High speed reflected impedance telemetry system for implantable medical device

• US 5,264,843 A
• Filed: 03/05/1991
• Issued: 11/23/1993
• Est. Priority Date: 04/05/1989
• Status: Expired due to Term

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Original Assignee: Siemens Pacesetter, Inc. (Abbott Laboratories Incorporated)
• Inventors: Silvian, Sergiu
• Primary Examiner(s): Yusko, Donald J.
• Assistant Examiner(s): Horabik, Michael
• Application Number:
  US07/665,627
• Time in Patent Office: 994 Days
• Field of Search: 340/870.18, 340/870.19, 340/870.26, 340/870.39, 340/870.42, 340/870.28, 340/870.31, 128/419 PT, 128/696, 128/903, 335/1 R, 335/15.02
• US Class Current: 340/870.18
• CPC Class Codes: A61N 1/37223 Circuits for electromagnetic coupling
• CPC Class Codes: A61N 1/3727 characterised by the modulation technique
• CPC Class Codes: G08C 17/04 using magnetically coupled devices
• CPC Class Codes: Y10S 128/903 Radio telemetry

Abstract

A telemetry system which will transmit data at a relatively high rate while retaining a high degree of accuracy is disclosed which utilizes a servo feedback loop in conjunction with a conventional reflected impedance receiving front end parallel LC circuit, an AM demodulator, and an oscillator to drive the LC circuit at a desired frequency. The feedback loop operates to keep the voltage across the LC circuit constant over time, and does not affect short term variations in the voltage across the LC circuit which are caused by the variations in the reflected impedance. While frequency response of the system without the loop declines from a maximum value at zero frequency, the frequency response of the system of the present invention with the loop is shifted which permits data transmission at a substantially higher rate.

Claims

1. A high speed reflected impedance telemetry system for use with an implantable medical device comprising:

an implantable medical device that includes means for performing a specified medical function;

an external transceiver; and

bidirectional data telemetry means for transmitting data between said implantable medical device and said external transceiver, said bidirectional data telemetry means including;

generating means within one of said implantable medical device or said external transceiver for generating an information-containing magnetic field;

receiving means within the other of said implantable medical device or said external transceiver for receiving and demodulating said information-containing magnetic field, and for establishing a wide bandwidth between said implantable medical device and said external transceiver, said receiving means comprising;

a first inductor for receiving said information-containing magnetic field,

a first capacitor connected in parallel across said first inductor,

an oscillator connected to drive said first inductor and said first capacitor with an oscillating output signal,

demodulator means connected across said first inductor and said first capacitor, said demodulator means having an output, and

feedback loop means connected from the output of said demodulator means to said oscillator for controlling the amplitude of said oscillating output signal so as to widen the bandwidth through which information may be transferred between said implantable medical device and said external transceiver,

whereby a high rate of change of said information-containing magnetic field may occur within said wide bandwidth, whereby a high data rate transfer of information may occurs between said implantable medical device and said external transceiver.

2. The system as set forth in claim 1, wherein said generating means includes:

a second inductor;

a second capacitor connected in parallel across said second inductor;

switch means for selectively connecting a low impedance or a high impedance across said second inductor and said second capacitor, said low impedance comprising a closed state of said switch means, and said high impedance comprising an open state of said switch means; and

encoding means for opening and closing said switch means in accordance with desired serial data to be transferred between said implantable medical device and said external transceiver device.

3. The system as set forth in claim 2, wherein said second inductor is magnetically coupled to said first inductor when information is transferred between said implantable medical device and said external transceiver device.

4. The system as set forth in claim 3, wherein said first inductor and said first capacitor have values to tune them to a frequency f₀, and said oscillator is set to oscillate at a frequency which is approximately f₀, and further wherein said second inductor and said second capacitor also have values to tune them to frequency f₀.

5. The system as set forth in claim 4, wherein said feedback loop means comprises:

an operational amplifier having a positive input, a negative input, and an output, said positive input of said operational amplifier being supplied with a DC reference voltage;

a first resistor connected between the output of said demodulator means and said negative input of said operational amplifier;

a third capacitor having a first terminal and a second terminal, said first terminal of said third capacitor being connected to said negative input of said operational amplifier; and

a second resistor having a first terminal and a second terminal, said first terminal of said second resistor being connected to said second terminal of said third capacitor, said second terminal of said second resistor being connected to the output of said operational amplifier, the output of said operational amplifier also being connected to drive said oscillator.

6. The system as set forth in claim 4, wherein said demodulator means comprises an AM demodulator.

7. The system as set forth in claim 6, further including:

an FM demodulator connected across said first inductor and said first capacitor, said FM demodulator having an output;

an amplifier and bandpass filter having as an input the output from said FM demodulator, said amplifier and bandpass filter providing an output; and

a decoder having as an input the output from said amplifier and bandpass filter.

8. A bidirectional telemetry system for transferring data between an implantable medical device and an external transceiver device, comprising:

transmitting means within one of said implantable medical device or said external transceiver device for generating an information-containing magnetic field in accordance with said data being transferred; and

receiving means within the other of said implantable medical device or said external transceiver device for receiving and demodulating said information-containing magnetic field in order to obtain the data being transferred;

said transmitting means including;

a first inductor for transmitting said information-containing magnetic field,

a first capacitor connected in parallel across said first inductor,

a switch connected in parallel across said first inductor and said first capacitor, and

means responsive to said data being transferred for periodically, sequentially opening and closing said switch to modulate a voltage across said first inductor and said first capacitor;

said receiving means including;

a second inductor for receiving said information-containing magnetic field,

a second capacitor connected in parallel across said second inductor,

an oscillator connected to drive said second inductor and said second capacitor,

said second inductor, second capacitor and oscillator causing said telemetry system to assume a first bandwidth,

an AM demodulator connected across said second inductor and said second capacitor, said AM demodulator having an output containing an AC component and a DC component, and

feedback loop means connected from the output of said AM demodulator to said oscillator, said feedback loop means for maintaining the DC component of the output of said AM demodulator constant and for further causing said telemetry system to assume a second bandwidth, said second bandwidth being wider than said first bandwidth;

whereby a higher rate of change of said information-containing magnetic field may occur through said second bandwidth than could occur through said first bandwidth, and hence a higher data rate transfer may be achieved between said generating means and said receiving means.

9. The bidirectional telemetry system, as set forth in claim 8, wherein said means for periodically, sequentially opening and closing said switch in said transmitting means comprises a digital encoder that closes and opens said switch in accordance with a desired data modulation pattern of the data being transferred.

10. The bidirectional telemetry system, as set forth in claim 9, wherein said desired data modulation pattern comprises a non-return-to-zero (NRZ) data format wherein an NRZ data bit of "0" is represented as two excursions from a first voltage signal level to a second voltage signal level, and an NRZ data bit of "1" is represented to be a single excursion from the first voltage signal level to the second voltage signal level.

11. In an implantable medical device coupled to an external transceiver, an improved reflected impedance telemetry system for transmitting data at a high rate between the implantable medical device and the external transceiver;

a first of said implantable medical device or said external transceiver including means for generating an information-containing magnetic field, and a second of said implantable medical device or said external transceiver including means for sensing said information-containing magnetic field and generating a demodulated signal representative of the information contained therewithin;

said means for sensing said information-containing magnetic field including a first inductor adapted to be magnetically coupled to said information-containing magnetic field, a first capacitor connected in parallel across said first inductor, an oscillator connected to drive said first inductor and said first capacitor at a frequency f₀, and an AM demodulator connected across said first inductor and said first capacitor, said AM demodulator generating said demodulated signal, said demodulated signal containing an AC component and a DC component;

said first inductor, first capacitor and oscillator providing a first bandwidth that permits the information contained within said information-containing magnetic field to be detected up to a first data transfer rate;

the improvement comprising;

a feedback loop circuit that connects the demodulated signal of said AM demodulator to said oscillator, said feedback loop circuit causing the DC component of the demodulated signal of said AM demodulator to be maintained substantially constant, and said feedback loop means also providing a second bandwidth wider than said first bandwidth that permits the information contained within said information-containing magnetic field to be detected up to a second data transfer rate, said second data transfer rate being greater than said first data transfer rate;

whereby information contained within said information-containing magnetic field may be transferred between said implantable medical device and said external transceiver at a higher data transfer rate than is possible without said feedback loop circuit.

12. The improved reflected impedance telemetry system for use between an implantable medical device and an external transceiver, as set forth in claim 11, wherein said feedback loop circuit comprises:

an operational amplifier having a positive input, a negative input, and an output, said positive input of said operational amplifier being supplied with a DC reference voltage;

a first resistor connected between said AM demodulator and said negative input of said operational amplifier, whereby said demodulated signal is applied to said negative input of said operational amplifier through said first resistor;

a second capacitor having a first terminal and a second terminal, said first terminal of said second capacitor being connected to said negative input of said operational amplifier; and

a second resistor having a first terminal and a second terminal, said first terminal of said second resistor being connected to said second terminal of said second capacitor, said second terminal of said second resistor being connected to the output of said operational amplifier, the output of said operational amplifier also being connected to drive said oscillator.

13. The improved reflected impedance telemetry system for use between an implantable medical device and an external transceiver, as set forth in claim 11, wherein said means for generating an information-containing magnetic field comprises:

a second inductor for transmitting said information-containing magnetic field;

a third capacitor connected in parallel across said second inductor;

a switch connected in parallel across said second inductor and said third capacitor; and

means responsive to the information to be transferred in said information-containing magnetic field for periodically, sequentially opening and closing said switch to modulate a voltage across said second inductor and said third capacitor.