[ 0009 ] fig1 illustrates the functions of a pulse oximetry system 100 . the sensor 110 has both red and infrared ( ir ) light - emitting diode ( led ) emitters 112 and a photodiode detector 114 . the monitor 160 has led drivers 162 , a front - end 164 and a signal processor 168 . the monitor 160 determines oxygen saturation by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor emitters 112 , as is well - known in the art . the led drivers 162 provide drive current which alternately activates the red and ir led emitters 112 . the patient cable 200 conducts the led drive current over drive wires 250 connecting the led drivers 162 to the led emitters 112 . the photodiode detector 114 generates a signal corresponding to the red and ir light energy attenuated from transmission through a tissue site . the patient cable 200 conducts the detector signal over detector wires 260 connecting the detector 114 to the front - end 164 . the front - end 164 has input circuitry for amplification , filtering and digitization of the detector signal , which is then input to the signal processor 168 . the signal processor 168 calculates a ratio of detected red and infrared intensities , and an arterial oxygen saturation value is empirically determined based on that ratio . a pulse oximetry sensor is described in u . s . pat . no . 6 , 088 , 607 entitled low noise optical probe , which is assigned to the assignee of the present invention and incorporated by reference herein . a pulse oximetry signal processor is described in u . s . pat . no . 6 , 081 , 735 entitled signal processing apparatus , which is assigned to the assignee of the present invention and incorporated by reference herein . in a pulse oximetry system , the detector typically generates a low - level signal that is susceptible to corruption from various noise sources , such as electromagnetic interference ( emi ) and internal noise sources that originate in the sensor , the patient cable and the monitor . one internal noise source is due to the triboelectric effect , which is the static charge generated when two materials are rubbed together . triboelectric noise is induced in the detector signal when , for example , the detector wires of the patient cable rub together , such as when the patient cable is flexed or is impacted . triboelectric noise spikes can be orders of magnitude larger than the detector signal . fig2 a - b illustrate a patient cable 200 designed for a pulse oximetry system 100 ( fig1 ). the patient cable 200 has an outer jacket 210 , an outer shield 220 , an inner jacket 230 , a graphite coating 240 , detector wires 250 configured as a twisted pair , emitter wires 260 and textile fillers 270 . the twisted pair 250 has detector conductors 252 and associated insulation 254 . the emitter wires 260 have emitter conductors 262 and associated insulation 264 . the shield 220 and the twisted pair configuration of the detector wires 250 reduce noise due to emi and crosstalk . because of the proximity of the twisted pair insulation 254 , however , the detector wires 250 are prone to rubbing and , hence , triboelectric noise . the graphite coating 240 provides a conductive layer along the outside of the detector wires 240 , reducing triboelectric noise by draining the triboelectric induced charge away from the detector wire insulation 254 . the coating 240 is formed by drawing the twisted pair 250 through a solvent bath containing graphite . the solvent is allowed to evaporate , depositing the conductive graphite coating 240 on the twisted pair 250 . a deposited graphite coating 240 , however , has several drawbacks . the coating 240 is difficult to precisely manufacture because the deposition process is difficult to control . as a result , the cable 200 itself is relatively expensive to manufacture . also , preparation of the cable 200 for connector attachment involves cutting and stripping the cable layers to expose the conductors 252 , 262 , which are difficult procedures to perform . in particular , the deposited coating 240 has to be selectively cleaned - off with a solvent and mechanical abrasion to expose the conductor ends 252 , which is time consuming and which may subject the cable 200 to damage . fig3 a - b illustrates a low noise patient cable 300 , which has an outer jacket 310 , an outer shield 320 , an inner jacket 330 , an inner shield 340 , a polymer layer 350 , detector wires 360 configured as a twisted pair , emitter wires 370 and textile fillers 380 . the twisted pair 360 has detector conductors 362 and associated insulation 364 . the emitter wires 370 have emitter conductors 372 and associated insulation 374 . the low noise patient cable 300 functions in a pulse oximetry system 100 ( fig1 ) in a manner similar to that of the patient cable 200 ( fig2 ) described above . in particular , the emitter wires 370 electrically connect the led drivers 162 ( fig1 ) to the leds 112 ( fig1 ), and the twisted pair 360 electrically connects the detector 114 ( fig1 ) to the monitor front - end 164 ( fig1 ). further , the shields 320 , 340 and twisted pair 360 reduce emi and crosstalk . the polymer layer 350 , however , is advantageously disposed around the detector wires 360 instead of a graphite coating as described with respect to fig2 above . the polymer layer 350 is formed by coextruding the twisted pair 360 with a conductive polymer . in one embodiment , the polymer layer 350 is a conductive pvc . in a particular embodiment , the conductive pvc utilizes a flexible conductive vinyl compound , such as abbey # 100 - 1 available from abbey plastic corporation and is coextruded to a diameter in the range of about 0 . 058 ± 0 . 003 inches . a coextruded conductive polymer has several advantages over a deposited graphite coating for reducing triboelectric noise . as with the graphite coating , the polymer layer 350 drains the triboelectric induced charge away from the detector wire insulation 364 . the coextrusion process , however , is easier to control and less expensive accordingly . further , during cable preparation for connector attachment the polymer layer 350 can be easily cut from the twisted pair 360 . in addition , better triboelectric noise reduction can be achieved with the polymer layer 350 than with a graphite coating . in addition to the foregoing , disposing the polymer layer 350 around the twisted pair of detector wires 360 has several advantages over disposing the polymer layer 350 around individual wires . for example , disposal around the twisted pair can be less expensive than disposal around individual wires and can produce an end product cable having a smaller diameter . moreover , disposal around the twisted pair in the embodiment of the low noise patient cable 300 being used for at least pulse oximetry , can increase the eventual signal quality output from signal processing circuitry , such as , for example , a differential amplifier . for example , formation of the polymer layer 350 in a manner that maintains the close physical proximity of the twisted pair tends to ensure external noise applied to the patient cable 300 is applied substantially equally ( or common ) to each conductor of the twisted pair . thus , the differential amplifier ( not shown ) of the monitor 160 can effectively filter the applied external noise through , for example , the amplifier &# 39 ; s common mode rejection . the low noise patient cable has been disclosed in detail in connection with various embodiments . these embodiments are disclosed by way of examples only and are not intended to limit the scope of the claims that follow . one of ordinary skill in the art will appreciate many variations and modifications .
a low noise patient cable has a plurality of emitter wires configured to communicate a drive signal between a monitor and at least one emitter . a plurality of detector wires is also configured to communicate a physiological signal between a detector responsive to the emitter and the monitor . a polymer layer is disposed around , and adapted to conduct a triboelectric charge away from , the detector wires .