PC Printer Port Controls I-V Curve Tracer

This article describes an I-V curve tracer circuit that uses a computer for display and control. The circuit is controlled via the PC parallel port. Software is provided, written in BASIC, to control the measurement and display the results on the monitor. When connected to the printer port of a PC, the circuit shown in Figure 1 enables you to determine the current-voltage (I-V) characteristics of an active component or integrated circuit. A short BASIC program* drives the port and displays the I-V characteristic as a graph on the monitor. The result is a very useful diagnostic tool for IC fault analysis.
PC Printer Port Controls I-V Curve Tracer - schematic

Figure 1. A 12-bit serial-data DAC (IC4) and ADC (IC3) form an interface that enables the printer port of a PC to control this I-V curve tracer. The 12-bit digital-to-analog converter (DAC), IC4, is configured for bipolar outputs to ±2.048V. Op-amp IC6A multiplies this signal with a gain of +2V/V, and op-amp IC7 converts the result to a current that passes through the device under test (DUT). This current ranges from ±40µA to ±40mA, according to the resistor value selected for RSENSE. For any combination of DUT and selected range, the maximum current available equals (approximately) the IC6A output (±4.096V max) divided by RSENSE. Current through the DUT produces a bipolar voltage that is sensed by the differential amplifier IC6B. To avoid the variable-offset error that would otherwise occur with a change in switch position, this amplifier's inverting-input signal is taken from the low-impedance, noninverting input of IC7 rather than its inverting input. The penalty for this choice is the fixed input-offset error of IC7. The differential amplifier's gain plus the offset supplied to it result in a maximum output swing (0V to 4.096V) compatible with the unipolar input range of the 12-bit analog-to-digital converter (ADC), IC3. IC3's 3.3k input resistor limits input current in the event of an applied overvoltage. IC7 requires ±15V supply rails to provide sufficient...

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