Fluke 45 - 2-31. Active Filter

45
Service Manual
2-16
mA
F1 F5
A1R4
ACTIVE
FILTER
A/D
A1R2
10
10A
A1F2
A1R8
A1K1
S36
RMS
CONVERTER
ACTIVE
FILTER
DCA
ACA
A1R3
.01
COM
A/D
A1K3
AC
BUFFER
RMS
CONVERTER
RESET
qb06f.eps
Figure 2-6. DC mA and Amps Simplified Schematic
2-31. Active Filter
Refer to Figure 2-7. The two-pole active filter consisting of A1R21, A1R22, A1C12, and
A1C13 filters noise on the a-d converter input signal for the DCV, DCmA, DCA,
OHMS, kOHMS, and Diode Test functions. Resistor A1R22 provides a 200-kΩ input
impedance for the filter, except as follows:
• Resistors A1R6 and A1R7 provide the 200-kΩ input impedance for the 100 mV dc,
300 mV dc, 1000 mV dc, 3 V dc, ohms, and kilohms ranges.
• Resistor A1R10 and the 100.5 kΩ resistor in A1Z1 provide the 200 kΩ impedance
for the 10 V and 30 V dc ranges.
• Resistor A1R4 provides the impedance for the two DCmA ranges.
Analog Measurement Processor switch S82 shorts out A1R21 during ranging. Switch
S87 shorts out A1R22 both during ranging and in the measurements listed above. For the
1000 kΩ, 3 MΩ, 10 MΩ, 30 MΩ, 100 MΩ, and 300 MΩ ranges, the active filter is
bypassed, and switch S83 selects C14 as the filter.