A method for determining a sheet resistance of a sample (1) by using point probes. The method includes: (a) positioning (101) five point probes (2a, 2b, 2c, 2d, 2e) on the sample (1) at selected positions which are distanced from an edge of the sample (1): (b) connecting (102) the five point probes (2a, 2b, 2c, 2d, 2e) in five configurations wherein each configuration comprises a different set of four point probes (abcd, bcde, cdea, deab, eabc) and measuring (103) a resistance (r.sub.1=r.sub.abcd, r.sub.2=r.sub.bcde, r.sub.3=r.sub.cdea, r.sub.4=r.sub.deab, r.sub.5=r.sub.eabc) between the four point probes for each configuration; (c) determining (104) the sheet resistance (ρ.sub.0).

A method for determining a sheet resistance of a sample (1) by using point probes. The method includes: (a) positioning (101) five point probes (2a, 2b, 2c, 2d, 2e) on the sample (1) at selected positions which are distanced from an edge of the sample (1): (b) connecting (102) the five point probes (2a, 2b, 2c, 2d, 2e) in five configurations wherein each configuration comprises a different set of four point probes (abcd, bcde, cdea, deab, eabc) and measuring (103) a resistance (r.sub.1=r.sub.abcd, r.sub.2=r.sub.bcde, r.sub.3=r.sub.cdea, r.sub.4=r.sub.deab, r.sub.5=r.sub.eabc) between the four point probes for each configuration; (c) determining (104) the sheet resistance (ρ.sub.0).

An impedance sensing circuit includes first and second current sources and first and second bias current sources that are appropriately coupled to first and second resistors. The impedance sensing circuit also includes a comparator that compares a first voltage based on the first terminal of the first resistor to a second voltage based on the first terminal of the second resistor to generate a comparator output signal. Either the comparator output signal or a digital signal based on the comparator output signal operates to regulate the current signals output from the first and second current sources so that the first voltage is same as the second voltage. The comparator output signal and the digital signal is representative of a difference between the first voltage and the second voltage that is based on an impedance difference between the first resistor and the second resistor.

An apparatus and method are disclosed for detecting the disconnection of a vascular access device such as a needle, cannula or catheter from a blood vessel or vascular graft segment. A pair of electrodes is placed in direct contact with fluid or blood in fluid communication with the vascular segment. In one embodiment, the electrodes are incorporated into a pair of connectors connecting arterial and venous catheters to arterial and venous tubes leading to and from an extracorporeal blood flow apparatus. Wires leading from the electrodes to a detecting circuit can be incorporated into a pair of double lumen arterial and venous tubes connecting the blood flow apparatus to the blood vessel or vascular graft. The detecting circuit is configured to provide a low-voltage alternating current signal to the electrodes to measure the electrical resistance between the electrodes, minimizing both the duration and amount of current being delivered. Detection of an increase in electrical resistance between the electrodes exceeding a pre-determined threshold value may be used to indicate a possible disconnection of the vascular access device.

An apparatus and method are disclosed for detecting the disconnection of a vascular access device such as a needle, cannula or catheter from a blood vessel or vascular graft segment. A pair of electrodes is placed in direct contact with fluid or blood in fluid communication with the vascular segment. In one embodiment, the electrodes are incorporated into a pair of connectors connecting arterial and venous catheters to arterial and venous tubes leading to and from an extracorporeal blood flow apparatus. Wires leading from the electrodes to a detecting circuit can be incorporated into a pair of double lumen arterial and venous tubes connecting the blood flow apparatus to the blood vessel or vascular graft. The detecting circuit is configured to provide a low-voltage alternating current signal to the electrodes to measure the electrical resistance between the electrodes, minimizing both the duration and amount of current being delivered. Detection of an increase in electrical resistance between the electrodes exceeding a pre-determined threshold value may be used to indicate a possible disconnection of the vascular access device.

The present invention discloses an AC impedance measurement circuit with a calibration function, which is characterized in that only one calibration impedance is needed, associated with a switch circuit. Based on the measurement results of the two calibration modes, an equivalent impedance of the switch circuit, circuit gain and phase offset can be calculated. Based on the above results, the equivalent impedance of the internal circuit is deducted from the measurement result of the measurement mode to accurately calculate an AC conductance and a phase of the AC conductance for impedance to be measured. In addition, by adjusting a phase difference between an input sine wave signal and a sampling clock signal, impedance of the same phase and impedance of the quadrature phase can be obtained, respectively, and the AC impedance and phase angle of the impedance to be measured can be calculated.

An insulation resistance calculation apparatus according to an embodiment of the present disclosure includes a switching unit including a first switch and a second switch that are independently controlled, a first protective resistor and a first reference resistor connected in series between a positive electrode terminal of a battery and a ground when the first switch is turned on, a second protective resistor and a second reference resistor connected in series between a negative electrode terminal of the battery and the ground when the second switch is turned on, a voltage measuring unit configured to measure a first detection voltage applied to the first reference resistor and a second detection voltage applied to the second reference resistor, and a processor configured to determine if a measurement error occurred in measured values of the first detection voltage and measured values of the second detection voltage.

An insulation resistance calculation apparatus according to an embodiment of the present disclosure includes a switching unit including a first switch and a second switch that are independently controlled, a first protective resistor and a first reference resistor connected in series between a positive electrode terminal of a battery and a ground when the first switch is turned on, a second protective resistor and a second reference resistor connected in series between a negative electrode terminal of the battery and the ground when the second switch is turned on, a voltage measuring unit configured to measure a first detection voltage applied to the first reference resistor and a second detection voltage applied to the second reference resistor, and a processor configured to determine if a measurement error occurred in measured values of the first detection voltage and measured values of the second detection voltage.

A circuit includes a temperature-sensitive voltage divider. The temperature-sensitive voltage divider includes a temperature-sensitive resistor and a second resistor having a first terminal coupled to a first terminal of the temperature-sensitive resistor. A temperature signal is generated at a first node coupled to the first terminal of the temperature-sensitive resistor. Detection logic is coupled to the first node to generate a detection signal responsive to the temperature signal.

A circuit includes a temperature-sensitive voltage divider. The temperature-sensitive voltage divider includes a temperature-sensitive resistor and a second resistor having a first terminal coupled to a first terminal of the temperature-sensitive resistor. A temperature signal is generated at a first node coupled to the first terminal of the temperature-sensitive resistor. Detection logic is coupled to the first node to generate a detection signal responsive to the temperature signal.