Low power A/D converter

A comparator is arranged to compare a series of analog voltage signal samples on a first capacitor with a voltage on a second capacitor which is linearly increased or decreased to equal the sample value. The comparator's single output freezes the count of the counter at counts which are proportional to the voltage of the respective samples. In this manner, analog to digital conversion can be accomplished using a single line between the analog and digital sides of a circuit, thereby reducing parasitic capacitance.

FIELD OF THE INVENTION

The subject invention relates to electronic circuitry and more particularly to analog-to-digital conversion circuitry particularly applicable to subcutaneous implantable cardioverter defibrillators.

BACKGROUND OF THE INVENTION

Defibrillation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing.

Defibrillation/cardioversion systems include body implantable electrodes that are connected to a hermetically sealed container housing the electronics, battery supply and capacitors. The entire system is referred to as implantable cardioverter/defibrillators (ICDs). The electrodes used in ICDs can be in the form of patches applied directly to epicardial tissue, or, more commonly, are on the distal regions of small cylindrical insulated catheters that typically enter the subclavian venous system, pass through the superior vena cava and, into one or more endocardial areas of the heart. Such electrode systems are called intravascular or transvenous electrodes. U.S. Pat. Nos. 4,603,705, 4,693,253, 4,944,300, 5,105,810, the disclosures of which are all incorporated herein by reference, disclose intravascular or transvenous electrodes, employed either alone, in combination with other intravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, the disclosures of which are incorporated herein by reference. A sensing epicardial electrode configuration is disclosed in U.S. Pat No. 5,476,503, the disclosure of which is incorporated herein by reference.

In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed. For example, U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which are incorporated herein by reference, teach the use of a pulse monitor/generator surgically implanted into the abdomen and subcutaneous electrodes implanted in the thorax. This system is far more complicated to use than current ICD systems using transvenous lead systems together with an active can electrode and therefore it has no practical use. It has in fact never been used because of the surgical difficulty of applying such a device (3 incisions), the impractical abdominal location of the generator and the electrically poor sensing and defibrillation aspects of such a system.

Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs which are small enough to be implanted in the pectoral region. In addition, advances in circuit design have enabled the housing of the ICD to form a subcutaneous electrode. Some examples of ICDs in which the housing of the ICD serves as an optional additional electrode are described in U.S. Pat. Nos. 5,133,353, 5,261,400, 5,620,477, and 5,658,321 the disclosures of which are incorporated herein by reference.

ICDs are now an established therapy for the management of life threatening cardiac rhythm disorders, primarily ventricular fibrillation (V-Fib). ICDs are very effective at treating V-Fib, but are therapies that still require significant surgery.

As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, especially children at risk of cardiac arrest, the requirement of ICD therapy to use intravenous catheters and transvenous leads is an impediment to very long term management as most individuals will begin to develop complications related to lead system malfunction sometime in the 5-10 year time frame, often earlier. In addition, chronic transvenous lead systems, their reimplantation and removals, can damage major cardiovascular venous systems and the tricuspid valve, as well as result in life threatening perforations of the great vessels and heart. Consequently, use of transvenous lead systems, despite their many advantages, are not without their chronic patient management limitations in those with life expectancies of >5 years. The problem of lead complications is even greater in children where body growth can substantially alter transvenous lead function and lead to additional cardiovascular problems and revisions. Moreover, transvenous ICD systems also increase cost and require specialized interventional rooms and equipment as well as special skill for insertion. These systems are typically implanted by cardiac electrophysiologists who have had a great deal of extra training.

In addition to the background related to ICD therapy, the present invention requires a brief understanding of a related therapy, the automatic external defibrillator (AED). AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defibrillation under the direction of a bystander user who treats the patient suffering from V-Fib with a portable device containing the necessary electronics and power supply that allows defibrillation. AEDs can be nearly as effective as an ICD for defibrillation if applied to the victim of ventricular fibrillation promptly, i.e., within 2 to 3 minutes of the onset of the ventricular fibrillation.

AED therapy has great appeal as a tool for diminishing the risk of death in public venues such as in air flight. However, an AED must be used by another individual, not the person suffering from the potential fatal rhythm. It is more of a public health tool than a patient-specific tool like an ICD. Because >75% of cardiac arrests occur in the home, and over half occur in the bedroom, patients at risk of cardiac arrest are often alone or asleep and can not be helped in time with an AED. Moreover, its success depends to a reasonable degree on an acceptable level of skill and calm by the bystander user.

What is needed therefore, especially for children and for prophylactic long term use for those at risk of cardiac arrest, is a combination of the two forms of therapy which would provide prompt and near-certain defibrillation, like an ICD, but without the long-term adverse sequelae of a transvenous lead system while simultaneously using most of the simpler and lower cost technology of an AED. What is also needed is a cardioverter/defibrillator that is of simple design and can be comfortably implanted in a patient for many years.

One factor which has added complexity to ICD design is the necessity to digitize an analog electrocardiogram (ECG) signal. For example, it may be desired to sample an ECG signal at intervals of 2 milliseconds or 4 milliseconds, i.e. at either a 250 Hz. or 500 Hz. sampling frequency.

Typically, an analog to digital converter (A/D) circuit is employed in such applications. In some cases, the environment includes an analog chip optimized for analog functions and a digital chip optimized for digital functions. Data may be transferred from the analog chip to the digital chip via, for example, an 8 bit A/D converter employing various known A/D conversion techniques, for example, successive approximation techniques, resistive ladders, or slope converters. In such an application, there would typically be a bus having 8 parallel lines connecting, for example, a microprocessor to an A/D converter located on an analog chip. A read/write control signal is then used to bring all 8 bits over a digital bus to the microprocessor.

One problem with this approach is that each of the bus lines has a parasitic capacitance associated with them. With respect to an eight bit bus, from 1 to all 8 of the parallel bus lines may toggle up or down on each cycle. Every time a line toggles it is necessary to charge up and discharge the parasitic capacitance associated with that line. The power lost due to this parasitic capacitance may be represented by the expression:[n2+1]·Cp·f·V2(1)
where “n” is the number of lines toggled, Cp is the value of the parasitic capacitance, f is the frequency, V is the voltage and “1” represents the parasitic capacitance associated with a read/write line, e.g., from a microprocessor. Equation (1) further employs the expression N over 2 because, on average, only half the bus signals will change state. If one increases the number of bits of the conversion to increase resolution, additional power will be lost. In some cases, the power loss can be worse because, if 10 bits are transferred to an 8 bit microprocessor, two transfers would be required and possibly another read/write signal line.

SUMMARY OF THE INVENTION

According to the invention, the value of an analog voltage sample derived on an analog side of an interface is used to control a count developed on a digital side of the interface. In this manner, a single control line crossing the analog/digital interface is used to develop a count corresponding to the value of the analog sample. In this manner, only a single signal line is subject to parasitic capacitance, as opposed to, for example, 8 or more parallel bus lines.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

An illustrative embodiment is shown inFIG. 1, wherein the circuit is schematically divided by a line15into an analog side17and a digital side19. The analog side17of the circuit includes a comparator21having an inverting input connected to a first terminal of a first capacitor C1and a non-inverting input connected to the first terminal of a second capacitor C2. The second terminals of the respective capacitors C1, C2are grounded.

The first terminal of the first capacitor C1is arranged to be connected via operation of respective switches23,25to either a first charging current source27or a second discharging current source29. The switches23,25are controlled by respective control signals B, A.

The second capacitor C2is arranged to capture a sample of an analog input voltage Vi (t) which is to be converted to a digital value by the circuit. The sample is provided by momentarily closing a switch31in response to application of a third control signal C.

The three control signals A, B, C are provided by a control circuit33, which receives a clock input CLK/N and an input from the output35of the comparator21which output35supplies a control signal UP/DOWN.

On the digital side19of the circuit, the output35of the comparator21is supplied to an n-bit up-down counter41, which provides a binary count on a number of parallel output lines43to a latch circuit45. The number of parallel lines43may be, for example, eight in number. The latch45is enabled to latch the count of the counter41by a control signal LE supplied on a signal line47by control logic49. This logic49receives input signals including the UP/DOWN control signal on line35, a clock signal CLK, and a count signal. The functionality of the digital side circuitry19can, if desired, be embodied as part of a programmed digital processor100, e.g., a microprocessor.

The clock signal CLK is a system clock signal, which may be generated in conventional fashion. The signal is divided by a divisor N at a divider block51to produce a signal denoted CLK/N. Again, production of such a clock and divided clock signals may be accomplished by conventional techniques well-known in the art.

An illustrative example of operation of the circuit ofFIG. 1will now be provided, assuming that the dynamic range of Vi(t) is zero to one volt, that the capacitor voltages VC1and VC2are initially zero, and that the counter41is an 8 bit counter (0 to 255). Assuming Vi(t) rises to ½ volt and is sampled at that value by application of the control signal C, the voltage on the sampling capacitor C2will be higher than that on the first capacitor C1, which will result in a “true” or “positive” output from the comparator21. The production of a “true” output turns on switch B, causing the current from the current source29to linearly charge the first capacitor C1. The “true” signal on the output35further causes the up/down counter41to begin counting up. When the voltage on the first capacitor C1, reaches the value of the voltage on the second or sample capacitor C2, the output35of the comparator21changes state causing the count of the UP/DOWN counter41to stop at a binary value representative of ½ volt, which is then captured by the latch45. Thus, an eight bit count has been developed by a change of state on only one analog signal line35.

Next, assume that at the next sample time, Vi(t) drops by 5 millivolts. VC1is then smaller than VC2, resulting in a false or negative signal on the output35of the comparator21, which causes the UP/DOWN counter41to begin counting down and further causes supply of a control signal A to the switch25, thereby beginning to linearly reduce the voltage on the first capacitor C1. When this voltage again equals the voltage on the sampling capacitor C2, the signal count on the comparator output35freezes the UP/DOWN counter41, whose output is then latched by the latch45.

With respect to clock frequencies, a 32 KHz clock is a frequency typical of those running on typical digital chips. For an eight bit UP/DOWN counter42, the sample period is then 7.8 milliseconds. The control signal C thus has a frequency of 32.768 KHz/256=128 Hz.

FIG. 2depicts illustrative control logic for implementing block33ofFIG. 1so as to generate the sample signal C and control the operation of the current sources27,29. This logic includes a monostable multivibrator61, three flip-flops63,65,67, two AND gates69,71, an OR gate73, and an inverter75.

A conversion begins on each rising edge of the 128 Hz sample clock shown in FIG.4. The sample signal C is generated on this rising edge by the monostable61. The {overscore (Q)} output72of the monostable61goes low on this rising edge, resetting the flip-flops65and67such that their Q outputs are low and there is no “DONE” signal on the output of the OR gate73. If the UP/DN signal from the comparator21changes state, the Q output of one of the flip-flops65,67will go true, such that the “DONE” output of the OR gate73will go true also.

The UP/DN signal also is supplied to the flip-flop63whose Q and {overscore (Q)} outputs form respective inputs to the two AND gates69,71. Each of these gates69,71receives the output of the inverter75(“NOT DONE”) as its second input. Thus, the output B of the AND gate69will be true if a comparison is underway and the comparator output35is positive, while the output A of the AND gate71will be true if a comparison is underway and the comparator output35is negative. As noted above, when the comparator21changes state, i.e., when the voltage on the capacitor C2equals the sample voltage, the DONE output goes true, thereby disabling the AND gates69,71and, as the case may be, terminating charging or discharging of the capacitor C1.

FIG. 4is a waveform diagram useful in illustrating operation of the circuitry of FIG.2. The waveform shows a 128 Hertz clock signal, with a monostable pulse Q from the monostable vibrator, which is high for a brief time period following the upward change of the clock signal. During this brief high time, the analog signal is sampled and, as explained above, the latches shown inFIG. 2are reset.

FIG. 3depicts illustrative control logic for implementing block49ofFIG. 1so as to generate the latch enable signal LE and control supply of the COUNT signal to the UP/DOWN counter41. This logic includes three flip-flops79,81,83, an inverter85, an OR gate87, an inverter89, and an AND gate91. The flip-flop79generates Q and {overscore (Q)} each cycle of the 128 Hz clock. The flip-flop79thereby resets the active low reset flip-flops81,83on the rising edge of the sample clock pulse, and generates the latch enable signal LE on the falling edge of the sample clock pulse.

The three input AND gate91controls the 32 KHz clock signal COUNT provided to the up-down counter41. The three inputs to the AND gate91are the {overscore (Q)} output of the flip-flop79, the 32 KHz clock signal, and the “NOT DONE” output of the inverter89.

In operation of the logic ofFIG. 3, when no conversion is underway, the DONE signal is “true,” which gates off the clock as a result of the “false” input provided by the inverter89to the AND gate91. When a conversion begins, the UP/DN signal input to the flip-flop81causes the NOT DONE signal to go “true,” thereby permitting the 32 KHz clock signal to pass through the AND gate91, thereby causing the UP/DOWN counter41to begin counting. When the output35of the comparator21changes state, the input of the UP/DN signal to the flip-flop81causes the DONE signal to again go true, freezing the count of the counter41at a value representative of the value of the analog sample of Vi(t) currently held by the sample capacitor C2.

FIG. 5is a waveform diagram useful in illustrating operation of the circuitry of FIG.3. The latch enable signal LE is shown, LE being the Q output of flip-flop79. It can be seen that, since the flip-flop79(FIG. 3) goes high only when both the 128 hertz signal and the 32 KHz signal rise, the latch45(FIG. 1) periodically reads the output of the counter41(FIG.1).

FIG. 6is a schematic block diagram of an embodiment employing a programmed digital processor. Line15dividesFIG. 6into digital and analog sides. Referring back toFIG. 1, the microprocessor100(FIG. 6) is shown embodying the digital side19of the circuit inFIG. 1, receiving an output signal35from the up/down comparator21on the analog side17.

While the present invention has been described above in terms of specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the following claims are intended to cover various modifications and equivalent methods and structures included within the spirit and scope of the invention.