Fluke 8508A, Руководство по обслуживанию

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Theory of Operation
Overall Functional Description
2
2-5
A 16-pin cable (W3) enables serial control and measurement data to move between the
A4 Digital PCA and the A1 DC PCA. Opto-isolators on the A1 DC PCA optically isolate
the external control and display circuits of the Multimeter from the measurement circuits.
This isolation minimizes problems associated with noise and current loops in the
measurement circuits. Throughout this chapter, this isolation is referred to as in-guard
(measurement circuits) and out-guard (control and data transfer circuits).
Control data from the A4 Digital PCA passes through the opto-isolators and serves as
input to a series of tri-state latches. These latches retain the control data, and they provide
the input to a series of relay drivers, which in turn drive a series of configuration relays.
These relays establish the necessary connections between the various assemblies to
perform the measurement called for from the front panel keyboard (or GPIB inputs).
These relays also route analog data (and associated guarding) from the input terminals
(front and rear) to the appropriate assemblies as defined by the measurement control data.
The A1 DC PCA, in addition to switching the measurement configurations and the
routing of input and measurement signals, includes the circuits to perform the following
functions:
•
DC measurements
Handles both DC voltage and dc current measurements
•
Analog to digital conversion (ADC)
The ADC processes all measurement data (volts, ohms, and current) before routing it
(in digital form) to the A4 digital PCA for display and GPIB access.
•
Current to voltage conversion
Both the ac and dc current measurement functions (IDC and IAC) use the same
current shunts.
•
Self-Test
Produces a variety of reference signals from -10 to +10 volts and places them on the
internal signal bus for measurement and display. These tests provide a high degree of
certainty that the Multimeter is functioning correctly, and they are useful for
troubleshooting to the board level.
The A2 AC PCA performs the ac voltage and ac current measurements. The ac voltage
portion of the assembly includes an associated collection of range, configutation, and
filter circuits, as well as an ac preamplifier, and an rms-to-dc converter. The output of the
rms-to-dc converter is a dc voltage proportional to the true-rms value of the input signal.
The multiplexer on the A1 DC PCA receives the dc voltage and routes it to the ADC
(Analog-to-Digital Converter) which, in turn, sends the digital results to the opto
isolators. The opto isolators couple the digital data to the A4 Digital PCA for access by
the display and GPIB.
The A1 DC PCA provides the configuration control required to connect the ac input
signal to the A2 AC PCA. For ac voltage measurements the input signal is routed in
much the same manner as a dc measurement signal. For ac current measurements, the ac
input current is routed through the same set of current shunts used to make dc current
measurements. To measure the ac current, the A2 AC PCA measures the voltage drop
across the shunt and sends the results to the A4 Digital PCA as when making an ac
voltage measurement.
The A3 Ohms PCA performs 2-wire, 4-wire, high-voltage, and ratio measurements. All
of these measurements derive the unknown resistance value by passing a constant
stimulus current through the unknown resistor and measuring the resulting voltage drop
across it. By limiting the current to a value of 1X10
-n
, the voltage drop is equal to the
resistance value, e.g., 1 mA through 2.12 k
Ω
= 2.12 V. To accommodate range changes,
the reference current changes to match the selected range. The reference current starts at
2nA for the 2 G
Ω
range and increases to .1 mA for the 2
Ω
range. A high-gain inverting
amplifier with low-input current and low-offset voltage performs the voltage