Power supply rail controller

[0001] The present application claims priority benefit under 35 U.S.C. §119(e) from U.S. Provisional Application No. 60/351,751, filed Jan. 25, 2002, entitled “Differential Input, Power Efficient Pulse Oximeter Preamplifer,” which are incorporated herein by reference.

[0002] A detector preamplifier is an optical receiver component utilized in a variety of applications including pulse oximetry. Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for monitoring blood oxygen that has gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care. A pulse oximeter sensor detects the light absorption characteristics of blood perfused tissue, and a corresponding monitor performs a blood spectral analysis based upon the sensor signal. By monitoring for decreases in the arterial oxygen supply, pulse oximeters reduce the risk of accidental death and injury.

[0003] FIG. 1 illustrates an analog portion of a pulse oximetry system 100 having a sensor 110 and a monitor 150. The sensor 110 has emitters 120 and a detector 130. The emitters 120 typically consist of a red LED (light emitting diode) and an infrared LED that project light through blood vessels and capillaries underneath a tissue site, such as a fingernail bed. The detector 130 is typically a photodiode positioned so as to detect light transmitted through or reflected from the tissue site. The signal current generated by the detector 130 is proportional to the intensity of the light emerging from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.

[0004] Also shown in FIG. 1, the monitor 150 has drivers 160, a preamplifier 200, signal conditioning 180 and digitization 190. The drivers 160 alternately activate the emitters 120. The preamplifier 200 provides an amplified detector signal for signal conditioning 180, which typically includes filtering and additional amplification. The digitization 190 performs an analog-to-digital conversion (ADC) of the conditioned detector signal. The resulting digitized signal is then analyzed by a digital signal processor (not shown) to determine oxygen saturation based upon the differential absorption by arterial blood of the two wavelengths projected by the emitters 120. A pulse oximetry signal processor is described in U.S. Pat. No. 6,081,735 entitled “Signal Processing Apparatus,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.

[0005] FIG. 2 illustrates a circuit for a conventional pulse oximeter preamplifer 200. Other names for this circuit are a current-to-voltage converter, photodiode amplifier and transimpedance amplifier. An operational amplifier (op amp) 210 provides the signal gain. The op amp 210 sets the output 216 so that the current flowing through the feedback impedance R₄, C₂ 240 is equal to the current flowing through the input resistors R₁, R₂ 220, R₃ 230 and the photodiode detector 130. The detector generated current creates a negative voltage at the op amp output 216. The preamp DC gain (output voltage/detector current) is approximately R₄, and the high frequency response of the preamp is approximately F_cutoff=1/(2πR₄C₂).

[0006] Preamp power supply rails need to provide enough headroom to allow a signal to be measured under high ambient light conditions. That is, the magnitude of the power supply voltage needs to be large enough to avoid signal saturation when the signal DC component is large. A larger supply voltage, however, results in higher preamp power consumption. A power supply rail controller advantageously reduces preamplifier power consumption by operating the preamp from a lower voltage power supply under low ambient light conditions and switching to a higher voltage power supply when necessary during high ambient light conditions.

[0007] One aspect of a power supply rail controller is a preamplifier having a signal input, a power input and a signal output. The signal input is adapted to receive a detector response to optical radiation. A first power supply generates a first voltage and a second power supply generates a second voltage, where the magnitude of the second voltage is greater than the magnitude of the first voltage. A voltage selector provides an enable output responsive to the signal output, and the power input is adapted to receive power from either the first power supply or the second power supply as determined by the enable output.

[0008] Another aspect of a power supply rail controller is a method comprising the steps of preamplifying a detector signal input utilizing a power input so as to generate a signal output, generating a low voltage for the power input so as to lower power consumption and generating a high voltage for the power input so as to reduce distortion on the signal output. Further steps include setting a threshold voltage, monitoring the signal output in relation to the threshold voltage, and selecting one of the low voltage and the high voltage for the power input in response to the monitoring step.

[0009] A further aspect of a power supply rail controller comprises a preamplifier means for inputting a detector signal and providing gain to generate an output signal. A first supply means generates a low supply voltage to the amplifier means. A second supply means generates a high supply voltage to the amplifier means. A voltage selector means enables one of said first and second supply means in response to the output signal.

[0010] Yet another aspect of a power supply rail controller operates on an analog component having a signal input, a power input and a signal output. A voltage controller provides a control output responsive to the signal output. A power supply generates a voltage for the power input, where the voltage is responsive to the control output. The voltage is reduced in magnitude to reduce power dissipation and increased in magnitude to avoid signal distortion.

[0017] FIG. 3 generally illustrates a power supply rail controller 300 having an preamplifier (preamp) 310, a low voltage power supply 320, a high voltage power supply 330 and a voltage selector 340. The preamp 310 has a signal input 312, a signal output 314 and a power input 318. The controller 300 advantageously lowers the preamp 310 power consumption by operating from the low voltage power supply 320 when possible and the high voltage power supply 330 when necessary to avoid saturation of the signal output 314 as the signal amplitude approaches the power supply rail. This situation can potentially occur during high ambient light conditions, as described above. The voltage selector 340 monitors the signal output 314 and generates an enable output 342 accordingly. Specifically, the enable output 342 nominally selects the low voltage power supply 320 for low preamp power dissipation and switches to the high voltage power supply 330, raising the preamp power supply rail to prevent signal distortion. The voltage selector 340 may utilize hysteresis, time delay, filtering or other mechanisms to reduce the sensitivity of the enable output 342 to the signal output 314. The enable output 342 may select the low voltage power supply 320 or the high voltage power supply 330 for the power input 318 by selectively connecting and disconnecting the supply outputs 322, 332 to the power input 318; selectively enabling or disabling one or both power supplies 320, 330; or selectively connecting or disconnecting one or both power supplies 320, 330 from a power source.

[0018] FIG. 4 illustrates one embodiment of a power supply rail controller 300 having a preamp 310 with a signal input 312, a signal output 314, a low voltage power supply 320, a high voltage power supply 330 and a voltage selector 340, as described with respect to FIG. 3, above. The signal input 312 is a differential input connecting to a photodiode detector 401. The preamp 310 has a negative power input 318 and a positive power input 418. The negative power input 318 is selectively powered by one of the power supplies 320, 330, as described with respect to FIG. 3, above. In particular, the low voltage power supply 320 provides a negative voltage output V-lo 322, and the high voltage power supply 330 provides a negative voltage output V-hi 332. The high voltage power supply 330 is selectively enabled by the enable output 342. The low voltage power supply 320 is always enabled. A supply isolation element 470 connects the V-lo and V-hi outputs 322, 332 to the negative power input 318 and allows the high voltage power supply 332, when enabled, to pull the negative power input 318 to a more negative voltage. The preamp 310, signal conditioning 460 and ADC 444 provide a sensor front-end for the DSP 450, such as described with respect to FIG. 1, above.

[0019] As shown in FIG. 4, the voltage selector 340 has a comparator 430, ADC 448 and a digital signal processor (DSP) 450. The DSP 450 advantageously processes a voltage selection algorithm in addition to signal processing of the detector 401 output, such as pulse oximetry spectral analysis as described with respect to FIG. 1, above. The comparator 430 compares a threshold value 403 to the amplifier output 314. The threshold 430 sets a trigger point for switching the preamp power input 318 from V-lo to V-hi. In particular, when the preamp output 314 exceeds the threshold 403, the comparator output 432 is asserted, generating an interrupt INT input to the DSP 450. The threshold 403 is set so that the DSP 450 will have time to respond to this potential signal saturation condition. Responding to the interrupt, the DSP 450 changes the state of the enable output so as to enable the high voltage power supply 330. The DSP 450 then monitors the preamp output 314 via the ADC 448. A DSP algorithm determines when to disable the high voltage power supply 330 with the power control output 342 so as to reduce power consumption. In particular, the DSP 450 determines if the amplifier output 314 is below a predetermined magnitude so as to insure that the comparator 430 is unlikely to trigger an interrupt shortly after the preamp power input 318 reverts back to the V-lo output 322.

[0020] In a particular embodiment, the ADCs 440 incorporate a dual channel device, such as a PCM3002 dual 20 bit CODEC available from Texas Instruments, which allows continuous digitization of the conditioned detector signal 462 by a first channel ADC 444 and intermittent monitoring of the amplifier output 314 as one of multiple diagnostic inputs to a second channel ADC 448. The enable output 342 is set and reset as one flag bit in a shift register providing output signals. The comparator threshold 403 is set to a predetermined value so that the DSP interrupt 432 is triggered when the preamp output 314 has exceeded about 60% of the allowable voltage range at the V-lo power supply voltage 322. Before reverting back to the low voltage power supply 322, the DSP 450 determines that the preamp output 314 will be no greater than 30% of the allowable voltage range at the V-lo power supply voltage 322. In one embodiment, the 30% threshold is monitored for a period of one minute before reverting back to the low voltage.

[0021] FIG. 5 illustrates an preamp 310 and comparator 430 circuit embodiment for a power supply rail controller 300 (FIG. 4), as described with respect to FIG. 4, above. The preamp 310 is configured around an operational amplifier (op amp) 510 such as an AD820, available from Analog Device. The comparator 430 is configured around a comparator 560, such as a LMC7211, available from National Semiconductor. The op amp negative power input 318 is operated from V-lo output 322 having a nominal value of about −3VDC and a V-hi output 332 having a nominal value of about −25VDC. The V-lo output 322 is provided by a Maxim 1721 charge pump voltage supply. The V-hi output 332 is provided by a Linear Technologies LT1611 inverting switching regular. Isolation between the two outputs is achieved by a diode pair D1 570, such as an industry standard BAT54A. Because the op amp output is always negative when there is detector current, the positive supply voltage may be a low voltage to reduce power. The op amp 510 and comparator 560 positive supply voltage V+ 418 is nominally about +3VDC. The comparator 560 negative supply voltage V− is nominally about −3VDC.

[0022] As shown in FIG. 5, the threshold 403 is determined by the voltage divider R₈, R₉ 550. In a particular embodiment, R₈=100K and R₉=2.74K. Resistors R₆ and R₇ scale the preamp output 314 to fit within the range of the ADC 444 (FIG. 4). In a particular embodiment, R₆=100KΩ and R₃=5.23KΩ. Capacitors C₃, C₇, and C₈ are power supply bypass capacitors, such as C=1.0 μF. Resistors R₅ and capacitors C₄ and C₅ filter the preamp negative supply. In a particular embodiment, R₅=20Ω, C₄=3.3 μF and C₅=0.1 μF. The comparator hysterises is determined by R₁₀ and C₆, such as, in a particular embodiment, R=1 MΩ and C=22 pF. Resistors R₁ and R₂, in conjunction with a choke (not shown) filter EMI and ESD from the preamp input and suppress emissions from the circuit board containing these components. In a particular embodiment, R₁=R₂=1KΩ.

[0023] Also shown in FIG. 5, the preamp circuit 310 is advantageously configured with differential inputs. In particular, the noninverting load resistance R₃ 530 matches the feedback resistance R₄ 520, and C₁=C₂, providing improved common mode noise rejection as compared with a prior art single-ended preamp such as described with respect to FIG. 2, above. The preamp DC gain (output voltage/detector current) is R₃+R₄, and the high frequency response is F_cutoff=1/[2π(R₄C₂)]. In a particular embodiment, R₃=R₄=162KΩ and C₁=C₂=100 pF. The resistor and capacitor values can be adjusted for different gain or bandwidth requirements, as long as the R and C values match each other. The differential input preamp achieves better common mode rejection (CMR) as compared with a single-ended preamp (FIG. 2). That is, there is higher attenuation of common-mode noise. In pulse oximetry applications, a source of common-mode noise include triboelectric noise introduced on the photodiode wires of the patient cable interconnecting the sensor and monitor. Another source of common-mode noise is 50 and 60 Hz power line noise. Another advantage of the differential input preamp includes a lower DC offset.

[0024] FIG. 6 illustrates a generalized power supply rail controller 600. A power supply rail controller has been described above with respect to a preamp having a signal output with a potentially large DC component. Further, only one power supply rail was varied, and it varied between two discrete voltages. A power supply rail controller 600, however, may operate to reduce power dissipation in other active analog components 610, such as amplifiers, filters and buffers to name a few, and over other operating conditions. For example, a power supply rail controller 600 may monitor a component signal output 614 having a positive DC component, a negative DC component or no DC component. A voltage controller 620 may be a microprocessor, a DSP, or discrete analog or digital components or a combination of these. The voltage controller 620 provides a positive rail control 622 or a negative rail control 624 or both, which are responsive to the signal output 614. That is, the power supply rail controller 600 may operate on both the positive power supply rail 616 and the negative power supply rail 618, either in tandem or separately. In this manner, the voltage controller 620 increases a positive power supply voltage 630, a negative power supply voltage 640 or both to increase the rail-to-rail maximum signal when the signal magnitude is large and reduces one or both power supply voltages 630, 640 when the signal magnitude is small. In this context, signal magnitude is a combination of DC offset and signal strength. In addition, the rail voltage may be varied discretely using more than two voltages from one or more power supplies, or may be varied continuously, again from one or more power supplies.

[0025] A power supply rail controller has been disclosed in detail in connection with various embodiments. These embodiments are disclosed by way of examples only and are not to limit the scope of the claims that follow. One of ordinary skill in the art will appreciate many variations and modifications.