Abstract:

This circuit generates a precision linear ramp from "5V to +5V that could be stopped within 1mV of the DUT (device under test) desired output. The DUT in this example is the ALD1108 MOSFET. The VGS voltage of this device is defined as that gate voltage at which the drain current is 1µA and the drain voltage is at 100mV. The challenge in this case is to measure that gate voltage VGS at which VDS=100mV and drain current IDS=1µA simultaneously. This is accomplished by using a precision current source that supplies a calibrated 1µA to the drain of the DUT while setting up a precision comparator to continuously comparing the drain voltage VDS to that of a precision reference voltage set at 100mV. When the ramp voltage increments from V-=-5V, it charges the holding capacitor C2, and VGS measurement accuracy is determined by the accuracy and response time of the comparator and the time it takes to subsequently control the logic and open the analog switch to stop the ramp generator. The comparator selected