Fluke 2680A, Руководство по обслуживанию

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Theory of Operation
A/D Theory of Operations
2
2-15
Analog Measurement Processor
The Analog Measurement Processor (A3U30) provides input signal conditioning,
ranging, and frequency measurement. This custom integrated circuit is controlled by the
inguard A/D Microprocessor (A3U5). The A/D Microprocessor communicates with the
main Controller/System Power PCA over a serial interface. Isolation is achieved through
the A/D power supply daughter card with a unique dc to dc converter.
Input Protection
This circuitry protects the instrument measurement circuits during overvoltage
conditions.
Input Signal Conditioning
Each input is conditioned and/or scaled to a dc voltage for measurement by the A/D
converter. DC voltage levels greater than 3 V are attenuated. To measure resistance, a dc
current is applied across a series connection of the input resistance and a reference
resistance to develop dc voltages that can be ratioed. DC volts and ohms measurements
are filtered by a passive filter. AC voltages are first scaled by an ac buffer, converted to a
representative dc voltage by an RMS converter, and then filtered by an active filter.
Analog-to-Digital (a/d) Converter
The dc voltage output from the signal conditioning circuits is applied to a multi-slope
A/D converter.
The input voltage is applied to a buffer/integrator that charges a capacitor for an exact
amount of time. During this time, positive and negative reference voltages are alternately
applied to the integrator. The references are switched in a sequence controlled by the A/D
Electrically Programmed Logic Device (EPLD) (A3U18), which prevents the integrator
from saturating.
The amount of time that each reference is applied to the integrator, and the amount of
time required to discharge the capacitor, are measured by digital counter circuits in the
A/D EPLD (A3U18). These times are used by the inguard microprocessor (A3U5) to
calculate the level of the unknown input signal.
Inguard Microcontroller
This microprocessor (A3U5) and associated circuitry controls all functions on the A/D
Converter PCA and communicates with the Controller/System Power PCA. Upon request
by the Controller/System Power PCA (also called the out guard processor), the inguard
microprocessor selects the input channel to be measured through the channel selection
circuitry, sets up the input signal conditioning, commands the A/D EPLD (A3U18) to
begin a conversion, stops the measurement, and then fetches the measurement result. The
inguard microprocessor manipulates the result mathematically and transmits the reading
to the digital kernel.
Channel Selection
This circuitry consists of a set of relays and relay-control drivers. The relays form a tree
that routes the input channels to the measurement circuitry. Two of the relays are also
used to switch between two-wire and four-wire operation. For signal switching and
selection, the PAI uses reed relays, while the FAI uses solid-state relays.