Method for ambient light subtraction in a photoplethysmographic measurement instrument

An improved photoplethysmographic measurement system is disclosed in which a portion of a time division multiplexed (TDM) signal represents an ambient light level, and other TDM signal portions represent detected levels of two or more centered wavelengths of transmitted light. The ambient and detected light portions of the signal are simultaneously applied to the inputs of an instrumentation amplifier(s) so as to produce a continuous output voltage that is proportional to a difference in voltage between the ambient and detected light portions of a TDM signal. Such an approach provides for ambient light level subtraction with reduced noise and componentry.

FIELD OF THE INVENTION 
This invention relates to systems that utilize time division multiplexed 
(TDM) signals and, more particularly, to an improved photoplethysmographic 
measurement instrument in which an ambient light component is subtracted 
from a TDM signal. The invention is particularly apt for implementation 
using instrumentation amplifiers. 
BACKGROUND OF THE INVENTION 
In the field of analog data transmission, one efficient data transmission 
technique is to utilize a TDM signal in which information corresponding 
with a plurality of sources is transmitted over a single data line. Data 
corresponding with each source is transmitted over the line in dedicated 
intervals which are generally regular in duration and sequenced. That is, 
at one particular point in time, data present on the line corresponds with 
only one of the sources. If the dedicated interval rate is sufficiently 
rapid, an apparency of continuous data transmission corresponding with 
each source is realized at the receiving end of the data line. In this 
regard, the TDM signal is de-multiplexed at the receiving end so as to 
separate the data into parallel channels, one corresponding with each 
source. De-multiplexing is generally performed in a synchronous switching 
operation. 
In some systems, after de-multiplexing, a first series of signal 
conditioning steps is performed which operate on the parallel channel 
source data. Thereafter, a second series of steps is performed which, once 
again, require the signal to be in a TDM form. Re-multiplexing of the 
parallel channels is necessary to regain the TDM signal format required 
for the second series of steps. After the second series of signal 
conditioning steps, the signal is de-multiplexed a second time into 
parallel channels for completion of analog signal processing. The 
performance of each multiplexing/de-multiplexing iteration introduces 
switching noise into the resultant signal(s). As can be appreciated, such 
noise presents system design considerations and limitations. 
Other limitations are also introduced by the performance of multiple 
de-multiplexing/multiplexing iterations. Specifically, each time either of 
these operations is performed, the overall parts count in the system is 
increased. Such an increase may significantly limit the reliability of the 
system and increase manufacturing costs. Moreover, the associated increase 
in signal line length resulting from the additional parts, along with 
their interconnections, may serve to couple still further noise into the 
system from the ambient environment, thereby reducing system performance. 
The noted design considerations/limitations are of particular importance in 
medical instruments that determine pulse rate and blood oxygen saturation 
level via measurement of certain blood analytes such as, for example, the 
concentration (as a percentage of total hemoglobin) of oxyhemoglobin 
(O.sub.2 Hb), deoxyhemoglobin (RHb), carboxyhemoglobin (COHb) and 
methemoglobin (MetHb) of a patient. Such photoplethysmographic measurement 
instruments are configured to emit light of at least two different, 
predetermined wavelengths through a selected portion of a patient's 
anatomy (e.g., a finger tip). The analytes to be identified within the 
patient's blood must each have unique light absorbance characteristics for 
at least two of the emitted wavelengths. By measuring changes in intensity 
of the transmitted (the light exiting an absorber is referred to as 
transmitted) light from the patient's finger (or other suitable area of 
anatomy) at these wavelengths, each analyte may be determined. Thereafter, 
characteristics such as blood oxygen saturation may be determined based on 
these analytes. Other characteristics such as pulse rate may be determined 
based on certain components of the transmitted light signal which passes 
through the patient's anatomy. Specifically, the transmitted light 
includes a large DC component and a smaller AC or pulsatile component. By 
using the pulsatile component, the patient's pulse rate may be determined, 
since fluctuations in the pulsatile component are a function of arterioles 
pulsating with the patient's heart rate. 
In one photoplethysmographic measurement system known as a pulse oximeter, 
at least two wavelengths of light may be emitted during dedicated, 
alternating intervals. The transmitted light from the selected body 
portion is detected by a light-sensitive element (e.g., a photodiode). The 
light-sensitive element then outputs a TDM signal that includes portions 
corresponding with each wavelength of the transmitted light. As will be 
appreciated, the photodiode is also sensitive to light which is present in 
the ambient environment. Consequently, the TDM output signal can include a 
corresponding ambient light component. Such component must be removed from 
the TDM signal for proper processing. For this purpose, at least one 
interval within a TDM signal is typically dedicated to measuring a 
component corresponding with only the detected ambient light. 
For example in one known pulse oximeter, each emitted light level is 
immediately preceded by an ambient light interval which may also be 
referred to as a "dark time" interval. The system first de-multiplexes the 
TDM signal into parallel channels. Signal processing then proceeds wherein 
a first series of steps performs preliminary filtering. Immediately 
following the first series of steps, the parallel channels are 
re-multiplexed. Next, a second series of steps is performed in which the 
re-multiplexed signal facilitates subtraction of the dark time signal from 
the signal corresponding with each emitted light interval in a manner 
known in the art. Such subtraction process relies on a dark time interval 
immediately preceding each and every emitted light interval in a TDM 
format. Following the second series of steps, in which ambient light 
subtraction is accomplished, the TDM signal is de-multiplexed a second 
time into parallel channels prior to the completion of signal processing. 
Such multiple de-multiplexing/multiplexing raises the very noise 
introduction and cost concerns noted above. 
SUMMARY OF THE INVENTION 
Accordingly, primary objectives of the present invention are to provide an 
improved photoplethysmographic measurement system wherein ambient light 
subtraction from a TDM signal is achieved with reduced noise and/or 
reduced componentry. 
In order to achieve such objectives, a system is provided having at least 
one TDM signal that includes at least a first identifiable portion that 
corresponds with detected light from at least one predetermined, light 
source plus any ambient light present in the system, and a second 
identifiable portion that corresponds with only the detected ambient light 
present in the system. In one aspect of the invention, the system further 
includes amplification means having first and second inputs and an output. 
The amplification means is configured to produce an amplified output on 
its output proportional to a difference between signals present on its 
first and second inputs. Means are provided for substantially 
simultaneously applying the first TDM signal portion to the first input 
and for applying the second TDM signal portion to the second input, at 
substantially the same time, such that the amplified output produced by 
the amplification means is proportional to the difference between the 
first and second signal portions, thereby achieving contemporaneous 
subtraction of the ambient light component and desired signal 
amplification. The contemporaneous amplification and ambient light level 
removal may be advantageously performed using an instrumentation 
amplifier. 
In another aspect of the invention, the system is configured for emitting 
light through a region of interest at two or more different primary 
wavelengths in an environment which includes an ambient light level. The 
system separately detects the ambient light level and transmitted light 
level for each primary wavelength that has passed through the region of 
interest, such that the levels of detected ambient light and transmitted 
light form corresponding portions of a TDM signal. The level of 
transmitted light detected at each primary wavelength includes the ambient 
light level. To prepare for removing the ambient light level from such 
transmitted light levels, demultiplexing means is provided for 
demultiplexing the TDM signal to provide (i) a first portion signal 
corresponding to the transmitted light level at the first primary 
wavelength, (ii) a second signal portion corresponding to the transmitted 
light level at the second primary wavelength and (iii) an ambient light 
signal portion corresponding to the detected ambient light level. A first 
subtraction means then produces a first ambient compensated output 
corresponding to the first primary wavelength by removing the ambient 
light level from the first signal portion. Similarly, while second 
subtraction means, separate from the first subtraction means, produces a 
second ambient compensated output corresponding to the second primary 
wavelength by removing the ambient light level from the second signal 
portion. Such separate first and second subtraction means may, for 
example, comprise first and second instrumentation amplifiers. A processor 
means is employed to determine the value of the characteristic(s) of 
interest within the region of interest based on the first and second 
ambient compensated outputs. 
In a primary embodiment of the invention, a photoplethysmographic 
measurement system includes means for emitting light through a portion of 
a patient's anatomy at two or more different, predetermined and centered 
wavelengths (e.g. by intermittent emission). The transmitted portions of 
the emitted light for each centered wavelength and the ambient light level 
are detected so as to form respective ambient and detected light signal 
portions within the TDM signal. First and second amplification means are 
provided, each of which includes a first input, a second input and an 
output for producing an amplified output. The output produced by each 
amplification means is proportional to a difference between signals 
present on its first and second inputs multiplied by a predetermined and 
variable gain. The system is configured to apply the ambient light signal 
to the first input of each amplification means while, at substantially the 
same time, applying the transmitted detected light signals to the second 
input of the first and second amplification means. Consequently, the first 
amplification means produces a first output that is proportional to the 
difference between the detected signal corresponding to a first 
predetermined, centered wavelength and the ambient light level; and the 
second amplification means produces a second output that is proportional 
to the difference between the detected signal corresponding to a second 
predetermined, centered wavelength and the ambient light of interest. The 
first and second amplification means may advantageously comprise separate 
first and second instrumentation amplifiers, each having a substantively 
linear response over a frequency range that accommodates the detected 
light. The first and second outputs from the amplification means are then 
processed to determine/output certain characteristics including, but not 
limited to, a patient's pulse rate and blood oxygen saturation level 
and/or specific blood analyte information such as, for example, the 
concentration (as a percentage of total hemoglobin) of oxyhemoglobin 
(O.sub.2 Hb), deoxyhemoglobin (RHb), carboxyhemoglobin (COHb) or 
methemoglobin (MetHb) or to otherwise provide an indication when one of 
such measures exceeds a predetermined level of interest. 
The concentrations of a plurality of the noted analytes of interest may be 
determined by using at least a common plurality of emitted wavelengths, 
provided that the analytes exhibit unique absorbance behavior at the 
emitted light wavelengths. By measuring changes in intensity of the 
transmitted light, for example, from a finger at the emitted wavelengths 
and based on the corresponding outputs of the amplification means, the 
aforementioned analytes are among those which may be determined in 
processing. Thereafter, characteristics such as blood oxygen saturation 
may be determined based on these analytes. Other characteristics such as 
pulse rate may be determined based on certain components of the 
transmitted light signal which passes through the patient's anatomy. 
Specifically, the transmitted light includes a large DC component and a 
smaller AC or pulsatile component. By using the pulsatile component, the 
patient's pulse rate may be determined, since fluctuations in the 
pulsatile component are a function of arterioles pulsating with the 
patient's heart rate. 
As will be appreciated, the present invention allows a system to be defined 
that employs only a single demultiplexing step within the overall system. 
In such a system, a TDM, such as described above, is demultiplexed into 
(i) a first signal corresponding to transmitted light at the first primary 
wavelength, (ii) a second output corresponding to the transmitted light at 
the second primary wavelength and (iii) an ambient light signal 
corresponding to the detected ambient light level. Thereafter, first and 
second ambient compensated outputs can be produced by removing the ambient 
light level from the first and second signals, respectively, using the 
ambient light signal. The first and second ambient compensated outputs may 
then be separately conditioned and combinatively processed to determine 
one or more of the noted characteristics of interest. 
Switching noise can be reduced in the present invention since a TDM signal 
need only be demultiplexed a single time into individual channels. 
Additionally, parts count and complexity can be reduced. Finally, as noted, 
the invention is particularly apt for implementations using 
instrumentation amplifiers, thereby further yielding improved system 
performance and reliability.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a diagrammatic illustration of a photoplethysmographic 
measurement system embodiment, generally indicated by reference numeral 
10, constructed in accordance with the present invention. As will be 
described, the embodiment utilizes a time division multiplexed (TDM) 
signal in conjunction with an instrumentation-type amplifier. The system 
is configured to apply specific portions of the TDM signal to the inputs 
of the instrumentation amplifiers so as to produce continuous output 
voltages that are proportional to differences in voltage between different 
portions of the TDM signal. 
System 10 includes a sensor probe 12 and a signal conditioning/processing 
assembly 30 mounted in housing 14. Probe 12 is configured for emitting 
light 16 centered about a first wavelength and light 18 centered about a 
second wavelength. Light of the first and second wavelengths is 
alternately emitted at regular intervals from first and second light 
sources 20 and 22, respectively, which may, for example, comprise light 
emitting diodes or laser diodes. One known combination of first and second 
wavelengths comprises light centered about 660 nm and 940 nm, 
respectively. It is to be understood, of course, that many other 
combinations can be employed. Furthermore, it should be appreciated that 
the present invention can be employed in systems utilizing light of more 
than two centered, or primary, wavelengths of light. 
Continuing to refer to FIG. 1, a portion of the emitted light is 
transmitted through a portion of a patient's anatomy, such as a finger 24, 
and is detected by a light-sensitive device. In the described embodiment, 
a photodiode 26 is utilized. Other areas of the patient's anatomy may also 
be used provided that the transmitted light suitably passes through such 
areas. In this regard, the output indications provided by system 10 
pertain to arterial blood flow data. More particularly, based upon the 
absorption of light at the emitted wavelengths certain characteristics may 
be determined including, but not limited to, a patient's pulse rate and 
blood oxygen saturation level, including the concentration (as a 
percentage of total hemoglobin) of oxyhemoglobin (O.sub.2 Hb), 
deoxyhemoglobin (RHb), carboxyhemoglobin (COHb) or methemoglobin (MetHb). 
Sensor probe 12 is electrically connected to the signal 
conditioning/processing assembly 30 via multi-conductor cable 32. A first 
set 34 of conductors within cable 32 carries drive signals to light 
sources 20 and 22, while a second set 36 of conductors is used to bias 
photodiode 26 and to carry a TDM signal to the signal 
conditioning/processing assembly 30. 
Referring to FIG. 2 in conjunction with FIG. 1, the TDM signal 38 includes 
a series of pulse groups 40 output by the photodiode 26 in response to the 
detection of light passed through finger 24. Each pulse group includes, in 
this case, a negative going "light 1" (hereinafter "LT1") portion, or 
interval, and a negative going "light 2" (hereinafter "LT2") portion, or 
interval, corresponding to the detected levels of light at each of the two 
transmitted wavelengths. Ambient light is also detected by photodiode 26 
together with the detected light corresponding with the light at the first 
wavelength 16 and light of the second wavelength 18. This ambient light is 
manifested within the output signal of the photodiode 26 as an offset 
voltage. That is, the LT1 and LT2 portions each include an offset which 
results from ambient light that is incident upon the photodiode 26 during 
the time that the LT1 and LT2 signal portions are generated. In order to 
facilitate removal of the offset, TDM signal 38 includes a "dark 
1"(hereinafter "DK1") portion, or interval, immediately preceding LT1, and 
a "dark 2"(hereinafter "DK2") interval 48 immediately preceding LT2. The 
voltage level during each of the DK1 and DK2 intervals represents the 
ambient light level incident upon photodiode 26 in the absence of 
transmitted light at the first wavelength 16 or transmitted light at the 
second wavelength 18. 
By way of example and in total darkness, a signal corresponding to 
V.sub.dark is output by the photodiode which may be offset slightly from 
the zero voltage level V.sub.zero. The difference in voltage between 
V.sub.zero and DK1, and between V.sub.zero and DK2, illustrated as 
V.sub.amb, represents the overall ambient light offset present in TDM 
signal 38. Such an ambient light level may result from any light source 
including, for example, room lighting or sunlight. As will be appreciated, 
subtraction of the DK1 and DK2 voltages from the LT1 and LT2 portions, 
respectively, will result in elimination of both the ambient light data 
and the photodiode dark current from the signal of interest, i.e., data 
corresponding with the transmitted light 16 and 18 which has passed 
through finger 24. It should also be appreciated that ambient light may 
not produce a D.C. offset as illustrated in FIG. 2. For example, certain 
common types of lighting induce a time-varying A.C. offset. One such type 
of lighting is 120 Volt A.C., 60 Hz. powered fluorescent lighting which 
may produce ambient light pulses at a frequency of 120 Hz. Therefore, 
circuitry for providing ambient light level subtraction should also be 
effective in the removal of such time-varying offset signals, as will be 
described in further detail below. 
It is to be understood that TDM signal 38 may be configured in a number of 
different ways in accordance with the present invention. In the described 
embodiment, each pulse group 40 includes a dark time interval preceding 
each light time interval. It is noted that some prior art systems require 
this signal configuration to perform ambient light subtraction. The 
present invention also contemplates ambient light subtraction utilizing 
one dark time interval per each pulse group 40, as will be described 
further below. 
Referring to FIGS. 1 through 3, signal conditioning/processing assembly 30 
will now be described. A processing/control section 50 is included which 
provides drive signals to a light source driver section 52 through a 
control signal line 53. The light source driver section may be configured, 
for example, to drive LEDs, laser diodes or other such suitable light 
sources which may become available. The light source driver section 52 
provides drive signal waveforms to probe 12 so as to excite sources 20 and 
22 to emit light 16 (of the first wavelength) and light 18 (of the second 
wavelength). In turn, photodiode 26 detects the light passing through the 
selected body portion to output the TDM signal 38 of FIG. 2. 
As shown in FIG. 3, TDM signal 38 is coupled to a preamp section 54 via 
conductors 36 from probe 12. Preamp 54 converts the relatively small 
magnitude current of TDM signal 38 to a voltage level more useful for 
processing. A silicon switch 56 is connected to preamp 54 through resistor 
58 (e.g., a 2.1 KW resistor). In the present example, silicon switch 56 
comprises a single pole, quadruple throw switch which is controlled by 
processing/control section 50 by means of control lines 59. Synchronous 
control of switch 56 is coordinated by processing/control section 50 with 
drive signals provided to light source driver section 52 such that TDM 
signal 38 is de-multiplexed. Specifically, silicon switch 56 outputs four 
data channels A, B, C and D wherein channel A comprises the LT1 signal 
portion, channel B comprises the DK1 signal portion, channel C comprises 
the LT2 signal portion and channel D comprises the DK2 signal portion. 
Following de-multiplexing, the signal on each channel, A-D, charges one of 
four holding capacitors 60a-d(e.g., 1.0 .mu.F capacitors). These holding 
capacitors are configured with resistor 58 to form part of a sample and 
hold circuit (as well as a low-pass filter) in which an average value of 
each channel is stored for that cycle. 
Thereafter, signals on each of channels A-D are filtered by one of four 
first order low-pass filters 62a-d. Each filter includes a resistor 64 
(e.g., 60.4 K.OMEGA. resistors) and a capacitor 66 (e.g., 0.1 .mu.F 
capacitors). In accordance with the present invention, the sample and 
hold/low pass circuit comprised of resistor 58, capacitors 60 and silicon 
switch 56 cooperates with low-pass filters 62 so as to simultaneously and 
continuously apply signals LT1', DK1', LT2' and DK2' to an amplification 
section 68. It should be appreciated that the values of resistor 58 and 
resistors 62, in the low pass filter, are selected along with the values 
of capacitors 60 and capacitors 62, in the low pass filter, to filter out 
common, time-varying offset voltages such as those produced by fluorescent 
lighting to effectively remove the time-varying ambient light signal 
component. The values of these various passive components may be modified 
as required by the TDM signal being processed and, in fact, the components 
may be of different values from one channel to the next for a particular 
application. For example, the circuitry of FIG. 3 may readily be modified 
for a TDM signal which includes a single dark time interval per pulse 
group (not shown). In such a case for a two-wavelength system, channel A 
may process the LT1 signal, channel B may process the LT2 signal and 
channel C may process a DK signal, with channel D not being required. 
Continuing to refer to FIG. 3, amplification section 68 includes first and 
second instrumentation amplifiers 70a and 70b. As employed herein, the 
term "instrumentation amplifier" refers to an amplifier which has an 
output, V.sub.out =(V.sub.+ -V.sub.-) G+V.sub.ref ! where V.sub.+ and 
V.sub.- are the inverting and non-inverting inputs, respectively. 
V.sub.ref is a reference voltage which may be set to ground. G is the 
gain. Typically, the common mode rejection ratio, CMRR, is very high, for 
example, greater than 100 dB. The instrumentation amplifier may be a 
single integrated circuit or made up of a group of integrated circuits 
and/or discrete transistors. 
A plurality of control lines 72 connect processing/control section 50 with 
amplification section 68. Each amplifier 70a, 70b includes an inverting 
input, indicated by a minus sign, and a non-inverting input, indicated by 
a plus sign. LT1' and DK1' are applied to the inverting and non-inverting 
inputs of amplifier 70a, respectively, while L' 2' and DK2' are applied to 
the inverting and non-inverting inputs of amplifier 70b, respectively. 
Alternatively, where a single dark time interval is presented within each 
pulse group, the dark time channel is applied to both of the non-inverting 
inputs of amplifiers 70a and 70b. 
The gain, G, of each amplifier 70a, 70b is adjusted by varying the 
resistance between a pair of terminals. Specifically, a first variable 
resistor 74 is connected between terminals 76 and 78 of amplifier 70a, 
while a second variable resistor 80 is connected between terminals 82 and 
84 of amplifier 70b. Variable resistors 74 and 80 are adjusted by 
processing/control section 50 using control line sets 72a and 72b, 
respectively. Amplifier 70a includes an output 86 and is configured to 
output a voltage according to the difference in voltage level between its 
inverting and non-inverting inputs multiplied by the gain of the 
amplifier, as determined by the setting of the variable resistor. 
Amplifier 70b includes an output 88 and is configured in the manner of 
amplifier 70a. Amplifiers 70a and 70b each include a reference input 90 
and 92, respectively. The reference inputs may be grounded but, 
alternatively, they may be provided with offset voltages V.sub.OFFa and 
V.sub.OFFb on control lines 72c and 72d, respectively, which are added to 
each amplifier's output voltage. Thus, each amplifier outputs a voltage 
V.sub.AMPa or VAMP.sub.b, respectively, in accordance with the equations: 
EQU V.sub.AMPa =V.sub.OFFa +GAIN.sub.a .times.(LT1-DK1)!; or 
EQU V.sub.AMPb =V.sub.OFFb +GAIN.sub.b .times.(LT2-DK2)! 
wherein V.sub.OFFa and V.sub.OFFb are provided from the processing/control 
section and wherein GAIN.sub.a and GAIN.sub.b are, likewise, determined by 
the processing/control section and implemented via settings of variable 
resistors 74 and 80. 
V.sub.AMPa 86 and V.sub.AMPb 88 are provided to an amplification/filtering 
section 91, then to processing/control section 50. 
Following processing, data is provided by processing/control section 50 to 
a display 92 including a display screen 94 of a suitable configuration 
including, for example, LCD and CRT types. Information and related 
warnings are provided in conjunction with or as an alternative to visual 
display. For example, in the event that the determined value of a 
monitored characteristic falls above and/or below predetermined threshold 
values an audio alarm may sound to alert attending medical personnel. 
One advantage in the configuration of the circuitry of FIG. 3 relates to 
control of the gain and offset voltage settings of amplifiers 70 by the 
processing/control section. Specifically, to achieve a relatively high 
signal to noise ratio, the gain and offset settings provided by 
processing/control section 50 should cooperatively center each amplifier's 
output within the input range of the processing/control measurement 
system, at the same time, maximizing the swing of the output voltage 
therebetween without clipping either the top or bottom of the waveform. 
Processing/control section 50 may accomplish this task by monitoring the 
V.sub.AMpa' and V.sub.AMPb' signals received from 
amplification/filtering section 91, or V.sub.AMPa and V.sub.AMPb, received 
directly from amplification section 68 using, for example, automatic gain 
control techniques which have previously been implemented in the art 
using, for example, control algorithms. 
FIG. 4 illustrates an amplification section 68. In that all of the 
instrumentation amplifiers which form part of system 10 are arranged in a 
directly analogous manner to amplifier 70a, only instrumentation amplifier 
70a and its associated circuitry including channels A and B for producing 
V.sub.AMPa will be described in detail. De-multiplexed signals LT1 and DK1 
of channels A and B are first applied to capacitors 60a and 60b, 
respectively, which form the sample and hold circuit in conjunction with 
resistor 58 (see FIG. 3). Next, the signals are filtered by low-pass 
filters 62a and 62b to present signals LT' and DK1' to the inverting and 
non-inverting inputs of amplifier 70a , as previously described. Power 
supply voltages are applied to amplifier 70a with each of V.sub.+ and 
V.sub.- being filtered by decoupling capacitors 100 (e.g., 0.1 .mu.F 
capacitors). 
Continuing to refer to FIG. 4, variable resistor 74 (indicated within a 
dashed line) is connected to amplifier 70a, as previously described, at 
terminals 76 and 78. Control line set 72a from processing/control section 
50 provides signals to drive four single pole, single throw switches 
106a-d within a readily available solid state switch 108. Each switch 
106is closed by providing a drive voltage to respective ones of a 
plurality of drive terminals 109a-d. For example, when voltage is 
connected to drive terminal 109a, switch 106a closes in solid state switch 
108. By selectively closing switches 106, predetermined resistance values 
are connected across terminals 76 and 78. These resistance values are 
formed by four pairs of resistors 112, 114, 116 and 118 in which each 
resistor within a pair is of equal resistance (e.g., 24.9 K.OMEGA. 
resistors, 8.25 K.OMEGA. resistors, 2.87 K.OMEGA. resistors, and 976 
.OMEGA. resistors, respectively). It is also noted that switches 106 
themselves present a series resistance (e.g., 45 ohms). Thus, the load 
seen. by amplifier 70a, appearing across terminals 76 and 78 from each 
pair of resistors, is balanced with respect to any reactance in switch 108 
and is equal to the total resistance of the pair of resistors plus the 
resistance of the switch itself. In the described embodiment, when switch 
106a is closed with switches 106c-d open, the value of the variable 
resistor 74 is approximately (24.9 K.OMEGA..times.2)+45 .OMEGA.! or 
approximately 49.8 K.OMEGA.. It should be appreciated that the 
configuration of variable resistor 74 permits the connection of any 
combination of these resistance values in parallel across terminals 76 and 
78 of the instrumentation amplifier. Therefore, with the four switches 
shown here, sixteen different combinations are available, each of which 
produces a predetermined gain when connected with the amplifier. For a 
typical instruentation amplifier, these gains are calculated using the 
approximate formula: 
EQU Gain=1+49.4 K/R 
where R is the resistance applied to terminals 76 and 78 by a particular 
parallel combination of resistor pairs including the series resistance of 
switches 106 . Table 1 below illustrates the various available gains for 
the described embodiment. 
TABLE 1 
______________________________________ 
SWITCH 106 SETTINGS 
0 = OPEN, 1 = CLOSED 
SWITCH SWITCH SWITCH SWITCH 
GAIN 106a 106b 106c 106d 
______________________________________ 
1 0 0 0 0 
1.991 1 0 0 0 
3.986 0 1 0 0 
4.977 1 1 0 0 
9.539 0 0 1 0 
10.53 1 0 1 0 
12.53 0 1 1 0 
13.52 1 1 1 0 
25.74 0 0 0 1 
26.73 1 0 0 1 
28.72 0 1 0 1 
29.71 1 1 0 1 
34.28 0 0 1 1 
35.27 1 0 1 1 
37.26 0 1 1 1 
38.25 1 1 1 1 
______________________________________ 
Still referring to FIG. 4, offset voltage V.sub.OFFa, is provided to 
terminal 140 of amplifier 70a from an offset control section 120 which 
forms part of processing/ control section 50. The offset control section 
120 is readily configurable to provide a range of values as offset 
voltages V.sub.OFF. 
Referring once again to FIGS. 1 and 3, it is readily apparent that in 
processing of the TDM signal received from probe 12, only one 
de-multiplexing operation is performed by silicon switch 56. As previously 
discussed, each multiplexing and/or de-multiplexing step potentially 
introduces switching noise having significant frequency content. Such 
noise may result in overall degradation of system performance. Thus, one 
advantage of the present invention resides in the fact that switching 
noise is minimized by performing a single de-multiplexing step, as 
compared with prior art systems which perform more than one 
de-multiplexing step and more than one multiplexing step. 
Another advantage resides in the fact that a decrease in the number of 
multiplexing and/or de-multiplexing steps is attended by a reduction in 
the number of parts required in the manufacture of a photoplethysmographic 
measurement system. This reduction is particularly significant in a two 
channel system. Reducing the overall parts count of the system results in 
lower manufacturing costs, improved performance, higher reliability and, 
possibly, a smaller overall instrument outline. Additionally, since 
printed circuit boards may be reduced in size whereby to reduce the length 
of signal paths on the board, noise coupled into the system along the 
signal path from the ambient environment may also be reduced, resulting in 
still frrther performance improvements. 
Still another advantage is realized by improving signal to noise ratio 
values through optimizing gain and offset voltage settings. This advantage 
is implemented through the use of the instrumentation amplifier, as 
described above. Moreover, prior art systems have generally used gain 
stages which amplify more than one or, in fact, all of the channels in a 
system. In these prior art systems, gain settings were basically a 
compromise in view of all of the data present on the channels being 
amplified. Such compromise has frequently resulted in increased signal to 
noise ratios. 
It should be understood that a system for performing ambient light 
subtraction using an instrumentation amplifier in conjunction with a time 
division multiplexed signal and its associated method may be embodied in 
many other specific forms without departing from the spirit or scope of 
the present invention. For example, the present invention is readily 
adaptable for use in any system which utilizes at least one time division 
multiplexed signal wherein it is desirable to simultaneously apply 
different portions of a TDM signal to the inputs of an instrumentation or 
other such amplifier. Therefore, the described embodiment is to be 
considered as illustrative and not restrictive, and the invention is not 
to be limited to the details given herein.