SPO4025 FAQs

June 2011

Can we trust the hardware of the SPO4025?

The SPO4025 is a 4-layer board with a ground plane. It is an industrial product and conforms to RoHS.
All sensor side inputs have ferrite beads in accordance with current EMC regulations. The board has no electrolytic capacitors and MTBF is estimated to yield one failure after 10000 units operating 10 years 24 hours a day. The board contains extensive internal hardware test logic that is capable to identify partial failures during operation. Example: Finger probe LED currents and voltages are always measured with good precision. Thus any problem with the finger probe LED control can be detected, e.g. broken cable, false wiring, bad LED, low supply voltage. The board also measures ambient light level, CPU temperature, photo current ADC noise etc. This logic is used during production to generate a complete test protocol for each unit. The protocols are delivered on CD-R with the boards.
The board automatically controls each LED current independently and also the photoamplifier gain, resulting in a large dynamic transmission measurement range. It can be used with new bright finger probes as well as older finger probe families producing less light. The board can be adapted to produce LED currents of up to 200 mA as necessary for operating the oximeter with a fiber cable (nuclear spin tomograph).


Can we trust the firmware of the SPO4025?

The firmware is modular and strongly based on data compression technologies in order to save processing power and energy. It has been checked using many different sets of recorded plethysmograms.
Various host interface modules for different data protocols are readily available and can be adapted to special needs. One of those implements bluetooth connectivity using an additional certified RF/antenna hardware module. Currently the SPO4025 does not include a display filter for saturation and pulse rate, nor alarm generation/management. Such filters can be implemented according to specification.
As of now we have about 3500 units operating in body boxes used in sleep analysis (polysomnograph). That is a demanding application, since the oximeter has to run 6 hours producing valid results every 5 seconds without any false desaturation of more than 2 % and without supervision by any expert.
The oximeter is fast: under normal stable conditions with reasonable perfusion, it will report the first saturation result after 3 heartbeats and the first heart rate result after 4 heart beats. There is a prompt reaction to both lost finger and lost sensor state changes. The software has an internal time horizon limited to 10 seconds and it will hold previous results at most 5 seconds. Under normal conditions it will report new patient model results after each heartbeat.

Certainly there is nothing like a perfect pulse oximeter, and we are always investing into better artefact suppression methods. We think that the ECG signal available in many monitors and somnography environments should be used to efficiently remove motion artefacts.


Do you provide test equipment for the SPO4025?

We do offer a test kit and two types of simulators.

a) The test kit is a small box with a RS232 cable to plug into a PC plus a DB-9 connector for the probe. There is Win32 application software on this site to be used with this hardware. The software runs the oximeter and displays a rich set of information on the PC screen. The software can be used with USB-to-Serial adaptors. A similar setup is used by us for production tests. There is a detailed description on this site.
b) The electrical simulator is connected instead of the finger probe.
c) An electronic finger simulator to be used with finger probe.

Both simulators run on 4 rechargeable AA batteries and include logic to turn on and off automatically. They work only when an active oximeter is connected.
Both simulators are "complete", i.e. they can be used with almost any pulse oximeter, even other brands. Our simulators support switch selectable simulation of two two different pulse rates (e.g. 68 and 103 bpm), two different SpO2 levels (98 and 84 %) and two different HbCO levels (e.g. 0 and 5%). Basic accuracy is 1 %. The simulators do have a heartbeat LED indicator. They are low noise designs and generate a perfusion amplitude of 3 %.


When testing the SPO4025c we noticed extra beeps. Why?

Probable you generate a beep as soon as the SPO4025c sends an extended record with new patient model data. The SPO4025c logic recalculates the patient model as soon as the probe data indicate a potential arterial blood wave. The results of this calculation are sent by the SPO4025c, even if the potential blood wave was not identified as a heartbeat. Anyway, pulse wave candidates will affect the patient model.
There are two possible ways to produce correct beep signals: a) Looking at some flag bits in the SPO4025c output data stream or b) using the module's heartbeat output pin. This output pin operates once per heartbeat, as identified according to the current patient model.


When testing the SPO4025c we received patient model data without perfusion. Shouldn't you avoid measuring into the noise?

The SPO4025c data interface was designed to be as general as possible. It includes both quantitative patient model data and patient status flags. With low perfusion, the patient model gets partially unreliable. Obviously the determination of saturation is not possible without perfusion. And obviously, somewhat higher perfusion levels will be necessary with large, continuous artefacts.
The SPO4025c applies a cut of 0.1% perfusion in order to detect a low perfusion state. Besides that it provides an estimate of the probability of the current patient model. This quantity measures to which extent the recent probe data stream can be explained using the patient model. Please look at the state information that will also indicate disconnected probe, probe loss (missing finger) and use the patient model information according to the extra information.
The SPO4025c firmware was optimized to detect state changes as fast as possible. Delays for disconnected sensor and missing finger are less than 1 second, delay until the first valid patient model is about 4 to 6 seconds.
The SPO4025c is unique in the sense that it guarantees a limited time horizon of the patient model. Therefore is has a limited reaction time of at most 10 seconds. Example: When you run the oximeter with a simulator and suddenly change the saturation, the SPO4025c will show the previous patient model for some seconds with reducing probability, then prefer the new patient model with increasing probability. It will not show invalid intermediate saturation levels.


How many ADC bits does the SPO4025c use?

The SPO4025c uses a high speed 12 bit ADC. The full dynamic range is reached by an analog front end including a sampled servo circuit for ambient light photo current. The SPO4025c also uses innovative oversampling and noise cancellation technics to reach an overall resolution of about 21 bits, resulting in a noise limit equivalent to about 0.002 % perfusion with a 70-fold ambient light margin.
This adds to a 16 bit dynamic range available for LED current control and photocurrent amplifier gain. This OEM board can be used under a large variety of conditions and with probes of largely different brightness.
Many engineers were impressed by the clean and reliable plethysmogram data that our modules deliver under varying conditions. Many modules are used for academic studies, where scientists need an efficient and reliable way of correctly driving the finger probe, while receiving valid and easy to use raw optical transmission data.
The 12 bit ADC has enough speed to sample yet other signals, e.g. ECG at 900 Hz for PTT determination. PTT is a very interesting quantity for apnoe/arousal detection, much faster than saturation. Or you can use the ECG sync to perform oximetry on an ergometer. We have firmware modules available for such applications.


How about electrical safety of the SPO4025?

Our OEM board is a low voltage device that cannot by itself contain any electrical safety hazard. The same statement applies when the SPO4025 is used in a handheld oximeter with no electrical interfaces other than the probe input. When using the SPO4025 in mains-driven or computerized monitoring devices, though, there are certain rules to observe. According to international safety standards there must be two insulation barriers. One is given by the construction of the finger probe. Another second barrier of certified quality must be present between the SPO4025 and mains, resp. any other device of unknown insulation status. We recommend the addition of a DC-DC insulator and an optocoupler. Due to the low power consumption of the SPO4025c, the DC-DC insulator can be of the smallest available size, e.g. 4 x 8 x 8 mm for 1 kV insulation. Or one of those integrated isolators that are able to provide enough power for the SPO4025. Another strategy is to use the insulation barrier of the monitor power supply and only implement insulated external data interfaces.
The LED driver of the SPO4025 remains safe even after failure of parts. It will under no circumstances heat or burn probe LEDs.


Can we use the testkit with our finger probe?

There are three finger rpobe attributes to match an oximeter:
a) Cable plug
b) Optical wavelength of LEDs
c) Brightness of LEDs
Our testkits come with DB-9 connectors for the finger probe. You can find many "compatible" finger probes on the market. Once the plug matches, the SPO4025 can recognize the finger probe and adapt its calculations to the specifications of that finger probe family. But this cannot possibly work when using the testkit with an unknown finger probe.
In this case the compatibility issues b and c apply. While b) is a minor problem affecting the accuracy of the measurement at the 1 or 2 % level, c) may pose a real problem, since with a very bright sensor the finger may not be detected in the probe. Please read the next paragraph.


Our testkit does not work. It displays LED currents but no pulse or saturation values.

The SPO4025c is designed to work with almost any existing finger probe. It can adjust both LED current levels in 256 steps independently for each LED. It can also adjust photo current amplifier gain in 256 steps. Since all numbers are known, the SPO4025 can calculate the optical transmission of the finger probe in order to detect the finger.
Now, this calculation depends on the brightness of the finger probe. There are quality finger probes with 50 times the brightness of other quality finger probes. This originates from the large conflict between hardware compatibility issues and silicon technology advances during the last 25 years. Nowadays certain families of quality finger probes need to produce low brightness in order to remain compatible with existing oximeters.
If you are using a very bright sensor with a OEM module that was made for a standard sensor, there will be more light on the photodiode even with the finger in the probe. Then the Win32 application will display Finger? in the status line indicating the missing finger status. Then you could try using the thumb in order to perform the measurement. Or use another older finger probe with less brightness.
In general, our OEM module will recognize certain finger probes by measuring a coding resistor in the finger probe plug and then adapt the calculations to the specification of that probe family.


Without finger in the finger probe, our testkit shows a red curve with continuous high frequency oscillations.

The SPO4025c is designed to work with almost any existing finger probe and with various other sensors, even in reflective mode. It can adjust both LED current levels in 256 steps independently for each LED. It can also adjust photo current amplifier gain in 256 steps. During a measurement, whenever the oximeter needed to change one of these controls, the Win32 application will display the plethysmogram in red color for a short time. If the screen exhibits a continuous red curve with rapid oscillations, this means the oximeter cannot find a good setting.
This can possibly happen when using a very bright sensor with a module made for an older sensor family of less brightness. Then, without a finger in the probe, brightness in the lowest LED current setting may be to small and brightness in the second lowest current setting may be to high. Then the module will start switching between these two settings. Of course, such oscillations will stop as soon as a finger appears in the finger probe.


How can we reduce power consumption of the SPO4025?

In revision SPO4025h it operates at about 7 mA out of 3 V plus LED currents. LED currents depend on the brightness of the finger probe and are typically 0.2 to 1 mA with a standard quality finger probe.
In revision SPO4025i supply current was reduced from 7 mA to 3 - 4 mA, keeping similar technical specs. In general processor supply current will be less than probe LED currents. We also added an automatic power down mode, based on "finger in probe" detection. Power saving was based on the observation that ambient light reserve can be reduced for somnography applications.
A similar, host controlled power down mechanism is available for a "no plethysmogram" operation mode that takes saturation samples once every 10 seconds. These developments are available to market on short notice.
Supply current also depends somewhat on the amount of data output to the host.


Your software shows parts of the plethysmogram curve in red color. What are the red parts?

Whenever the oximeter needs to adjust LED currents and photoamplifier gain, the curve turns red for a short time. The adjustments are made in steps and may cause artefacts. For example, when we increase LED current by 2 %, the photometric output of that LED may increase by 2.2 %. This is absolutely realistic. Then there will be a 0.2 % artefact, since the 2 % current change is subtracted out of the curve. With a normal perfusion amplitude of 5 %, a 0.2 % artefact is hardly noticeable, and the pulsoximeter will easily measure over this. Under critical low perfusion conditions the artefacts will be more visible. Normally the oximeter quickly reaches a good adjustment and it will readjust settings rarely, maybe once per minute or so.


In your specs the supply voltage is to be 3 V. Can we use 3.3 V instead?

Sure, supply voltage range is 3 V to 3.6 V. While the processor operates reliably with voltages as low as 1.8 V, the finger probe red LED operating voltage plus the current source operating voltage require about 3 V minimum. The device has a built-in supply voltage supervisor that is programmed for a shutdown limit of 2.8 +/- 0.05 V. When operating the device at 3.0 V, please observe tolerances in order to stay off the SVS limit.



Copyright: www.cadt.de, 2011