Sometimes, analog is better

-November 01, 2005

Project Profile

Device Under Test
Quad- or dual-channel zero-threshold matched-pair MOSFET arrays. Each MOSFET features a low threshold voltage, about 0.0 V ±2 mV. The device's nearly zero-threshold gate voltage (VGS) lets it operate with lower power-supply voltages than most other MOSFETs.

The Challenge
Shorten test time because using computer-controlled test equipment takes too long to find the threshold voltage (VGS) required to fully turn on the device. Design an analog circuit that lets a bench DMM accurately measure VGS while reducing test time.

The Tools
Advanced Linear Devices: quad SPST analog switch, dual comparator, dual op amp, timer, dual and quad MOSFET arrays. www.aldinc.com.
Agilent Technologies: source-measure unit and DMM. www.agilent.com.
Keithley Instruments: source-measure unit. www.keithley.com.

Project description

Advanced Linear Devices (ALD, Sunnyvale, CA, www.aldinc.com) recently introduced the ALD1108 zero-threshold MOSFET. To characterize a device, ALD engineers must apply a gate voltage of –5 V to +5 V and find VGS at the point where VDS is 100 mV ±1 mV and IDS is 1 mA. The engineers need a way to accurately measure VGS, VDS, and IDS.

When the first devices were available for engineering evaluation, ALD engineers used source-measure units to apply the gate voltage and the 1 mA of drain current (ID). "We started by programming the source-measure units to apply an ever-increasing voltage to the gate and to measure VGS and VDS and IDS," said ALD president Bob Chao, "but finding VGS took over one minute on each device, which was too long."

The delays occurred because the source-measure unit had to increment its output voltage in 1-mV steps across the +5 V to –5 V span and take an ID measurement at each step until ID reached 1 mA with VDS at 100 mV ±1 mV. A computer programmed the source-measure unit to produce each value of VGS and make the measurements.  

To solve the problem, ALD engineers designed a simple analog circuit from op amps, comparators, MOSFETs, a timer, and an analog switch (ALD manufactures all of these devices), as well as several passive components from other manufacturers. The figure depicts a simplified schematic.

The circuit uses a ramp generator that applies a voltage from –5 V to +5 V to the device under test's (DUT's) gate. A current source pumps 1 mA into the DUT's drain. When the ramp voltage reaches the DUT's threshold voltage, the DUT turns on, which lets current from the source pass through.

A precision comparator monitors the drain voltage through a resistor.

An analog circuit finds the value of gate voltage that produces the full 1-mA drain current at a drain voltage of 100.0 mV.
View a detailed schematic.

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When ID reaches the full 1 mA and the drain voltage reaches 100 mV ±0.2 mV, the comparator changes state, causing a switch on the ramp generator's output to open. The polypropylene capacitor holds the gate voltage long enough for a high-resolution DMM to measure it. A power supply or source-measure unit provides a reference voltage for the comparator.

A second comparator (level shifter) converts the precision comparator's output to standard logic levels. The digital-control circuit uses the level-shifter's output to start and stop the test. To start a test, an engineer pushes a button that starts the timer. The timer forces the switch closed, which connects the ramp generator to the DUT. When the comparator and level shifter change state, indicating ID = 1 mA, the level-shifter's output signal resets the timer, opens the switch, and lights an LED indicating that the measurement is complete.

Lessons learned

ALD engineers learned that using computer-controlled test equipment isn't always the best solution to a measurement problem. With off-the-shelf active and passive components, you can sometimes build a simple supplementary circuit that, in conjunction with automated test instruments, can cut test time and improve measurement accuracy.

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