PC Printer Port Controls I-V Curve Tracer

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...