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National Instruments PXIe-4480 Скачать руководство пользователя страница 23

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3

Signal Acquisition

This chapter contains information about signal acquisition concepts, including analog-to-digital
converters, operation modes, filtering, timing, triggering, and synchronization.

Analog-to-Digital Converter

The PXIe-4480/4481 analog-to-digital converters (ADCs) use the Delta-Sigma modulation
conversion method. This approach involves oversampling the input signal and then decimating
and filtering the resulting data to achieve the desired sample rate.

Nyquist Frequency and Nyquist Bandwidth

Any sampling system, such as an ADC, is limited in the bandwidth of the signals it can measure.
Specifically, a sampling rate of

fs

can represent only signals with frequencies lower than

fs

/2.

This maximum frequency is known as the

Nyquist frequency

. The bandwidth from 0 Hz to the

Nyquist frequency is the

Nyquist bandwidth

.

Operation Modes

The PXIe-4480/4481 supports two acquisition modes: frequency domain mode and time domain
mode.

In a frequency domain mode acquisition, oversampled data is decimated to the requested user
sample rate, and digital filters are applied to filter out frequency content above the Nyquist
frequency. Frequency domain mode is used any time the requested sample rate is between
100 S/s and 1.25 MS/s. Refer to the

Frequency Domain Mode Acquisitions

section for details on

the acquisition process and filters used in frequency domain mode.

The PXIe-4480/4481 also supports time domain mode, in which the oversampled data is loosely
filtered so that a maximum amount of bandwidth can be measured. Data may or may not be
decimated. Time domain mode is used any time the requested sample rate is above 1.25 MS/s
and can operate at up to 20 MS/s. Refer to the

Time Domain Mode Acquisitions

and the

Domain Mode Considerations

sections for details on the acquisition process and filters used in

time domain mode.

The ability for the module to sample at a requested rate is limited by the module’s sample rate
generation circuitry and, as a result, a sample rate resolution specification is provided. The
sample rate resolution specification indicates how many samples per second are between
two adjacent sample rates that the module is capable of producing. Because of the decimation

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Содержание PXIe-4480

Страница 1: ...Dynamic Signal Acquisition PXIe 4480 4481 User Manual 6 Channel 24 bit 1 25 MS s DSA Analog Input PXIe 4480 4481 User Manual October 2016 376368A 01...

Страница 2: ...support phone numbers email addresses and current events National Instruments Corporate Headquarters 11500 North Mopac Expressway Austin Texas 78759 3504 USA Tel 512 683 0100 For further support infor...

Страница 3: ...TLE OR NON INFRINGEMENT AND ANY WARRANTIES THAT MAY ARISE FROM USAGE OF TRADE OR COURSE OF DEALING NI DOES NOT WARRANT GUARANTEE OR MAKE ANY REPRESENTATIONS REGARDING THE USE OF OR THE RESULTS OF THE...

Страница 4: ...ependent from NI and have no agency partnership or joint venture relationship with NI Patents For patents covering NI products technology refer to the appropriate location Help Patents in your softwar...

Страница 5: ...nsducer Electronic Data Sheet 2 11 Voltage Excitation 2 11 Fault Conditions 2 13 IEPE Open and IEPE Short 2 13 Voltage Excitation Current Limit 2 13 Overtemperature Detection 2 14 Chapter 3 Signal Acq...

Страница 6: ...Mode 3 8 Data Throughput 3 9 Timing and Triggering 3 10 Sample Clock Timebase 3 10 External Clock 3 10 Timing Engines 3 11 Digital Triggering 3 11 Analog Triggering 3 11 Triggering and Filter Delay 3...

Страница 7: ...on and harshness NVH measurements High speed blast and pressure events Underwater transient acoustic measurements The PXIe 4480 offers a large set of sensor conditioning features including charge meas...

Страница 8: ...cessories Accessories for the PXIe 4480 and PXIe 4481 include the following shielded InfiniBand 12x cables SHB12X 6BNC 6 BNC analog inputs SHB12X 6MXLRM 6 mini XLR analog inputs SHB12X 6RJ50 6 RJ50 an...

Страница 9: ...I 0 5 G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G17 G18 G19 G20 G21 G22 G23 G24 G25 AI 0 AI 0 AI 1 AI 1 AI 2 AI 2 AI 3 AI 3 AI 4 AI 4 AI 5 AI 5 S1 S3 S5 S7 S9 S11 S13 S15 S17 S19 S21 S23...

Страница 10: ...to 5 AI 0 5 G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G17 G18 G19 G20 G21 G22 G23 G24 G25 AI 0 AI 0 AI 1 AI 1 AI 2 AI 2 AI 3 AI 3 AI 4 AI 4 AI 5 AI 5 S1 S3 S5 S7 S9 S11 S13 S15 S17 S19 S2...

Страница 11: ...ESERVED Reserved for accessories or other use Table 2 2 RJ50 Connector Pin Assignments RJ50 Modular Plug Pin Number Diagram Pin Signal Name Signal Description 1 NC No connect 2 AI Positive analog inpu...

Страница 12: ...ption 1 AI Positive analog input signal 0 to 5 2 AI Negative analog input signal 0 to 5 Table 2 4 mXLR Connector Pin Assignments mXLR Male Connector Pin Diagram Pin Signal Name Signal Description 1 GN...

Страница 13: ...ber The module is being accessed The Access LED flashes amber for 75 ms when the module is accessed Green Module is ready to be accessed by software Active Off Module is not acquiring or preparing to...

Страница 14: ...nalog Input Block Diagram Filter Select Charge Amplifier Lowpass Filter Charge Out Charge In Excitation Voltage 10 V 25 V Adj 9 5 V to 12 V EX 0 5 GND EX 0 5 Voltage Charge Voltage Charge 4mA 10mA 20m...

Страница 15: ...lock diagram Depending on the connector type connect the signals as described in the Connecting Signals section Figure 2 4 PXIe 4481 Analog Input Block Diagram AI 0 5 AI 0 5 1 62 M ADC 1 62 M 50 AC DC...

Страница 16: ...gnal source is floating use the pseudodifferential input configuration The pseudodifferential configuration provides a ground reference connection for the floating source by connecting a 50 resistor f...

Страница 17: ...setup Configure the channels based on the signal source reference or DUT configuration Refer to Table 2 7 to determine how to configure the channel For maximum flexibility the module s input configura...

Страница 18: ...oupling any DC offset present in the source signal is passed to the ADC The DC coupled configuration is usually best if the signal source has only small amounts of offset voltage or if the DC content...

Страница 19: ...able of parameters and sensor information TEDS sensors have two modes of operation Analog mode allows the sensors to operate as transducers measuring physical phenomena Digital mode allows you to writ...

Страница 20: ...V It is also possible to utilize the voltage excitation source to excite a passive full bridge strain gage as shown in Figure 2 9 Refer to Table 2 8 for a guide to connecting signals from the PXIe 448...

Страница 21: ...the attached signal source Common connection issues that can occur include open and short circuits An open circuit occurs if there is a disconnect between the signal source and the module resulting i...

Страница 22: ...ing acquired may be compromised while the PXIe 4480 4481 is displaying an overtemperature condition If an overtemperature condition is detected the hardware will turn the Active LED red and set a flag...

Страница 23: ...ta is decimated to the requested user sample rate and digital filters are applied to filter out frequency content above the Nyquist frequency Frequency domain mode is used any time the requested sampl...

Страница 24: ...ng frequency components above the Nyquist limit The undesirable effect of the digitizer modulating out of band components into the Nyquist bandwidth is aliasing The greatest danger of aliasing is that...

Страница 25: ...with that event is available at the output of the acquisition and filtering process This delay is called the group delay In order to simplify the process of acquiring data from the PXIe 4480 4481 modu...

Страница 26: ...passed through the same analog front end circuitry as in a frequency domain mode acquisition This means that time domain mode can use all of the same analog channel configurations available on the PXI...

Страница 27: ...e program it to the PXIe 4480 4481 It is required that the filter have the following characteristics Have between 1 and 36 coefficients Filters larger than 36 coefficients are not allowed due to hardw...

Страница 28: ...modulator noise Refer to the PXIe 4480 4481 Specifications for more information about the default filter and the performance of time domain mode when using the default filter Step response is affecte...

Страница 29: ...sample rate than 20 MS s When hardware decimates it does so by returning every nth sample coming out of the filter For example to produce a sample rate of 5 MS s n 4 the hardware drops the first thre...

Страница 30: ...al FIR filter always operates at 20 MS s regardless of the final rate returned to the user after decimation Therefore it is possible for aliasing to occur when using the lower sample rates supported b...

Страница 31: ...is transferred for every sample To improve efficiency the PXIe 4480 4481 supports packing the sample data so that this unused byte is eliminated This can be controlled by setting NI DAQmx Channel Pro...

Страница 32: ...idth for example at least 500 MB s slot bandwidth and a sufficient system bandwidth to operate all devices installed in the system If a PXI Express remote controller is selected it should use a x4 or...

Страница 33: ...n a reference triggered acquisition you configure the module to acquire a certain number of pre trigger samples and a certain number of post trigger samples Reference triggered acquisitions can theref...

Страница 34: ...eases as sample rates decrease Synchronization Some applications require tight synchronization between input and output operations on multiple modules Synchronization is important to minimize skew bet...

Страница 35: ...d Synchronization Synchronization Type to Slave and that of the Master to Master 7 Query DAQmx Timing More Synchronization Pulse Synchronization Time on all modules being synchronized choose the maxim...

Страница 36: ...y your systems accuracy and reliability requirements and provides warranty sparing and calibration services to help you maintain accuracy and minimize downtime over the life of your system Visit ni co...

Страница 37: ...tions Engineers make sure every question submitted online receives an answer Software Support Service Membership The Standard Service Program SSP is a renewable one year subscription included with alm...

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