Data Translation DT300 Series, User Manual

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Principles of Operation
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According to sampling theory (Nyquist Theorem), specify a frequency that is at least twice as
fast as the input’s highest frequency component. For example, to accurately sample a 20 kHz
signal, specify a sampling frequency of at least 40 kHz. Doing so avoids an error condition
called
aliasing , in which high frequency input components erroneously appear as lower
frequencies after sampling.
Note: If your channel list contains only digital input channel 16, the maximum frequency is
3 MHz (3 MSamples/s).
External A/D Sample Clock
The external A/D sample clock is useful when you want to pace acquisitions at rates not
available with the internal A/D sample clock or when you want to pace at uneven intervals.
Connect an external A/D sample clock to screw terminal TB48 on the STP300 screw terminal
panel (pin 22 on connector J1). Conversions start on the falling edge of the external A/D
sample clock input signal.
Using software, specify the clock source as external. For the DT300 Series board, the clock
frequency is always equal to the frequency of the external A/D sample clock input signal that
you connect to the board through the screw terminal panel.
Analog Input Conversion Modes
DT300 Series boards support the following conversion modes:
• Single-value operations are the simplest to use but offer the least flexibility and
efficiency. Using software, you can specify the range, gain, and analog input channel
(among other parameters); acquire the data from that channel; and convert the result. The
data is returned immediately. For a single-value operation, you cannot specify a clock
source, trigger source, trigger acquisition mode, scan mode, or buffer.
Single-value operations stop automatically when finished; you cannot stop a single-value
operation.
• Scan mode takes full advantage of the capabilities of the DT300 Series boards. In a scan,
you can specify a channel list, clock source, trigger source, trigger acquisition mode, scan
mode, and buffer using software. Two scan modes are supported: continuously-paced
scan mode and triggered scan mode (often called burst mode). These modes are described
in the following subsections.
DT304 400 kHz
DT321 250 kHz
DT322 250 kHz
Table 7: Maximum Frequency Supported (cont.)
Board Type Maximum Frequency