Fluke NetDAQ 2640A, Service Manual

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NetDAQ
Service Manual
2-58
Note that these numbers do not apply to measurement types that do not use the A/D
converter. They also do not apply to reference balance readings (see Reference Balance
Readings).
Table 2-20. A/D Readings to Average to Obtain a 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 4 20 (60 Hz)
24 (50 Hz)
Measurement Types 2-105.
There are several steps that you must perform at the beginning of any channel
measurement:
•
Set function relays. See “Function Relays” earlier in this chapter. This is a relatively
slow operation and should be done only if the relay positions actually need to
change.
•
Set tree and channel switches. See “Channel MUX” earlier in this chapter.
•
Program Stallion. See “Stallion Chip and Signal Conditioning” earlier in this
chapter.
•
Wait for channel switches to settle. See Table 2-14: Tree and Channel Switch
Settling Times.
•
Wait for signal conditioning circuitry to settle. See Table2-18: Signal Conditioning
Settling Time.
After these steps have been carried out, the sequence of operations depends on the
measurement function.
VDC, VAC, Ohms 2-106.
These types of measurements all use the A/D converter. After selecting the channel and
configuring the signal conditioning circuitry, the A/D is triggered and, depending on the
reading rate, one or more readings taken. The A/D counts are converted to a
floating-point value and stored in a buffer for later transmission to the outguard. The
channel and tree switches are then deselected.
VDC Fast Rate, 2645A 2-107.
Volts DC on the 2645A, fast rate represent a special case. To attain the required
throughput, you cannot perform the sequence of steps given above for each channel.
Instead, certain characteristics of VDC readings are exploited in order to allow the A/D
to be triggered continuously for all the channels in a VDC block. A VDC block consists
of a series of channels that are all defined as VDC, with “similar” ranges. Similar range
means either the low ranges (90 mV, 300 mV, 750 mV, and 3V) or the high ranges (30V
and HIV).