Fluke NetDAQ 2640A, Service Manual

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Theory of Operation
Inguard Software Description
2
2-61
The actual points where a channel up-ranges or down-ranges varies, depending on the
outguard calibration constants. This happens because the overrange/underrange
determination is made by the inguard, which is comparing uncalibrated, raw A/D counts.
These points are selected so that some overlap exists between ranges, to ensure a certain
amount of hysteresis when changing ranges. Also, when autoranging, on a given scan for
a given channel, the instrument only up-ranges or down-ranges, not both. This avoids
“infinite autoranging,” where a channel measurement could hypothetically take forever
as the instrument up-ranges and down-ranges continuously on a noisy input.
Overload 2-113.
A channel can be in either positive or negative overload, depending on the polarity of the
input signal. Overload limits are similar to autorange limits in that their actual values can
vary, depending on the outguard calibration constants.
Housekeeping Readings 2-114.
The following paragraphs describe the housekeeping functions, which are called Drift
Correction in the NetDAQ Logger for Windows software.
Reading Types 2-115.
There are two types of housekeeping readings: reference balance and zero offset
readings. There are two different reference balance readings and four different zero
offset readings.
Reference Balance Readings 2-116.
Reference balance readings are similar to VDC readings, except that no channel
selection is required, and no function relay switching is required. The A/D itself,
however, must be configured to operate in a different mode. See Control Signals. After
the Stallion chip is configured (to provide an input of zero volts to the A/D), the A/D can
be triggered and then read as normal. Note that a reference balance reading has different
timing than a normal reading on the 2645A; see “Timing” earlier in this chapter. The
number of readings to take and average for the different reading rates are given in Table
2-22.
Table 2-22. A/D Readings to Average to Obtain a Reference Balance Measurement
Instrument Reading Rate Fast Reading Rate Medium Reading Rate Slow
2640A 1 5 (60 Hz)
6 (50 Hz) 45 (60 Hz)
48 (50 Hz)
2645A 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.