Fluke 2680A, Service Manual

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Theory of Operation
A/D Theory of Operations
2
2-45
V ac Discharge Mode
After a frequency or V ac reading, the hardware is configured to discharge certain signal
conditioning capacitors, charged during the measurement. This is done to avoid
disturbing the measurement of a subsequent V ac or frequency measurement. The
function relays are set to the V ac discharge position, as given in Table 2-11, Function
Relays, and the DISCHARGE signal is set high. After 6 ms, the DISCHARGE signal is
once again set low.
Autoranging
The configuration of each channel includes the state of autoranging for that channel,
either enabled or disabled. When performing a measurement on a channel that has
autoranging enabled, the instrument first attempts a measurement on the range that was
used on that channel for the previous scan. If this results in an overrange or underrange,
the instrument up-ranges or down-ranges accordingly. Channels that are configured with
autorange enabled, but have not yet been measured start on the highest legal range for the
channel’s function type.
Autoranging may decrease the measurement rate, since readings on multiple ranges may
be required for a single channel. On the slow and medium rates, only the first A/D
reading of a measurement is used to determine whether a range change is required.
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
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
The following paragraphs describe the housekeeping functions, which are called Drift
Correction in the user software.
Reading Types
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
Reference balance readings are similar to V dc 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 0 V 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 FAI; see “Timing” earlier in this chapter. The number of readings to take
and average for the different reading rates are given in Table 2-18.