Fluke 2680A, Руководство по обслуживанию

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Theory of Operation
A/D Theory of Operations
2
2-39
A/D
The multi-slope A/D converter in the instrument uses a hardware state machine (A3U18)
to control the switching of the voltage references during the A/D conversion. This state
machine also contains the counters that measure how long each reference is switched in,
and provides the A3U5 A/D microprocessor with its interface to the A/D. A synchronous
serial port is used to transfer the counter contents from the state machine to the A3U5
A/D microprocessor. These counter values can then be manipulated to form an A/D
reading. There are two counters, NCOUNT and PCOUNT, which measure how long the
negative and positive references, respectively, are switched in.
Timing
The timing for the FAI and PAI A/Ds is shown in Figures 2-12 and 2-13. These figures
apply to normal readings. For Reference Balance readings, the timing for both FAI and
PAI is given by Figure 2-13.
After the Trigger signal from the A3U5 A/D microprocessor is recognized, the A/D goes
into the Autozero period. Immediately following this are the Integrate and Deintegrate
periods. The only time that the input signal is actually being measured by the A/D is
during the Integrate period. Therefore, the channel can be deselected and the Stallion
programming for the next channel begun during the Deintegrate period. Also, the signal
conditioning does not need to be settled until the beginning of integrate. At the end of
Deintegrate, if the Trigger signal is still asserted, the A/D immediately begins the
Autozero period for the next reading. Otherwise, it enters the Untimed Autozero period,
which lasts until the Trigger signal is once again asserted. To take higher resolution
measurements, the Trigger signal is left asserted until the required number of readings are
obtained. This is also done for V dc readings on the fast rate (FAI only).
AZ
Autozero
I
Integrate
DE
Deintegrate
UAZ
Untimed Autozero
Trigger
200.0 us 491.2 us 140.8 us
f2-14.eps
Figure 2-12. A/D Timing (FAI Normal Reading)
AZ
Autozero
I
Integrate
DE
Deintegrate
UAZ
Untimed Autozero
Trigger
140.8 us 244.8 us 2948.8 us
f2-15.eps
Figure 2-13. A/D Timing (PAI Normal Reading, PAI and FAI Reference Balance)