Autonulling AC bridge using differential and integration feedback
First Claim
1. An autonulling bridge circuit system for measuring impedance changes, comprising:
- an AC bridge circuit including two current dividing branches with a first branch including a test impedance to be measured and a second branch including a feedback controlled impedance means wherein each of sad first and second branches receives an alternating current first reference voltage, said bridge circuit further including a first and a second output voltage means each connected to a respective one of said first and second branches;
signal buffer means responsive to said first reference voltage and the outputs of said first and second voltage output means to provide at least one phase relate output signal said signal buffer means including means for amplifying the difference between said first and second output voltage and means for providing said at least one phase related output signal as DC output signal; and
feedback control means including an integrator means for receiving said at least one phase related DC output signal and outputting a respective at least one feedback output signal for controlling said feedback controlled impedance, said feedback control means also outputting a display signal indicating the impedance of said test impedance, wherein said signal buffer means includes an in-phase mixer for receiving both said first reference voltage and the output of said amplifier means and a quadrature-phase mixer for receiving both 90°
shifted first reference voltage and the output of said amplifier means and wherein said in-phase mixture and said quadrature-phase mixture respectively provide a first and second mixture output fed to a respective first and second low pass filter and wherein the outputs of said respective first and second low pass filters provide two phase related DC output signals to said feedback control means.
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Abstract
A self-contained autonulling bridge circuit based on phase-sensitive deteon of an impedance to be measured is described. The system utilizes a feedback structure to control a variable impedance in order to establish the null of the bridge. The system utilizes a voltage-variable impedance having both resistance and capacitance and provides the phase sensitive relationship in the control means in order to provide highly accurate measurement of an unknown test impedance. The ability to measure small changes in impedance provides particularly useful application to the area of biological sensor impedance measurement or electrochemical impedance measurement using silicon-based devices.
28 Citations
13 Claims
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1. An autonulling bridge circuit system for measuring impedance changes, comprising:
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an AC bridge circuit including two current dividing branches with a first branch including a test impedance to be measured and a second branch including a feedback controlled impedance means wherein each of sad first and second branches receives an alternating current first reference voltage, said bridge circuit further including a first and a second output voltage means each connected to a respective one of said first and second branches; signal buffer means responsive to said first reference voltage and the outputs of said first and second voltage output means to provide at least one phase relate output signal said signal buffer means including means for amplifying the difference between said first and second output voltage and means for providing said at least one phase related output signal as DC output signal; and feedback control means including an integrator means for receiving said at least one phase related DC output signal and outputting a respective at least one feedback output signal for controlling said feedback controlled impedance, said feedback control means also outputting a display signal indicating the impedance of said test impedance, wherein said signal buffer means includes an in-phase mixer for receiving both said first reference voltage and the output of said amplifier means and a quadrature-phase mixer for receiving both 90°
shifted first reference voltage and the output of said amplifier means and wherein said in-phase mixture and said quadrature-phase mixture respectively provide a first and second mixture output fed to a respective first and second low pass filter and wherein the outputs of said respective first and second low pass filters provide two phase related DC output signals to said feedback control means. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A sensor impedance detector, comprising:
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a bridge network including two branches with a first branch including a sensor impedance to be measured and the second branch including feedback controlled impedance means wherein each of said first and second branches receive an alternating current reference voltage and said bridge circuit further including a first and second output means each connected to a respective one of said first and second branches; feedback control means responsive to the output of said first and second output means and responsive to said first reference voltage to provide a phase-sensitive control signal output to said feedback controlled impedance means and said feedback control means further including a means for providing a second output indicating the measured value of said sensor impedance, wherein said feedback control means includes a difference amplifier means responsive to said first and second voltage output to provide a difference signal, said feedback control means further including a mixing means responsive to said reference signal and said difference signal to provide at lease one dc phase sensitive detection signal to said feedback controlled impedance means. - View Dependent Claims (10, 11, 12, 13)
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Specification