Data Translation DT9857, Руководство пользователя, Страница: 85 / 192

Principles of Operation
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For the analog output subsystem, use software to specify the internal clock source and the
output frequency. The output frequency can range between 30 kHz and 216 kHz.
Note: The sample frequencies for the A/D and D/A subsystems are independently
programmable and are derived from the same 48 MHz reference clock. Therefore, it is
possible to establish a fixed relationship between the A/D and D/A subsystem sample
frequencies, including setting them to the same frequency (30 kHz to 105.469 kHz). However,
the subsystems are not designed to be synchronous with each other.
The DT9857 and DT9857E modules require 36 sample clock periods once the output
subsystem has been triggered before the first D/A conversion is completed. The clock is
always running.
Output Conversion Modes
DT9857 and DT9857E modules support single-value, waveform, and continuous analog
output operations.
Single-Value Mode
Single-value mode is the simplest to use but offers the least flexibility and efficiency. Use
software to specify the analog output channel that you want to update, and the value to
output from that channel. The value is output from the specified channel immediately.
For a single-value operation, you cannot specify a clock frequency, trigger source, or buffer.
Single-value operations stop automatically when finished; you cannot stop a single-value
operation.
Waveform Generation Mode
The DT9857 and DT9857E modules support the ability to generate a waveform (specified in a
single buffer) from the output channels.
If you want the hardware to output the waveform without USB bus intervention, ensure that
the waveform is equal to or less than the size of the output FIFO on the module (64 kSamples).
In this case, the waveform pattern can range from 2 to 64K (65536) samples if you specify one
output channel, 2 to 32K (32768) samples for two output channels, or 21.3K (21845) samples
for three output channels (two analog output channels and the digital output port).
If your waveform is larger than the size of the output FIFO (64K samples), the driver will
manage the buffer for you. However, USB bus intervention is required in this case.
Use software to fill the output buffer with the values that you want to write to the channels in
the output channel list.