fig1 illustrates an embodiment of an oximetry system incorporating the present invention . a sensor 10 includes red and infrared leds and a photodetector . these are connected by a cable 12 to a board 14 . led drive current is provided by an led drive interface 16 . the received photocurrent from the sensor is provided to an i - v interface 18 . the ir and red voltages are then provided to a sigma - delta interface 20 incorporating the present invention . the output of sigma - delta interface 20 is provided to a microcontroller 22 . microcontroller 22 includes flash memory for a program , and sram memory for data . the processor also includes a microprocessor chip 24 connected to a flash memory 26 . finally , a clock 28 is used and an interface 30 to a digital calibration in the sensor 10 is provided . a separate host 32 receives the processed information , as well as receiving an analog signal on a line 34 for providing an analog display . fig2 is a block diagram illustrating the location of the filter according to an embodiment of the invention . shown is a sensor 10 that is driven by an led drive circuit 16 . the led drive circuit 16 alternately drives an ir led 40 and a red led 42 . a photodetector 44 provides a signal to a current - to - voltage ( i - v converter 46 ). the voltage signal is provided to high pass and anti - aliasing filter 48 . this block includes the band pass filter according to an embodiment of the invention . the output signal is then provided to a sigma - delta modulator 50 . the output of sigma - delta modulator 50 is provided to a demodulator 52 , which is then provided to filtering and decimating blocks 54 and 56 . fig3 illustrates a band pass filter 60 according to an embodiment of the invention . the filter includes an amplifier 62 and a resistor and capacitor circuit comprising capacitors c 2 , c 110 , c 111 , and c 40 and resistors r 7 , r 111 , r 112 , r 110 , and r 109 . an input to this circuit is provided from i - v converter 46 along a line 64 to a first switch 66 for an offset correction not relevant to the present invention . the signal is then provided to a second switch 68 , which is used for a calibration mode according to the present invention . a cross - talk control signal 70 couples the switch to an led current sense line 72 for calibration mode . in the design and manufacture of the band pass filter of fig3 , the corner frequencies are adjusted by varying the capacitor and resistor values to offset and minimize the cross - talk effect . the corner frequencies are the high pass and low pass ends of the band pass filter , which is in place to filter out ambient interferences . there is a major trade off involved in the design of the band pass filter . it is desirable to have the filter corners as close to the modulation frequency as possible . raising the frequency of the high pass corner makes the filter better able to reject any ac portion of ambient light . typically in the us , fluorescent lights have strong ac component at 120 hz and the harmonics of 120 hz . it is desirable to filter this out of the signal . lowering the cut off frequency of the low pass filter limits the high frequency noise from the i - v converter , and provides some anti - aliasing to keep ambient noise out of the system . however , any filtering spreads out the signal in the time domain , for example some of the ir pulse will leak into the dark pulse following it . this has two drawbacks . the first is cross - talk where the ir signal “ leaks ” into the red signal , and vice versa . the second is an offset resulting from a transient that occurs due to capacitances in the patient cable between the led wires and the detector wires . when this transient is filtered , part of it leaks into the sampled part of the signal causing an offset . both of these effects get worse as the corners of the filters are pulled closer to the modulation frequency . tuning the band pass filter to optimize for cross - talk is done when it is designed by adjusting the high pass filter corner and the low pass corner to force the cross - talk to be zero . the size of the red pulse is measured by comparing the sample p 5 ( see fig4 ) to the samples taken in the dark states p 4 and p 6 . since the signal from the ir pulse is still decaying in the dark 2 time period , the p 4 sample will be higher due to the low pass response and the lower due to the high pass response . the effect of the ir pulse on p 4 will affect the size of the measured red signal . this is a cause of cross - talk where the ir signal leaks into the red signal and vice versa . this effect is minimized if the filter is a band pass , with both high pass and low pass effects . the effect of the high pass filtering compensates for the effect of the low pass filtering . thus , the corners are adjusted so that the high pass and low pass signals shown in fig5 are adjusted so that the effect of the high pass filtering compensates for the effect of the low pass filtering to minimize cross - talk . the low pass filter causes a positive cross - talk , and the high pass filter causes an offsetting negative cross - talk . in one embodiment , the band pass filter consists of an rc high pass followed by a salen - key low pass configured as a second order butterworth filter . the impedance of the rc high pass section will have an effect on the transfer function of the salen - key circuit , however this effect is negligible if capacitance c 2 is much larger than c 110 and c 111 . the high pass filter cut off frequency is 32 hz ., and the low pass filter cut off frequency is 12 . 7 khz . in addition to designing the hardware of the band pass filter to reduce cross - talk , a calibration mode allows a further correction for cross - talk using a cross - talk calibration test . a subtle cross - talk effect arises from the filtering in the circuit causing light and dark pulses to spread out into each other in the time domain . fortunately the effects from the band pass filter are linear and measurable , and so can be compensated for in software . since this is the result of the filtering , the magnitude of the effect is known ahead of time . a constant is used to subtract the effects of the ir signal from the red signal and vice versa : fig5 is a circuit diagram of an embodiment of led drive circuit 16 of fig2 . included in the circuit are a connection to the red led on a line 80 , and a connection to the ir led on a line 82 . these are provided through mosfet transistors 84 and 86 to a 1 ohm resistor 88 . in the calibration mode , the led current sense signal on line 72 is taken from the current through this 1 ohm resistor with line 72 of fig5 connected to line 72 of fig3 as an input through switch 68 to the band pass filter . in addition to designing the hardware of the band pass filter to reduce cross - talk , the connection of line 72 in fig5 during a calibration mode allows a further correction for cross - talk using a cross - talk calibration test . while doing the cross - talk test , most of the analog circuits on the board are used and so this is a good test to check the integrity of the analog hardware . this test connects the 1ω current sense resistor 88 to the input to the band pass filter . this way a fixed led current can inject a signal into the signal acquisition circuits . this allows the operation of the led drive 16 , the band pass filter 60 and the sigma - delta modulator 50 to be verified . in addition , measuring the led current using the 1ω resistor allows the led &# 39 ; s current sense circuit to be calibrated more accurately than the 10 % tolerance capacitors in the circuit would ordinarily allow . thus , during the calibration mode , current is shunted into the current sense input from the led drive current . the only analog circuitry not being used is the photodetector and the i - v converter . in a preferred embodiment , whenever a sensor is connected , this is detected and the software automatically does the cross - talk calibration test . a 50 % drive signal is applied to the leds during the calibration circuit to give a sufficiently large signal without going to full range and risking too high of a signal being provided . alternately , other percentages of the drive current could be used . 1 ) set ir led to 50 %, red led to 0 ; then measure the 0 red signal ; 2 ) set red led to 50 %, ir led to 0 ; then measure the 0 ir signal . subsequently , during actual operation , the red cross - talk effect is determined by multiplying the percentage cross - talk times the red signal , and then it is subtracted from the ir signal . the corresponding action is done for the red signal . as will be understood by those of skill in the art , the present invention could be embodied in other specific forms without departing from the essential characteristic thereof . for example , the drive current could be obtained in a different manner and a different design could be used for the band pass filter . alternately , the band pass filter could be used alone , without the software calibration added . accordingly , the foregoing description is intended to be illustrative , but not limiting , on the scope of the invention which is set forth in the following claims .
a method and apparatus for reducing cross - talk in an oximeter . the oximeter includes a band pass filter . the amount of cross - talk through the band pass filter is estimated . based on this estimate , the corner frequencies of the band pass filter are adjusted when it is designed to minimize the cross - talk . in one embodiment , a calibration mode is performed when a sensor is attached to the oximeter . in the calibration mode , the signals are measured with first only the red led on and then with only the ir led on . any signal measured in the off channel is assumed to be a result of cross - talk from the other channel . the magnitude of the cross - talk is determined as a percentage , and subsequently the percentage is multiplied by the actual signal and subtracted from the other led signal as cross - talk compensation .