Fluke 2680A, Руководство по обслуживанию

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268XA
Service Manual
2-46
Table 2-18. A/D Reading to Average to Obtain a Reference Balance Measurement
Instrument Reading Rate Fast Reading Rate Medium Reading Rate Slow
PAI 1 5 (60 Hz)
6 (50 Hz) 45 (60 Hz)
48 (50 Hz)
FAI 1 1 5 (60 Hz)
6 (50 Hz)
There are two reference balance readings: one with both references on, and one with both
references off. These readings are intended to compensate for unequal voltage references
in the A/D. They are used to obtain a scale factor, which is then applied to the P counter
for normal measurements.
The scale factor is derived as follows:
S = 1 - {[(16P2 - N2) - (16P0 - N0)]K2}
where
S = scale factor
P2 = P counts (both references on)
N2 = N counts (both references on)
P0 = P counts (both references off)
N0 = N counts (both references off)
K2 = (0.1) / (16) / (1843) / (1.6)
Zero Offset Readings
There are four zero readings: one for each gain setting of the DC buffer amplifier. Zero
offset readings are similar to V dc readings, except that no channel selection is required
and no function relay switching is required. The Stallion chip must be configured. The
A/D is placed in normal measurement mode. The number of readings to average is the
same as for a normal reading (see Table 2-18, A/D Readings to Average to Obtain a
Measurement). Zero offset measurements are converted to volts in the same way as
normal channel measurements, except, of course, that no zero offset is subtracted. The
reference balance scale factor is used.
Housekeeping Schedule
Housekeeping readings are always taken in response to a “Do Housekeeping” command
from the outguard, as described in “Do Housekeeping” earlier in this chapter.
Setting the HK bit in a configuration command causes the inguard to schedule
housekeeping readings on a rotating basis, taking one at the end of each channel scan. It
also enables a timer, which is started at the end of a scan or after a configuration
command. Whenever the timer expires, the next housekeeping reading in the schedule is
measured, and the timer is restarted. The timer is set to expire after 17.476 seconds.
On the medium and slow rate, all six housekeeping readings are scheduled as described
in the preceding paragraph. On the fast rate, however, only zero offset readings are
scheduled. This is because a single reference balance reading is longer than a normal
reading on the fast rate, which would cause scans containing them to take longer.