Theory of Operation
Detailed Circuit Description
2
2-17
ACTIVE
FILTER
INPUT
A1R22
S87
A1C12 A1C13
S82
A1R21
A1C14
+
_
W
FILTER
A/D
M
qb07f.eps
Figure 2-7. Active Filter Simplified Schematic
2-32. A/D 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
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