232 AD Converter

Figure 2-8 shows the dual slope a/d converter used in the Fluke 45. A voltage level proportional to the unknown input signal charges (integrates) a capacitor for an exact amount of time. This capacitor is then discharged by a reference voltage of opposite polarity. The capacitor discharge time, which is proportional to the level of the unknown input signal, is measured by the digital circuits in the Analog Measurement Processor and sent to the microprocessor.

In the medium and fast measurement rates, the a/d converter uses the ±300 mV or ±3 V range. These ranges typically uprange at a display of 32,000 in the medium rate. In the slow rate, the a/d converter uses one of two additional ranges: ±100 mV and ±1000 mV. The uprange point for these additional ranges is a display of 99,999. In the ohms function only, a typical uprange point is a display of about 98,000.

During the integrate phase, the a/d buffer in Analog Measurement Processor A1U1 applies the signal to be measured to one of the four integrate resistors in resistor network A1Z3. The choice of resistor depends on the measurement range. Switch S69 connects output B.1 (pin 47) for the 100-mV range; S71 uses output B.3 (pin 48) for the 300-mV range; S73 uses output B1 (pin 49) for the 1000 mV range; and S75 uses output B3 (pin 50) for the 3 V range. Refer to Figure 8-1.

The current through the selected integrator resistor charges integrator capacitor A1C16. After the integrate phase, the buffer applies the opposite polarity reference voltage, and the integrator integrates back down until the comparator toggles. A counter measures the integrate reference time. If the a/d converter is overloaded and the integrator does not return to its starting point by the end of the integrate reference phase, S77 discharges integrator capacitor A1C16.

The reference voltage for volts, current, and diode test modes is provided by a 6.3 V zener diode, A1VR1. The reference amplifier in the Analog Measurement Processor provides 2 mA of current through A1VR1. Resistor network A1Z3 divides the 6.3 V down to 1.1 V.

INTEGRATE X^ INPUT

Figure 2-8. A/D Converter Simplified Schematic

INTEGRATE X^ INPUT

Figure 2-8. A/D Converter Simplified Schematic

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