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Theory of Operation Chapter 3
Lab-NB User Manual 3-6 © National Instruments Corporation
DAQ Rates
Maximum DAQ rates (number of samples per second) are determined by the conversion period
of the ADC plus the sample-and-hold acquisition time. During multichannel scanning, the DAQ
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-NB DAQ timing circuitry detects when DAQ 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-NB
Status Register. If the recommended DAQ rates in Table 3-2 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 3-1 shows
the recommended multiplexer and gain settling times for different gain settings.
Table 3-2 shows the maximum recommended DAQ rates for both single-channel and
multichannel 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 multichannel data acquisition, observing the DAQ rates given in
Table 3-2 ensures 12-bit accuracy.
Table 3-1. Analog Input Settling Time Versus Gain
Gain Setting Settling Time Recommended
1 16 µsec
2, 5 20 µsec
10, 20 30 µsec
50, 100 100 µsec
Table 3-2. Lab-NB Maximum Recommended DAQ Rates
Single-Channel Data Acquisition:
Any Gain Setting 62.5 ksamples/sec
Multichannel Data Acquisition:
Gain = 1 62.5 ksamples/sec
Gain = 2, 5 50.0 ksamples/sec
Gain = 10, 20 33.3 ksamples/sec
Gain = 50, 100 10.0 ksamples/sec