Magnetic field sensor for implantable medical device

Claim 2

An implantable medical device as claimed in claim 1, wherein said first and second split-drain FETs have two split-drain halves or three split-drain thirds.

Claim 3

An implantable medical device as claimed in claim 1 or 2 wherein said differential amplifier comprises a plurality of FET devices.

Claim 4

An implantable medical device as claimed in any preceding claim, wherein said first split-drain FET is a P-channel split-drain FET, and wherein said second split-drain FET is an N-channel FET.

Claim 5

An implantable medical device as claimed in any preceding claim, wherein said bias current is in a range from ten to one-hundred nano-amps.

Claim 6

An implantable medical device as claimed in any preceding claim, wherein said first and second planar channel regions have width-to-length ratios of 1:1.

Claim 7

An implantable medical device as claimed in any preceding claim, further comprising: a multiplexed active load circuit receiving a plurality of digital input signals and having a plurality of output lines, said multiplexed active load circuit comprising a plurality of said first split-drain FETs, said digital input signals for selecting one of said plurality of split-drain FETs to be coupled to said plurality of output lines, each of said plurality of split-drain FETs having a source, a gate defining a planar channel region, and first and second split-drain halves, said planar channel region of each of said plurality of split-drain FETs having different dimensions from others of said plurality of split-drain FETs; and wherein said sources of said plurality of split-drain FETs comprising said multiplexed active load are coupled to the power supply, said second split-drain FET comprising a differential pair and said source of said second split-drain FET being coupled to the current source, such that a bias current is conducted through said first and second split-drain halves of said selected split-drain FET and said third and fourth split-drain halves; and wherein a first portion of said bias current is conducted through said first split-drain half of said selected split-drain FET and said fourth split-drain half, and a second portion of said bias current is conducted through said second split-drain half of said selected split-drain FET and said third split-drain half; such that a magnetic field perpendicular to said planar channel region of said selected split-drain FET and said second planar channel region increases said first portion of said bias current and decreases said second portion of bias current; said differential amplifier output voltage changing in response to said increase in said first portion of bias current and said decrease in said second portion of said bias current, the extent of said change in differential amplifier output voltage varying depending upon which of said plurality of split-drain FETs is selected.

Claim 8

An implantable medical device, responsive to application of an external magnetic field to alter an aspect of its operation, comprising an integrated circuit implemented on a silicon substrate, said integrated circuit comprising: a split-drain FET having a source, a gate defining a first planar channel region, and first and second split-drain halves, said split-drain FET being coupled to a bias circuit including a power supply such that a bias current flows through said first and second split-drain halves; and a plurality of FET devices comprising a differential amplifier coupled between said first and second split-drain halves, said differential amplifier producing an output voltage; wherein an external magnetic field applied to said split-drain FET increases said bias current in said first split-drain half and decreases the bias current in said second split-drain half; and wherein said differential amplifier output voltage varies in proportion to the increase in bias current in said first split-drain half and said decrease in bias current in said second split-drain half; such that said application of said external magnetic field is manifested by said variation in said differential amplifier output voltage.

Claim 9

An implantable medical device as claimed in claim 8 wherein said bias current is in a range from ten to one-hundred nano-amps.

Claim 10

An implantable medical device as claimed in claim 8 or 9 wherein said planar channel region has a width-to-length ratio of 1:1.

Claim 11

An implantable medical device, responsive to application of an external magnetic field to alter an aspect of its operation, comprising an integrated circuit implemented on a silicon substrate, said integrated circuit comprising: a tri-drain FET having a source, a gate defining a first planar channel region, and a split drain having first, second, and third tri-drain thirds, said tri-drain FET being coupled to a bias circuit including a power supply such that a bias current is established in said tri-drain FET, a portion of said bias current being conducted through each of said first, second, and third tri-drain thirds; a plurality of FET devices comprising a differential amplifier coupled between said first and third split-drain thirds, said differential amplifier producing an output voltage; wherein an external magnetic field applied to said split-drain FET increases said bias current in said first tri-drain third and decreases the bias current in said third tri-drain third; and wherein said differential amplifier output voltage varies in proportion to the increase in bias current in said first tri-drain third and said decrease in bias current in said third tri-drain third; such that the presence of said external magnetic field is manifested by said variation in said differential amplifier output voltage.

Claim 12

An implantable medical device in accordance with claim 11, further comprising: an active load comprising a second tri-drain FET having fourth, fifth, and sixth tri-drain thirds, said fourth tri-drain third being coupled to said first tri-drain third, said fifth tri-drain third being coupled to said second tri-drain third, and said sixth tri-drain third being coupled to said third tri-drain third; said second tri-drain FET being coupled to said bias circuit such that said portion of said bias current conducted through said first tri-drain third is also conducted through said fourth tri-drain third, said portion of said bias current conducted through said second tri-drain third is also conducted through said fifth tri-drain third, and said portion of said bias current conducted through said third tri-drain third is also conducted through said sixth tri-drain third; such that said external magnetic field increases said portion of said bias current conducted through said first and fourth tri-drain thirds, and decreases said portion of said current conducted through said third and sixth tri-drain thirds; and such that said differential amplifier output voltage varies in proportion to said increase in bias current in said first and fourth tri-drain thirds and said decrease in bias current in said third and sixth tri-drain thirds.

Claim 13

An implantable medical device responsive to an externally applied magnetic field to change an aspect of its operation, comprising an integrated circuit implemented on a silicon substrate, said integrated circuit comprising: a first plurality of cascoded split-drain FETs coupled to a regulated current drain, and a second plurality of cascoded split-drain FETs coupled to said first plurality of split-drain FETs and said regulated current drain, said first plurality of cascoded split-drain FETs producing a first output current and said second plurality of cascoded split-drain FETs producing a second output current; a positive-feedback oscillator producing an oscillating output having a duty cycle which varies in proportion to said first and second output currents; wherein an external magnetic field applied to said integrated circuit increases said first output current and decreases said second output current; such that said application of said external magnetic field alters said duty cycle of said oscillating signal.

Claim 14

An implantable medical device as claimed in claim 13, further comprising a low-pass filter for receiving said oscillating signal, said low-pass filter producing an output voltage proportional to the average voltage of said oscillating signal, such that a change in said duty cycle of said oscillating signal produces a change in said output voltage.

Claim 15

An implantable medical device including a circuit responsive to application of an external magnetic field comprising a first P-channel FET having first and second split-drain parts and a second N-channel FET having third and fourth split-drain parts, said first and fourth drain parts being connected and said FETs being arranged and biassed such that an applied magnetic field perpendicular in one direction to the integrated circuit causes an increase in the current in said first drain and a decrease in the current in said fourth drain, accompanied by a corresponding respective decrease and increase in the current in said second and third drain parts, said circuit further comprising a means for monitoring the current change in some or all of the drain parts to determine the application of a magnetic field.

Claim 16

An implantable medical device as claimed in claim 15 wherein said second and third drain parts are connected and said monitoring means monitors the voltage difference between the connected drain parts.

Claim 17

An implantable medical device as claimed in claim 16 wherein said monitoring means comprises an oscillator means connected to said second and third drain parts which provides a duty cycle which varies in response to the current in said second and third drain parts.