National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ - Manual (Page 66)

Chapter 4 Theory of Operation
© National Instruments Corporation 4-7 Lab-PC+ User Manual
You must initialize two additional counters to operate in interval acquisition mode. In single-
channel interval acquisition mode, the Lab-PC+ samples a single channel a programmable
number of times, waits for the duration of the scan interval, and repeats this cycle. In the
scanned interval acquisition mode, the Lab-PC+ scans the selected samples, waits for the
duration of the scan interval, and repeats the cycle.
Data Acquisition Rates
Maximum data acquisition rates (number of samples per second) are determined by the
conversion period of the ADC plus the sample-and-hold acquisition time. During multiple-
channel scanning, the data acquisition rates are further limited by the settling time of the input
multiplexers and programmable gain amplifier. After the input multiplexers are switched, the
amplifier must be allowed to settle to the new input signal value to within 12-bit accuracy before
an A/D conversion is performed, or else 12-bit accuracy will not be achieved. The settling time
is a function of the gain selected.
The Lab-PC+ data acquisition timing circuitry detects when data acquisition rates are high
enough to cause A/D conversions to be lost. If this is the case, this circuitry sets an overrun error
flag in the Lab-PC+ Status Register. If the recommended data acquisition rates in Table 4-2 are
exceeded (an error flag is not automatically set), the analog input circuitry may not perform at
12-bit accuracy. If these rates are exceeded by more than a few microseconds, A/D conversions
may be lost. Table 4-1 shows the recommended multiplexer and gain settling times for different
gain settings. Table 4-2 shows the maximum recommended data acquisition rates for both
single-channel and multiple-channel data acquisition. Notice that for a single-channel data
acquisition, the data can be acquired at the maximum rate at any gain setting. The analog input
bandwidth, however, is lower for higher gains. For multiple-channel data acquisition, observing
the data acquisition rates given in Table 4-2 ensures 12-bit accuracy.
Table 4-1. Analog Input Settling Time Versus Gain
Gain Setting Settling Time Recommended
1 12
µ
s
2, 5, 10, 20, 50 16
µ
s typical, 18
µ
s maximum
100 50
µ
s