NZ538082A - Ambient light auto-zero circuit for photo-optical physiological monitoring equipment - Google Patents

Abstract

A power reduction circuit adapted to be used to measure one or more physiological properties of a patient at preset intervals by illuminating a portion of the patient with one or more light sources, which produces a reflected or transmitted modified light signal representative of the physiological property being measured; the modified light signal is detected by a photo-sensor which produces a raw output signal, which includes an ambient light component, that is representative of the modified light signal, characterised in that the power reduction circuit includes: (i) an adding unit adapted to produce a processed signal by adding the raw output signal to a second signal; (ii) a first signal buffer adapted to retain the processed signal; (iii) a processing unit which further processes the processed signal; (iv) an isolating switch which is controlled by the processing unit and is connected to the first signal buffer, said isolating switch being adapted to reversibly connect the first signal buffer to a feed back circuit; (v) said feedback circuit is adapted to generate the second signal from the processed signal, and includes a second signal buffer, which is adapted to retain the level of the processed signal inside the feedback circuit, and an inverting amplifier, which amplifies and inverts the processed signal in the second signal buffer generating the second signal; wherein the processed signal generated when the isolating switch is open, during the measurement of the desired physiological property, is effectively free of the ambient light component.

Description

538082 New Zealand Patent App. 538082 Filed: 4 February 2005 Patents Form No. 5 Patents Act 1953 COMPLETE SPECIFICATION AMBIENT LIGHT AUTO-ZERO CIRCUIT FOR PHOTO-OPTICAL PHYSIOLOGICAL MONITORING EQUIPMENT We, SENSCIO LIMITED, Unit 6, 10, Acheron Drive, Christchurch, New Zealand, a New Zealand company hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 (to be followed by 1a) Intellectual Property Office of N.Z. 2 0 APR 2006 RECEIVED TITLE: AMBIENT LIGHT AUTO-ZERO CIRCUIT FOR PHOTO-OPTICAL PHYSIOLOGICAL MONITORING EQUIPMENT Technical Field The present invention relates to photo-optical medical monitoring equipment. As used herein, the term "photo-optical medical monitoring equipment" means equipment designed to receive signals from sensors incorporating light emitting diodes; said sensors being connected to or associated with a subject, to monitor one or more physiological parameters of that subject. The term includes sensors, monitors, and adapters in the related field.

A wide range of physiological monitoring equipment is currently available, for monitoring one or more of a number of different physiological parameters. Typically these devices are used for medical monitoring. Medical monitoring equipment often has the ability to record the analysis of signals from sensors, and include an alarm system to alert medical staff of undesirable or dangerous changes in the patient's condition.

Background Art In the most commonly used types of photo-optical physiological monitoring equipment, a sensor designed to sense the selected physiological parameter is attached to the patient and is connected to a monitor by a cable. In use, the light-source(s) of the sensor shine through or into the target site. Transmitted or reflected light falling on a photo-detector component of the sensor causes the photo-detector to generate an electrical signal corresponding to the physiological parameter being sensed. The electrical signal is transmitted directly or digitised and transmitted to the monitor by various means including, but not limited to, a conducting wire or infrared, radio-frequency, or fibre-optic connections.

Photo-optical sensors require power to operate, and even relatively low-power light sources are often the major element in a sensor power budget.

Therefore, the light source power consumption becomes a limiting factor in continuous operational performance life-time for physiological monitoring devices with finite power supplies such as, but not limited to, batteries or solar cells. This limitation is particularly important in the burgeoning field of portable sensors, sensor adapters, and 1a portable monitors. The light source is therefore typically switched on and off rapidly during operation to minimise the power consumption. It should be noted that the term 'light' as herein used includes the electromagnetic frequencies from far infra-red through visible light to extreme ultraviolet.

During the time the light source is on, the photo-detector measures the light transmitted or reflected by the target site, and the ambient light falling on the sensor. This ambient light falling on the photo-detector increases the electrical signal from the photo-detector which can obscure the electrical signal representing the physiological 10 parameter being measured, making it more difficult to obtain an accurate reading; this ambient light signal can therefore be considered noise. In addition, the ambient light signal increases the overall magnitude of the electrical signal from the photo-detector and can therefore increase the power required to process and transmit the signal to the monitor.

In the past, physiological monitoring devices have had comparatively large power supplies such as mains supply or large batteries. With the recent development of very small portable and wireless monitoring devices and adapters it has become desirable to avoid these energy sources. This has meant that the portable monitoring device 20 often has a short operational life before the battery must be recharged or replaced. This recharging or replacing disturbs the subject and may mean that valuable monitoring data is lost during the changeover. Therefore means for achieving significant power savings need to be implemented to extend the useful operational life. In a wireless device, (including but not limited to infrared or radiofrequency 25 connections), the electrical signal has to be digitised prior to transmission, and the power required to digitise the electrical signal is dependent upon its magnitude. Thus reducing the magnitude of the ambient light signal relative to the physiological signal prior to digitisation conserves overall power consumption extends the useful operational life.

Disclosure of Invention It is therefore an object of the present invention to provide a means to reduce the power consumption of photo-optical physiological monitoring equipment, by using an 35 auto-zero circuit to reduce the relative magnitude of the ambient light component of the photo-detector signal, without significantly reducing the performance specification 2 for the equipment. A further object of the invention is to reduce the relative magnitude of the ambient light component in the photo-detector signal thus increase the dynamic range of the analogue to digital converter by maximising the signal to noise ratio.

The present invention provides a power reduction circuit for photo-optical physiological monitoring equipment adapted to be used to measure one or more physiological properties of a patient at preset intervals by illuminating a portion of the patient with one or more light sources, which produces a reflected or transmitted modified light signal representative of the physiological property being measured; the modified light signal is detected by a photo-sensor which produces a raw output signal, which includes an ambient light component, that is representative of the modified light signal, characterised in that the power reduction circuit includes : i. an adding unit adapted to produce a processed signal by adding the raw output signal to a second signal; ii. a first signal buffer adapted to retain the processed signal; iii. a processing unit which further processes the processed signal; iv. an isolating switch which is controlled by the processing unit and is connected to the first signal buffer, said isolating switch being adapted to reversibly connect the first signal buffer to a feed back circuit; v. said feedback circuit is adapted to generate the second signal from the processed signal, and includes a second signal buffer, which is adapted to retain the level of the processed signal inside the feedback circuit, and an inverting amplifier, which amplifies and inverts the processed signal in the second signal buffer generating the second signal; wherein the processed signal generated when the isolating switch is open, during the measurement of the desired physiological property, is effectively free of the ambient light component.

Preferably the feedback circuit includes a high frequency filter before the second signal buffer. In a further preferred form there is more than one frequency filtering stage included in the feedback circuit.

Preferably the power reduction circuit includes additional signal processing circuitry in 35 the feedback circuit or photo-detector output. 3 Preferably the second signal can be further processed prior to the adding unit Preferably a method of using the power reduction circuit includes the following steps in order: a. the isolating switch is opened by the processing unit, isolating the feedback circuit and locking the second signal level in the second signal buffer; b. the or each light source is turned on; c. the raw output signal is added to the second signal, generating the processed signal; d. the or each light source is turned off; e. the isolating switch is closed.

Brief Description of Drawings Figure 1 is a block diagram of the invention.

Figure 2 is a flowchart showing the steps involved in an active measurement cycle using the invention.

Figure 3 is a block diagram of a second embodiment of the invention which includes additional filters in the feedback circuit.

Figure 4 is a flowchart showing the steps involved in a further embodiment of the invention that carries out a second active measurement cycle using a second light source shortly after a first active measurement cycle with a first light source.

Best Mode for Carrying out the Invention Referring to Figure 1, the invention includes a photo-detector (3), a light source (1), an adder (4), a first signal buffer (5) and a processing unit (8), all connected in series in that order. The light source (1) is adapted to illuminate a portion of a subject (2) for a period of time; the light from the light source (1) interacts with the portion of the subject (2) and is modified. The photo-detector (3) is adapted to respond to certain properties of this modified light and generate an output signal whose properties are dependent upon the ambient light and modified light received. The output signal is transmitted to the adder (4) which combines it with a second signal producing an adder signal. The 4 first signal buffer (5) is adapted to retain the adder signal for a period of time prior to further processing.

The invention further includes an isolating switch (9), which is adapted to be controlled by the processing unit (8), and a feedback circuit (13). The isolating switch (9) is connected at a point between the first signal buffer (5) output and the processing unit (8). The feedback circuit (13) includes a high frequency filter (10), a second signal buffer (11) and an inverting amplifier (12), connected in that order in series wherein: i. the high frequency filter (10) is adapted to remove the high frequency noise from the first signal buffer (5) output signal, generating a filtered signal which is transmitted to the second signal buffer (11); ii. the second signal buffer (11) is adapted to retain the filtered signal for processing by the inverting amplifier (12); iii. the inverting amplifier (12) is adapted to invert and amplify the filtered signal from the second signal buffer (11) generating the second signal which is then transmitted to the adder (4).

The output signal, prior to the light source (1) being turned on, is representative of the ambient light impinging on the photo-detector (3). During this time the isolating switch (9) is closed and the second signal is representative of, and varies with, the ambient light level.

Referring to Figure 2; when a physiological measurement of the subject is to be undertaken, an active measurement cycle (20) is commenced and the isolating switch (9) is opened. When the isolating switch (9) is opened the second signal buffer (11) retains the filtered signal stored at that time, and the second signal is fixed at a level representative of the ambient light level immediately prior to the active measurement cycle (20) being commenced.

The light source (1) is turned on and the photo-detector generates an output signal containing ambient light and modified light components. The output signal from the photo-detector (3) during the active measurement cycle (20) is added to the second signal (which is representative of the ambient light level prior to the measurement cycle), so that the resulting adder signal is effectively free from the ambient light component. The adder signal is then transmitted via the first signal buffer (5) to the processing unit (8) for further processing. Once the further processing, which may involve multiple reading of the first signal buffer (5), has been completed the processing unit (8) turns the light source (1) off and closes the isolating switch (9) completing the active measurement cycle (20).

In a second embodiment, as shown in Figure 3, one or more additional signal filters (14) are placed in the feedback circuit (13), and the output signal is pre-processed prior to the adder (4).

In a further embodiment, as shown in Figure 4, a second active measurement cycle (2) is undertaken shortly after the first using a different frequency light source (1).

Claims (1)

1. A power reduction circuit for photo-optical physiological monitoring equipment adapted to be used to measure one or more physiological properties of a patient at preset intervals by illuminating a portion of the patient with one or more light sources, which produces a reflected or transmitted modified light signal representative of the physiological property being measured; the modified light signal is detected by a photo-sensor which produces a raw output signal, which includes an ambient light component, that is representative of the modified light signal, characterised in that the power reduction circuit includes: i. an adding unit adapted to produce a processed signal by adding the raw output signal to a second signal; ii a first signal buffer adapted to retain the processed signal; iii a processing unit which further processes the processed signal; iv an isolating switch which is controlled by the processing unit and is connected to the first signal buffer, said isolating switch being adapted to reversibly connect the first signal buffer to a feed back circuit; v said feedback circuit is adapted to generate the second signal from the processed signal, and includes a second signal buffer, which is adapted to retain the level of the processed signal inside the feedback circuit, and an inverting amplifier, which amplifies and inverts the processed signal in the second signal buffer generating the second signal; wherein the processed signal generated when the isolating switch is open, during the measurement of the desired physiological property, is effectively free of the ambient light component. The power reduction circuit as claimed in claim 1 wherein the feedback circuit includes a high frequency filter before the second signal buffer. The power reduction circuit as claimed in claim 1 wherein the feedback circuit includes more than one frequency filtering stage in the feedback circuit. The power reduction circuit as claimed in claim 1 wherein the feedback circuit includes additional signal processing circuitry. The power reduction circuit as claimed in claim 1 wherein the feedback circuit includes additional signal processing circuitry for the photo-detector output. The power reduction circuit as claimed in claim 1 wherein the second signal can be further processed prior to the adding unit A method of using the power reduction circuit as claimed in claim 1 which includes the following steps in order: a. the isolating switch is opened by the processing unit, isolating the feedback circuit and locking the second signal level in the second signal buffer; b. the or each light source is turned on; c. the raw output signal is added to the second signal, generating the processed signal; d. the or each light source is turned off; e. the isolating switch is closed. A power reduction circuit for photo-optical physiological monitoring equipment, substantially as hereinbefore described with reference to and as shown in Figures 1 and 2, or Figures 3 and 4 of the accompanying drawings. The method as claimed in claim 7 and substantially as hereinbefore described with reference to and as shown in Figures 1 and 2, or Figures 3 and 4 of the accompanying drawings. Intellectual Property Office of N.Z. 2 0 APR 2006 Received 8