Fluke Corporation

Douglas A Miller

Raymond D Zoellick

A low error, switchable measurement lead detect circuit (,) for providing accurate current measurement readings and reducing susceptibility to false measurement lead detections due to leakage current is described. A gate (,) that receives an input select voltage V,selectively connects either a supply voltage, V,, or ground to the input of a voltage divider (,). V,is connected during a measurement lead detection mode of operation or open fuse detection mode of operation, and a ground is connected during a current measurement mode of operation. During the measurement lead detection mode of operation, if the pin of a measurement lead is present in a split jack (,), the voltage divider (,) divides the supply voltage, V,. Conversely, if no measurement lead pin is present in the split jack (,), the output voltage, V,, of voltage divider (,) equals the level of V,. During open fuse detection mode of operation, a measurement lead is present in the split jack (,), and if a fuse (F,) is present, the voltage divider divides the supply voltage, V,. If a fuse (F,) is not present or is blown, the output voltage, V,, of voltage divider (,) equals the level of V,. During the current measurement mode of operation, the voltage divider (,) and shunt (,) form a series parallel circuit when a measurement lead pin is located in the split jack (,). The impedance of the shunt (,) is negligible in comparison to the impedance of the voltage divider (,), and the voltage drop across shunt (,) is proportional to the injected current. Because the voltage divider (,) is grounded through the gate (,) during the current measurement mode of operation, no error current from supply voltage, V,, passes through the shunt (,). As a result, the accuracy of current measurement readings made during the current measurement mode of operation is improved. Because the supply voltage does not produce an error current during the measurement mode of operation, the overall impedance of the voltage divider (,) can be reduced to reduce susceptibility to leakage current during the measurement lead detection mode of operation.

Low error, switchable measurement lead detect circuit

An improved capacitance measurement system employs a selectable constant current source which is switched into a capacitor in a charge measurement system. The capacitor produces a linear ramp voltage in response to the constant current. An analog-to-digital converter (ADC) measures the change in voltage along the ramp over a corresponding change in time. These values, together with the value of constant current, are used to calculate the capacitance. In the preferred embodiment, a multi-slope ADC is utilized for the measurement. This system offers a wide range of measurable capacitance, and is fast responding and accurate. In addition, visual feedback to the user on measurement progress can be provided, which is particularly advantageous when measuring large capacitors.

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Stevenson, WA, United States of America

Douglas A Miller