National Instruments 6711 User Manual

Also see for DAQCard-6715: User manual User manual

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National Instruments 6711 User Manual Manual pdf 31 page image

© National Instruments | 3-3TitleShort-Hidden (cross reference text)Minimizing Glitches on the Output SignalWhen you use a DAC to generate a waveform, you may observe glitches on the output signal.These glitches are normal; when a DAC switches from one voltage to another, it producesglitches due to released charges. The largest glitches occur when the most significant bit (MSB)of the DAC code switches. You can build a lowpass deglitching filter to remove some of theseglitches, depending on the frequency and nature of the output signal. Visit ni.com/supportfor more information on minimizing glitches.AO Data Generation MethodsWhen performing an analog output operation, there are several different data generationmethods available. You can either perform software-timed or hardware-timed generations.Hardware-timed generations can be non-buffered or buffered.Software-Timed GenerationsWith a software-timed generation, software controls the rate at which data is generated. Softwaresends a separate command to the hardware to initiate each DAC conversion. In NI-DAQmx,software-timed generations are referred to as On Demand timing. Software-timed generationsare also referred to as immediate or static operations. They are typically used for writing a singlevalue out, such as a constant DC voltage.Hardware-Timed GenerationsWith a hardware-timed generation, a digital hardware signal controls the rate of the generation.This signal can be generated internally on your device or provided externally.Hardware-timed generations have several advantages over software-timed generations:• The time between samples can be much shorter.• The timing between samples can be deterministic.• Hardware-timed generations can use hardware triggering. For more information, refer toChapter 10, Triggering.Hardware-timed operations can be buffered or non-buffered. A buffer is a temporary storage incomputer memory for acquired or to-be-generated samples.BufferedIn a buffered generation, data is moved from a PC buffer to the DAQ device’s onboard FIFOusing DMA or interrupts before it is written to the DACs one sample at a time. Bufferedgenerations typically allow for much faster transfer rates than non-buffered generations becausedata is moved in large blocks, rather than one point at a time. For more information on DMA andinterrupt requests, refer to the Data Transfer Methods section of Chapter 9, Bus Interface.One property of buffered I/O operations is the sample mode. The sample mode can be eitherfinite or continuous.