Chapter 4
Theory of Operation
©
National Instruments Corporation
4-9
DAQCard-1200 User Manual
For an example of a multiple-channel interval DAQ operation, refer to the
description of interval scanning signal timing in the
section of Chapter 3,
Data Acquisition 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 multiple-channel scanning, the 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 settle to the new input signal value to within 12-bit accuracy before
you perform an A/D conversion, or 12-bit accuracy is not achieved. The
settling time is a function of the selected gain.
Table 4-1 shows the recommended settling time for each gain setting
during multiple-channel scanning. Table 4-2 shows the maximum
recommended acquisition rates. For single-channel scanning, this rate is
limited by the ADC conversion period plus the sample-and-hold
acquisition time, which is specified at 10
µ
s (11
µ
s if EXTCONV* is used).
Table 4-1.
Analog Input Settling Time Versus Gain
Gain Setting
Settling Time Recommended
1, 2, 5, 10
10
µ
s max
20
12
µ
s typ, 15
µ
s max
50
25
µ
s typ, 30
µ
s max
100
60
µ
s typ, 80
µ
s max
Table 4-2.
DAQCard-1200 Maximum Recommended Data Acquisition Rates
Acquisition Mode
Gain Setting
Sampling
Rate
Single channel
1, 2, 5, 10, 20, 50, 100
100 kS/s
Multiple channel
(observe these settings
to ensure 12-bit
accuracy)
1, 2, 5, 10
100 kS/s
20
83.3 kS/s
50
40.0 kS/s
100
16.6 kS/s
