stop (or that bio-compatible layer mentioned above) rather than the PCB. 

System Integration Considerations for Optical Heart Rate Sensing Designs

Designing and implementing an optical heart rate monitoring (HRM) system, also known as photoplethysmography (PPG), is a complex, multidisciplinary project. Design factors include human ergonomics, signal processing and filtering, optical and mechanical design, low-noise signal receiving circuits and low-noise current pulse creation.

Wearable manufacturers are increasingly adding HRM capabilities to their health and fitness products. Integration is helping to drive down the cost of sensors used in HRM applications. Many HRM sensors now combine discrete components such as analog front ends (AFE), photodetectors and light-emitting diodes (LEDs) into highly integrated modules. These modules enable a simpler implementation that reduces the cost and complexity of adding HRM to wearable products.

Wearable form factors are steadily changing too. While chest straps have effectively served the health and fitness market for years, HRM is now migrating to wrist-based wearables. Advances in optical sensing technology and high-performance, low-power processors have enabled the wrist-based form factor to be viable for many designs. HRM algorithms also have reached a level of sophistication to be acceptable in wrist form factors. Other new wearable sensing form factors and locations are emerging, such as headbands, sport and fitness clothing, and earbuds. However, the majority of wearable biometric sensing will be done on the wrist.

No two HRM applications are alike. System developers must consider many design tradeoffs: end-user comfort, sensing accuracy, system cost, power consumption, sunlight rejection, how to deal with many skin types, motion rejection, development time and physical size. All of these design considerations impact system integration choices, whether to use highly integrated module-based solutions or architectures incorporating more discrete components.

Figure 1 shows the fundamentals of measuring heart rate signals, which depend on the heart rate pressure wave being optically extracted from tissue. Figure 1 shows the travel path of the light entering the skin. The expansion and contraction of the capillaries, caused by the heart rate pressure wave, modulates the light signal injected into the tissue by the green LEDs. The received signal is greatly attenuated by the travel through the skin and is picked up by a photodiode and sent to the electronic subsystem for processing. The amplitude modulation due to the pulse is detected (filtering out motion noise), analyzed and displayed.

Figure 1. Principles of operation for optical heart rate monitoring.

A fundamental approach to HRM system design uses a custom-programmed, off-the-shelf microcontroller (MCU) that controls the pulsing of external LED drivers and simultaneously reads the current output of a discrete photodiode. Note that the current output of the photodiode must be converted to voltage to drive most analog-to-digital (A/D) blocks. The Figure 2 schematic shows the outline of such a system. Note that the I-to-V converter creates a voltage equal to VREF at 0 photodiode current, and the voltage decreases with increasing current.

The current pulses generally used in heart rate systems are between 2 mA and 300 mA depending on the color of the subject’s skin and the intensity of sunlight with which the desired signal needs to compete. The infrared (IR) radiation in sunlight passes through skin tissue with little attenuation, unlike the desired green LED light, and can swamp the desired signal unless the green light is very strong or unless an expensive IR blocking filter is added. Generally speaking, the intensity of the green LED light where it enters the skin is between 0.1x and 3x the intensity of sunlight. Due to heavy attenuation by the tissue, the signal that arrives at the photodiode is quite weak and generates just enough current to allow for a reasonable signal-to-noise ratio (SNR) (70 to 100 dB) due to shot noise even in the presence of perfect, noise-free op amps and A/D converters. The shot noise is due to the finite number of electrons received for every reading that occurs at 25 Hz. The photodiode sizes used in the design are between 0.1 mm2 and 7 mm2. However, above 1 mm there are diminishing returns due to the effect of sunlight.

Figure 2. The basic electronics required to capture optical heart rate.

The difficult and costly function blocks to implement in an optical heart rate system design, as shown in Figure 2, are the fast, high-current V-to-I converters that drive the LED, a current to voltage converter for the photodiode and a reliable algorithm in the MCU that sequences the pulses under host control. A low-noise (75 - 100 dB SNR) 300 mA LED driver that can be set to very low currents down to 2 mA while still creating very narrow light pulses down to 10 µs is an expensive block to achieve with discrete op amps.

The narrow pulses of light down to 10 µs shown in Figure 2 allow the system to tolerate motion and sunlight. Typically two fast light measurements are made for each 25 Hz sample. One measurement is taken with the LEDs turned off and one with the LEDs turned on. The calculated difference removes the effect of ambient light and gives the desired raw optical signal measurement that is, most importantly, insensitive to flickering background light. 

The short duration of the optical pulses both allows and requires a relatively strong light pulse. It is essential to stay brighter than the sunlight signal, which may be present and not allow the PPG signal carrier to be dwarfed by the sunlight signal. If the sunlight signal is larger than the PPG carrier, then although it may be removed by subtraction, the signal can be so large that external modulation such as swinging an arm in and out of shadow can create difficult-to-remove artifacts. As a result, systems that use low-current LED drivers and large photodiodes to compensate suffer severely from motion artifacts in bright light situations     

Much of the desired HRM sensing functionality is available pre-designed and integrated into a single device. Packing most of this functionality into one piece of silicon results in a relatively small 3 mm x 3 mm package that can even integrate the photodiode (PD) itself.

Figure 3 shows an example of a schematic with an Si118x optical sensor from Silicon Labs. This HRM design is relatively easy to implement. The engineer just needs to focus on the optical portion of the design, which includes optical blocking between the parts on the board and coupling the system to the skin.

Figure 3. An integrated heart rate sensor requiring only external LEDs.

While the approach shown in Figure 3 results in a high-performance HRM solution, it is not as small or power efficient as some designers would like. To achieve an even smaller solution, the LED die and the control silicon must be integrated into a single package that incorporates all essential functions including the optical blocking and the lenses that improve the LED output. Figure 4 illustrates this more integrated approach, based on a Silicon Labs Si117x optical sensor.

No external LEDs are required for this HRM design. The LEDs and photodiode are all internal to the module, which can be installed right below the optical ports at the back of a wearable product such as a smartwatch. This advantageous approach enables a shorter distance between the LEDs and the photodiode than is possible with a discrete design. The reduced distance allows operation at extremely low power due to lower optical losses traversing the skin. 

Integrating the LEDs also addresses the issue of light leakage between the LEDs and the photodiode. As a result, the designer does not have to add optical blocking to the printed circuit board (PCB). The alternative to this approach is to handle the blocking with plastic or foam inserts and special copper layers on the PCB. 

Figure 4. A highly integrated HRM sensor module incorporating all essential components.

There is one more part of an HRM design that the developer does not necessarily need to create: the HRM algorithm. This software block residing on the host processor is quite complex due to the signal corruption that occurs during exercise and motion in general. End-user motion often creates its own signal that spoofs the actual heart rate signal and is sometimes falsely recognized as the heart rate beat.

If a wearable developer or manufacturer does not have the resources to develop the algorithm, third-party vendors provide this software on a licensed basis. Silicon Labs also offers a heart rate algorithm for its Si117x/8x optical sensors that can be compiled to run on most host processors.

It is up to the designer to decide how much integration is right for the HRM application. The developer can simplify the design process and speed time to market by opting for a highly integrated module-based approach using a licensed algorithm. Developers with in-depth optical sensing expertise, time and resources may opt to use separate components (sensors, photodiodes, lenses, etc.) and do their own system integration, and even create their own HRM algorithm. Ultimately, when it comes to HRM system design, the developer has a choice of doing it all or purchasing it all.