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Chapter 5
Note: 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 to avoid aliasing.
The modules support a wide pass band of 0.5 Hz (0.1 Hz for the DT9837A) to 25.8 kHz (0.49 x
sampling frequency) to eliminate aliasing, allowing you to measure low frequency signals
accurately at the Nyquist sampling rate.
The actual frequency that the module can achieve may be slightly different than the frequency
you specified due to the accuracy of the clock. You can determine the actual clock frequency
using software.
Internally, the value that you specify for the internal clock frequency is multiplied by 512 (for
frequencies of 52.734 kHz or less) or 256 (for frequencies greater than 52.734 kHz) to set the
oscillator on the module. For example, if you specify an internal clock frequency of 50 kHz,
the module sets the internal oscillator for the A/D converters to 25.6 MHz. The maximum
timebase is 27 MHz.
Once the sample clock is started, the module requires 39 clock pulses before the first A/D
conversion is completed (39/sample rate) due to the group delay of the converters. The
software automatically adjusts for the group delay to provide only valid data in each buffer.
The tachometer data (which does not have the 39 sample group delay) is synchronized with
the analog data stream. This is done through the firmware and device driver by caching the
tachometer data and aligning it in time with the analog data in the user's data buffers.
Analog Input Conversion Modes
DT9837 Series modules support single-value, single-values, and continuous scan conversion
modes. This section describes each of these conversion modes.
Single-Value Operations
Single-value operations are simpler to use than continuous operations. Using software, you
specify the analog input channel (0, 1, 2, or 3) and the gain that you want to use. The module
acquires the data from the specified channel and returns the data immediately.
For single-value operations, you cannot specify a clock source, trigger source, scan mode, or
buffer. Single-value operations stop automatically when finished; you cannot stop a
single-value operation.
Note: You cannot read the value of tachometer counter 0 (described on
counter 1 (described on
readback channel (described on
these values, specify the channels as part of the analog input channel list using continuous
scan mode, described on
70
page
84), gate counter 2 (described on
page
79) using a standard single-value operation. To read
page
71.
page
82), tachometer
page
85), or the analog output
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