The photodetector (PD) is the main input device of the pulse oximeter system.
These devices sense the intensity of light emitted by each LED after the light passes through the tissue.
The PD produces a current which is linearly proportional to the intensity of incident light.
This current is then converted to a voltage which is passed on to the pulse oximeter unit for processing.
The choice of PD depends on factors such as performance, packaging, size, and cost.
APMKorea produces PDs, DDN2094(4.46X4.46mm)/2092(2.8X2.8mm)/2090(2.03X2.03mm)/2091(noise filter)/3064(1.42X1.42mm)/2043(0.86X0.86mm) as active areas.
All of them are Si photodiode with high sensitive low capacitance PiN structure for pulse oximeters.
Your choosing a PD sensor must be considered,
1. Spectral response of device to different wavelengths
2. Linearity of the output signal, an output linearity proportional to the intensity of incident light is highly desirable.
3. Sensitivity or the ratio of the electrical output signal to the intensity of incident light.
4. Response time, how quickly the output of the device is able to response to a change in the incident light.
The p-i-n photodiode has an intrinsic (lightly doped) layer between the n and p layers.
This modified structure typically results in lower junction capacitance than for p-n diode of the same optical sensing area.
As a result, p-i-n photodiode response time is faster than p-n junction photodiodes.
Bandwidths typically run on the order of 10 MHz while p-n diode shows 50KHz.
Because of relatively low cost and linear output current response to incident light both the standard p-n diodes and p-i-n diodes are currently in use today as the PD in pulse oximetery systems.
There are several parameters serve as evaluation criteria for pulse oximeter designer.
1. Junction Capacitance
Junction capacitance is proportional to the junction area, it also decreases with increasing reverse bias voltage so it may be expressed at a specified reverse bias voltage across the photodiode.
The response speed of the photodiode depends on the RC time constant of the junction capacitance and the load resistance.
Therefore a higher response speed can be obtained by selecting small junction area and by a larger reverse bias to the photodiode.
APMKorea provides p-i-n photo detector with different area for oximeter designers.
DDN2094(4.46X4.46mm) ; 49pF
DDN2092(2.8X2.8mm) ; 45pF
DDN2090(2.03X2.03mm)/DDN2091(2.03X2.03mm noise filter) ; 25pF
DDN3064(1.42X1.42mm) ; 15pF
DDN2043(0.86X0.86mm) ; 5pF
2. Dark current
Dark current is the reverse leakage current that flows in a photodiode in the absence of light.
Although technically, no dark current should flow with zero, most zero bias applications have a small voltage across the photodiode such as the offset voltage of the op amp.
The dark current increases as the reverse voltage or ambient temperature increases.
Since the output current of the photodiode is linear, the sensitivity is normally expressed as the output current level for a known incident light level at a specified temperature.
In some cases, the sensitivity is determined with an LED optical source and thus the center frequency of the LED is specified.
In this case incident light is expressed in mW/cm2. If the light source is an International Commission on Illumination (CIE) standard, it is expressed in Lux.
4. Spectral response
Figure 4 shows that photodiodes have a spectral response and as such care should be taken when selecting a PD.
The designer should keep in mind the wavelengths of interest in pulse oximeter when deciding on an proper PD.
APMKorea provides PD with peak wave length 940nm, able to detect 400~1050 nm, following is characteristic of DDN2090M.
PD used in pulse oximeter probes has some unusual mounting and mechanical assembly requirement.
A variety of characteristics should be considered including cost, hermetic seal, package material and package type.
In general, PDs are available in three types of packages;
5.1 Can package(TO). APMKorea provides DDN3120 as Fig.5 Can package
The PD chip is mounted on a metallic stem and is sealed with a cap that has a window to allow incident light to reach the semiconductor surface.
5.2 Resin on high Tg package
The PD chip is mounted on a High Tg substrate and is coated with resin.
APMKorea provides high reliability surface mount device DCM01, DCM02, DCM03, DDN2090, DDN2094 with hermetic seal as shown Fig. 6
DCM01 DCM02 / DCM03 DDN2090 DDL2094
5.3 EMC mold package
The PD chip is mounted on a PCB or Lead frame and molded with Epoxy Mold Compound.
APMKorea provides high sensitive surface mount device DDN3064M, DDN2090M, DDN2092M as shown Fig. 7.
DDN3064M DDN2090M DDN2092M
There are excessive ambient lights such as surgical lamps, fluorescent lights, infrared heat lamps and direct sunlight.
Usually this type of interference will saturate the PD so that no pulse can be distinguished.
However some of noise sources may result in apparently normal but inaccurate readings.
Since oximetry is a system with an optical interface, it is important to minimize the effects from light other than the optical signals of interest.
Most of the light being transmitted (reflected) from the LEDs must not reach the photodiode unless it has passed through tissue containing arterial blood.
Pulse oximeter designer must attempt to limit the light reaching the PD to that which has traveled through tissue containing arterial blood.
This can be accomplished through mechanical proper LED/PD placement and noise filtering.
Light impervious barriers should be placed between LED and PD.
APMKorea DCM01 had black isolator to minimize such cross talk between them in Fig. 6. and DCM02 coated the housing around the PD with black material that does not scatter or reflect light.
APMKorea DDN2091M is able to filter the noise out to make sure clear proper output signal.