User's Manual

Theory of Operation
A/D Theory of Operations
2
2-43
Measurement Types
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 2-10, Tree and Channel Switch Settling
Times.
• Wait for signal conditioning circuitry to settle. See Table 2-14, Signal Conditioning
Settling Time.
After these steps have been carried out, the sequence of operations depends on the
measurement function.
V dc, V ac, Ohms
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.
V dc Fast Rate, FAI
Volts DC on the FAI, 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 V dc readings are exploited in order to allow the A/D to be triggered
continuously for all the channels in a V dc block. A V dc block consists of a series of
channels that are all defined as V dc, with “similar” ranges. Similar range means either
the low ranges (90 mV, 300 mV, 750 mV, and 3 V) or the high ranges (30 V and HI V).
For the channels within such a block, we can assume the following:
• No function relay switches are required.
• There is only one Stallion register that must be written to between channels.
• A channel can be selected at the beginning of the deintegrate period of the previous
channel, at the same time that the previous channel is deselected.
• There is sufficient time during the deintegrate period to configure the Stallion for the
next channel.
• There is sufficient time during the Autozero period of a channel for signal
conditioning settling.
• The N and P counters for a channel can be read during the Autozero period of the
next channel.
Thermocouples
A thermocouple channel is measured in the same way as a V dc channel, on the 90 mV
range. However, before deselecting the channel at the end of the measurement, an
open-thermocouple check may be done, if the channel is so configured. An “open”
indication from this check causes a value of OTC_VAL to be returned for the channel
measurement, regardless of the voltage measured. See Open-Thermocouple Detector.