Fluke 45 - 2-19. Input Protection; 2-20. Input Signal Conditioning

45
Service Manual
2-10
Table 2-1. Analog Measurement Processor Pin Name Description (cont)
Pin Name Description
51 VREF+ A/D reference plus
52 VREF- A/D reference minus
53 RAO A/D reference amplifier output
54 RA+ A/D reference amplifier noninverting input
55 RA- A/D reference amplifier inverting input
56 AFO Active filter output
57 MOF Megohms filter
58 AFI Active filter input
59 FAI Filter amplifier inverting input
60 FAO Filter amplifier output
61 RMSF RMS output, filtered
62 ARTN Analog return
63 (not used)
64 RMSO RMS converter output
65 BIAS2 Bias input
66 VSSF -5 V dc, filtered
67 BIAS1 Bias input
68 RMSI RMS converter input
2-19. Input Protection
Input protection safeguards the meter against a number of over-voltage and over-current
conditions. The various input protection schemes are as follows:
• In the Voltage mode of operation, MOVs A1RV1, A1RV2 and A1RV3 clamp input
voltage transients to about 1800 volts, and A1R5, A1RT1 and A1R6 limit the
current. In this mode, A1R7 and the 10-MΩ resistor in A1Z1 protect A1U1.
• In the Ohms and Diode Test modes, A1Q1 clamps voltage inputs of both polarities,
and A1R5 and thermistor A1RT1 limit the overload current. With large overloads,
A1RT1 heats up and increases in resistance. Components A1R9 and A1Z1 protect
A1U1.
• The [100 mA] input is protected by fuses F1 and F5. With this input, A1R4 protects
A1U1.
• The [10 A] input is protected by fuse A1F2, with A1R8 protecting A1U1.
2-20. Input Signal Conditioning
The input signal (voltage, current, or resistance) must be scaled or conditioned to a form
that can be measured by the a/d converter. High dc voltage levels must be attenuated.
Resistances, currents, and ac voltages must be converted to a representative dc voltage.
DC-type measurements (dc volts, dc current, ohms, and diode test) are then filtered by
the active filter. AC measurements (ac volts and ac current) are passively filtered after
being converted to a dc voltage. In the medium and fast measurement rates, the a/d
converter uses one of two ranges: ±300 mV and ±3 V full scale. In the slow rate, the a/d
converter uses one of two additional ranges (±100 mV and ±1000 mV full scale), for a
total of four ranges.