fig1 is a block diagram of a pulse oximeter system incorporating a calibration memory element 56 according to the invention . in one embodiment , memory element 56 is a two - lead semiconductor digital memory chip . the calibration element is part of the sensor 50 which also includes red and infrared leds 52 as in the prior art , along with a detector 54 . if desired , leds 52 may be replaced with other light emitting elements such as lasers . the oximeter includes read circuit 60 , drive circuit 66 , look - up tables 62 and 63 , controller 64 , amplifier 72 , filter 74 , and analog - to - digital converter 76 . read circuit 60 is provided for reading multiple coded values across the two leads 51 , 53 connected to calibration element 56 . one value is provided to a look - up table 62 to determine appropriate wavelength dependent coefficients for the oxygen saturation calculation , as in the prior art . the other value ( s ) are then provided to another look up table ( s ) 63 which provides input ( e . g ., coefficients ) to other calculations performed by controller 64 . these additional calculations may enhance the performance and / or safety of the system . controller 64 provides signals to a drive circuit 66 , to control the amount of drive current provided to leds 52 . memory 56 may , for example , be implemented as a random access memory ( ram ), a flash memory , a programmable read only memory ( prom ), an electrically erasable prom , a similar programmable and / or erasable memory , any kind of erasable memory , a write once memory , or other memory technologies capable of write operations . as in the prior art , detector 54 is connected through an amplifier 72 and a filter 74 to an a / d converter 76 . this forms a feedback path used by controller 64 to adjust the drive current to optimize the intensity range of the signal received . for proper operation the signal must be within the analog range of the circuits employed . the signal should also be well within the range of a / d converter 76 ( e . g ., one rule that may be applied is to adjust led drives and amplifier gains so that both red and ir signals fall between 40 % and 80 % of full scale reading of converter 76 ). this requires correct and independent settings for both the red and infrared leds . numerous types of data can be stored in memory chip 56 . some of these types of data are now discussed . the motion - signal algorithm here refers to the sensors designed to be used where “ motion provides the signal ”, i . e ., the cardiac pulse need not be present or discernible in order for the oximeter to provide sp 0 2 values . instead , the red and ir waveforms resulting from the motion itself are used for determining the arterial saturation ( see e . g ., u . s . pat . no . 6 , 018 , 673 ). this feature is possible for tissue beds that are well “ arterialized ” ( a large supply of arterial blood relative to the metabolic needs of the tissue ) resulting in a small aterio - venous saturation difference , as well as other signal characteristics that are not germane to this discussion . we have observed that the necessary degree of arterialization correlates well to being “ well perfused ” at the tissue site , which itself correlates well to the tissue bed being warm . thus by monitoring the temperature of the skin at the sensor site , and by knowing a value of temperature ( programmed into the memory chip ) at which the “ motion - signal ” algorithm can be utilized for the specific sensor design being used , improved reading accuracy through motion can be better accomplished . an electro - chemical or thermal device that senses and stores to memory the number of exposures ( zero , once , or potentially more than once or the actual number ) to sterilization cycles could be used to capture the history of the sensor . excessive exposure to sterilization cycles degrades a number of components in the sensor , and can affect its performance . a sensor exceeding a certain number of exposures could cause a display to indicate the sensor needs to be replaced . data encryption utilizes private and / or public keys to scramble the data written to the memory chip and later decipher the data so that only authorized devices are supported . to further prevent the use with a monitor that isn &# 39 ; t certified to provide correct results , the sensor manufacturing system could periodically change the private and / or public keys . the change in the key could be communicated to the instrument via the memory chip in encrypted form . the purpose of this feature is to elevate the level of security in the memory system . this might be used in a scheme in which the memory chip was on the bandage , not in the connector . this combines a mems accelerometer with any of several different chips that might usefully be placed in the sensor head ; local digitizing chip , preamp chip , memory chip . accelerometer data may be used to warn of the presence of motion ( in which case special algorithms may be called into play or oximetry may be suspended ), or actually to help correct for motion ( to the extent to which we can produce algorithms which can predict physio - optic effects of known motion ). the amount of optical shunting could be measured for each sensor , or family of sensors . the value would be stored in the sensory memory for the monitor to read and use to adjust the processing coefficients . this might be used , e . g ., in overseeing the operation of an actively warmed sensor ( i . e ., a sensor provided with a low - power heating or warming surface ). there is preferably a thermal cutout in the control system of actively warmed sensors , that causes operation to terminate if the sensor goes over a certain temperature . this is a necessary component of protecting the patient against burns . if the reason for a high sensor temperature is that the environment is warm , it could be quite acceptable to continue oximetry , even though warmer operation would be shut down . in the absence of knowledge about environmental temperature , a high temperature reading might have to be assumed to mean that something was wrong with the sensor , in which case all operation might have to cease . an environmental temperature sensor in the plug could help to tell which rule to apply . the memory chip could record the calibration of the device used for thermometry . in legacy oximetry sensors there is a resistor which is selected and installed in the sensor connector , to correspond to the wavelength of the red led , as described in u . s . pat . no . 4 , 700 , 708 . the wavelength difference from led to led has an impact on the calibration of the saturation measurement , if not compensated for . oximeters designed for such sensors will read the value of resistance and adjust its calculation accordingly . when adding the memory chip , memory compatible oximeters will be able to obtain the necessary calibration coefficients from the memory chip but the legacy instruments will still need a calibration resistor value . with a resistance properly built - in to the chip and trimmed or selected at sensor manufacture , only one device would need to be installed in the sensor connector . that would reduce the overall - cost , yet keep the sensor compatible with both the legacy instruments and the new memory compatible instruments . when making measurements of the resistance that is placed in the sensor , for calibration information purposes , one of the factors that can influence that measurement is the contact resistance of the connectors that are between the oximeter and the resistor itself . in order to compensate for connectors that are significant in their impact on the measure , one could encode the contact resistance of the connector and subtract that algorithmically from the measured resistance to get a more accurate measurement of the resistance of the calibration resistor . this would enhance the accuracy with which the resistance measurement is made and therefore make the instrument less prone to miscalculation and therefore inaccuracies in maturation calculation and display . one of the interfering noise sources that can have an effect on oximetry is that of common mode electrical noise . this can come from the surrounding electrical environment . other instruments , lights , drills etc . can produce electrical fields that can couple into the cable between the patient and the oximeter . once coupled - in , they can make measurements more difficult , less accurate or not possible , depending on the severity of the noise . to help reduce this common mode noise , differential amplifiers are used for amplifying the signal from the sensor . these amplifiers amplify only the difference between two signal wires . thus , if the common mode signal is coupled exactly the same into both wires , the amplifier will not amplify it because the same signal is present on both wires . if the two wires have different coupling to their electrical environment then they will present different signals and the difference will be amplified as if it were a signal . one component that influences this coupling is the capacitance of the lines to the outside world . this is affected by the manufacture of the cable , materials , twists in the wire , etc . if one measures the cable during manufacture and then stores that information in the memory chip , it can be read when the oximeter is operating . once the capacitance for the two wires to the shield are known the instrument can be provided with a tunable capacitance device that balances the two lines and makes the noise coupling to the lines better matched . this reduces the amount of susceptibility to the external noise that becomes coupled into the patient cable . reduced noise results in better measurements or the ability to make measurements on some patients at all . another potential source of interference with pulse oximetry sensors is the interference caused by ambient light in the environment reaching the sensor &# 39 ; s photodetector . this can be made worse when a sensor comes loose or the ambient light is extremely high in value . by characterizing the sensor during manufacture or by its design one can know the level of ambient light that can be tolerated , and give a warning to the operator when the level has been exceeded . an external measure of ambient light by the pulse oximeter monitor provides operators the opportunity to adjust the sensor , the light , or both to effect an improvement in the performance of the oximeter . this can be accomplished , e . g ., with a photodetector positioned on or near the pulse oximeter . the stronger the pulsatile signal the better the chances are of measuring the saturation accurately . one way to enhance the modulation percentage is to apply pressure in the range of the median pulsatile pressure or the mean arterial pressure . when implemented , one can use relatively low cost transducers and supply calibration coefficients in the memory to allow accurate pressure readings to be made . the memory can also contain the pressure settings and / or expected modulation enhancement capability to determine effectiveness of the pressure enhancement . a moisture sensor or impedance sensor can detect the amount of wetness of the sensor . this can be used for different purposes , and can be stored in the sensor memory for trending or monitoring . a ) to determine sensor malfunction ( e . g ., oxicliq ). the sensor can be disabled if the wetness exceeds a threshold , which could be stored in the sensor memory . b ) patient isolation . some sensors may not provide for isolation of the patient from the electronics for excessive wetness . the maximum allowable wetness could be stored in the sensor memory . the memory can store information about what parameters are to be analyzed and displayed when the extra wavelengths are used in the pulse oximeter sensor . oxygen saturation may be displayed when 2 wavelengths are used , while additional information could be displayed when an extra wavelength or more are used ( hct , cohb , etc .) while the present invention has been described herein with reference to particular embodiments thereof , a latitude of modification , various changes and substitutions are intended in the foregoing disclosure , and it will be appreciated that in some instances some features of the invention will be employed without a corresponding use of other features without departing from the scope of the invention as set forth . therefore , many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope and spirit of the present invention . it is intended that the invention not be limited to the particular embodiments disclosed , but that the invention will include all embodiments and equivalents falling within the scope of the claims .
the present invention provides a memory chip for use in an oximeter sensor , or an associated adapter or connector circuit . the memory chip allows the storing of different data to provide enhanced capabilities for the oximeter sensor . in addition to providing unique data to store in such a memory , the invention describes unique uses of data stored in such a memory . the data stored in the memory chip includes information relating to enhancing the performance of the oximetry system .