Fluke 45 - 2-22. DC Volts; 2-23. Ohms

45
Service Manual
2-12
ACTIVE
FILTER
A/D
HIGH
LOW
A1R9
A1Z1
10.01k
A1K2
S9
S3
S10
A1Z1
10M
A1R5 &
A1RT1
A1R6 &
A1R7
A1K1
S2
V
Ω
COM
qb03f.eps
Figure 2-3. DC Volts 300 V Range Simplified Schematic
2-22. DC Volts
For the mV and 3 V ranges, the input signal is applied to A1U1 through A1R6, A1R7,
and A1K1. The signal is routed directly through A1U1 to the active filter and the a/d
converter without attenuation. Higher voltages are attenuated by the dc input divider
A1Z1.
For the 300 V range, switches S3 and S9 in A1U1 connect the A1Z1 10-MΩ resistor to
the A1Z1 10.01-kΩ resistor, forming a 1000:1 divider. See Figure 2-3. The S10 switch in
A1U1 routes the signal to the active filter. Pin 3 of A1Z1 serves as divider common,
which is also the a/d converter low signal sensed through A1R9.
2-23. Ohms
Resistance measurements are made using the ratio ohms technique. See Figure 2-4. A
voltage source is connected in series with the reference resistor (A1Z1) and the unknown
resistor. Since the same current then flows through both resistors, the unknown
resistance is determined from the ratio of the voltage drops across the reference and
unknown resistors.