DAQ VXI-SC-1102 Manual de Instruções descarregar pdf (Página 122)

Chapter 20 DSA Devices

DAQ Hardware Overview Guide 20-4 www.ni.com

There are two hardware trigger sources—analog or digital. The device can

receive the analog trigger from any one of the input channels, even if that

channel is not included in the scan list of the acquisition. The analog trigger

circuitry generates a trigger when the selected input channel reaches a

specified slope and level. If you want a digital trigger, you can configure the

device to receive the trigger from the digital I/O connector or from the

RTSIbus.

When using a digital trigger, notice that the first sample after the trigger

does not appear in the acquisition buffer until 42 scans have been acquired.

This means that in posttrigger mode the acquisition buffer contains

42 pretrigger samples when the acquisition is finished. This behavior is a

feature of the digital antialiasing filter in the converter and cannot be

disabled.

PCI-4451, PCI-4453, and NI 4551 Analog Output

Channels

The PCI-4451, PCI-4453, and NI 4551 contain simultaneously updated

analog output channels based on delta-sigma DACs. The converters

accept 18-bit resolution samples. On the PCI-4451 and NI- 4551, each

channel also has software configurable attenuation settings of 0, 20, and

40 dB.

The converters continuously generate samples while the update clock

is running. A DDS clock generation circuit generates an extremely stable

clock signal that can drive the converters at update rates up to 51.2 kHz in

increments of 200 µHz. The anti-imaging filter in the converters adjusts to

the update rate to reject frequency components from frequencies above the

update rate. Because the converters require the stability of the DDS clock,

you cannot supply an external clock. Clock signals can be synchronized

between multiple 445X and 455X devices over the RTSIbus.

Waveform Generation

The waveform generation (WFM) functions can write values to either one or

both analog output channels continuously using the onboard DDS clock to

update the DACs at regular intervals. The values are contained in a buffer

that you allocate and fill. The resultant voltages produced at the analog

output channels depend on the value of the numbers in the buffer.

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DAQ VXI-SC-1102 Manual de Instruções Página 122