PXI Oscilloscopes/Digitizers

NI PXI Oscilloscopes and Digitizers

How PXI Oscilloscopes and Digitizers Work

A PXI digitizer converts an analog input waveform into a sequence of digital samples. It is installed in a compatible PXI or PXI Express chassis and controlled by an embedded controller or remote computer. Waveforms can be displayed, analyzed, transferred to system memory, stored to disk or evaluated automatically against test limits.

Unlike a standalone oscilloscope with an integrated display and front-panel controls, a PXI oscilloscope is software controlled. This makes it suitable for automated systems that combine waveform acquisition with signal generation, switching, programmable power, digital instruments and synchronized timing resources.

PXI Oscilloscope Compared with PXI Digitizer

The terms are often used together because the underlying hardware is similar. Selection should be based on bandwidth, sample rate, resolution, memory, triggering and software requirements rather than the word “oscilloscope” or “digitizer” in the product name.

Representative NI PXI Oscilloscope and Digitizer Models

This table represents major digitizer architectures rather than every model or hardware revision. Verify the exact part number, memory option, connector, chassis requirement, driver support and calibration status before ordering.

How to Select a PXI Oscilloscope or Digitizer

1. Start with Analog Bandwidth

Analog bandwidth defines the frequency range that the input front end can acquire with useful amplitude fidelity. Select bandwidth from the highest-frequency content that must be measured, not only the signal’s repetition frequency.

Fast digital pulses contain harmonics well above their clock or repetition rate. When rise-time measurement is important, a first planning estimate is:

Oscilloscope rise time ≈ 0.35 ÷ analog bandwidth.

The digitizer should normally have a faster rise time than the signal being measured. The exact bandwidth margin depends on the permitted rise-time and amplitude error.

2. Determine the Required Sample Rate

Sample rate defines how frequently the waveform is converted into digital data. Higher sample rates provide more samples across pulses, edges and high-frequency cycles, improving time-domain representation and measurement flexibility.

Nyquist theory establishes a minimum rate above twice the highest signal frequency, but practical waveform measurements commonly require additional sample-rate margin for accurate shape, timing and transient analysis.

The highest available sample rate is not automatically the best choice. Increasing the sample rate also increases memory consumption, bus traffic, storage requirements and processing workload.

3. Evaluate Vertical Resolution and Dynamic Performance

Vertical resolution determines the number of discrete amplitude levels available from the analog-to-digital converter. Higher resolution can improve the ability to distinguish small changes, but nominal ADC bits do not by themselves define measurement quality.

Also compare:

• Effective number of bits
• Signal-to-noise ratio
• Spurious-free dynamic range
• Total harmonic distortion
• DC gain and offset accuracy
• Input noise and range

A high-resolution digitizer such as the PXI-5922 may be preferable for precision and spectral measurements, while a higher-speed model may be more appropriate for fast edges and wideband transients.

4. Select the Number of Channels

Channel count should reflect the signals that must be captured at the same time. Typical multichannel applications include power-converter inputs and outputs, clock and data comparison, multiphase electrical systems, sensor arrays and parallel DUT testing.

When phase or time relationships are important, confirm that channels are sampled simultaneously. Also review channel-to-channel skew and whether all channels retain full performance when enabled.

5. Match the Input Range to the Signal

Select an input range that accommodates the expected waveform, DC offset, noise and transient overshoot. An unnecessarily wide range reduces the useful amplitude resolution, while an undersized range can cause clipping or overload.

Check whether the required range is supported with the selected input impedance and coupling mode. Maximum safe input and measurable full-scale range are separate specifications.

6. Confirm Input Impedance and Coupling

Common digitizer input configurations include 50 Ω and 1 MΩ impedance. A 50 Ω input is commonly used for terminated coaxial signal paths, while a 1 MΩ input is appropriate for compatible passive probes and higher-impedance sources.

Incorrect termination can change signal amplitude, introduce reflections or load the circuit being tested. Confirm whether the application requires AC or DC coupling and whether coupling options vary by input impedance or voltage range.

7. Calculate Record Length and Onboard Memory

Record length determines how long a waveform can be captured at the selected sample rate:

Required samples per channel = sample rate × acquisition duration.

Approximate waveform memory can be calculated as:

Memory = samples per channel × active channels × stored bytes per sample.

Use the transferred data width rather than only the ADC resolution. A 14-bit converter may transfer each sample in a 16-bit or wider data format.

Long records are useful for:

• Power-up and shutdown sequences
• Communication bursts
• Rare transient events
• Automated fault capture
• Intermittent production failures
• Long-duration waveform analysis

8. Review Triggering Requirements

Triggering determines which event initiates or qualifies waveform acquisition. Define the event of interest and confirm the required trigger source, level, slope, coupling and timing behavior.

Possible requirements include:

• Analog edge triggering
• Window or hysteresis triggering
• External digital triggering
• Software triggering
• PXI backplane trigger routing
• Pre-trigger and post-trigger acquisition
• Reference or stop triggering
• Multi-instrument synchronized triggering

Trigger performance should be considered together with the DUT control signals, stimulus generator, switch system and test sequence.

9. Determine Whether Segmented Acquisition Is Needed

Segmented or multirecord acquisition captures multiple triggered events while omitting unnecessary idle time between them. This can use onboard memory more efficiently and increase test throughput for repetitive pulses, packet bursts and production measurements.

Verify the minimum record length, rearm time, number of supported records and timestamp requirements for the selected model.

10. Calculate Continuous Data Throughput

High-speed digitizers can generate large data volumes. For continuous or repeated acquisition, estimate the payload rate:

Data rate = active channels × sample rate × transferred bytes per sample.

For example, two channels transferring two bytes per sample at 1 GS/s produce a theoretical payload of 4 GB/s before protocol, driver and file-system overhead.

The complete path must support the required sustained rate:

• Digitizer acquisition engine
• Module PCIe interface
• PXIe peripheral slot
• Chassis switch and backplane segment
• Controller or remote link
• System memory
• Storage device
• Analysis and display software

Bandwidth, Sample Rate and Resolution Tradeoffs

A single instrument does not necessarily maximize bandwidth, resolution, channel density and acquisition duration simultaneously. Selection should prioritize the characteristics that determine measurement quality for the actual signal.

Probes, Cables and Input Protection

The signal connection is part of the measurement system. Coaxial cables, probes, adapters, attenuation and external conditioning can limit bandwidth or introduce measurement error even when the digitizer itself meets the required specifications.

Confirm:

• Connector type and cable impedance
• Cable bandwidth and length
• Passive, active or differential probe requirements
• Probe attenuation and compensation
• Maximum input voltage and transient category
• Required DC blocking or external attenuation
• Grounding and common-mode voltage

Most PXI oscilloscope inputs are ground referenced and should not be treated as isolated inputs. Floating or high-common-mode measurements require an appropriate differential probe, isolation interface or other measurement method rated for the circuit.

PXI and PXIe Slot Compatibility

Traditional PXI digitizers use the PCI-based PXI backplane, while PXIe digitizers use PCI Express links. A traditional PXI module can be installed only in a compatible PXI or PXIe hybrid peripheral slot. A PXIe module requires a PXIe-compatible peripheral slot.

Before installation, verify:

• PXI, PXIe or hybrid slot type
• Available PCIe generation and lane width
• Bandwidth from the slot to the system controller
• Chassis power and cooling capability
• Number of adjacent slots required
• Front-panel cable clearance

High-speed digitizers should be positioned according to the chassis PCIe topology. Several modules installed behind the same chassis switch may share an upstream link and compete for bandwidth.

Multi-Digitizer Synchronization

Multiple digitizers can be synchronized when an application requires more channels than one module provides. A coordinated system may share reference clocks, sample-clock alignment and trigger signals through the PXI backplane.

NI-TClk supports synchronization of compatible modular instruments. The achievable alignment depends on the selected models, chassis, clock source, calibration, routing and software configuration.

Review:

• Reference clock frequency and accuracy
• Trigger distribution
• Channel-to-channel and module-to-module skew
• Sample-clock phase alignment
• Jitter requirements
• Single-chassis or multichassis architecture

Using a common chassis does not automatically guarantee phase alignment. Synchronization must be configured and verified for the complete system.

PXI Digitizer Compared with Other Measurement Modules

Use PXI data acquisition modules when the application primarily requires slower multichannel voltage or sensor measurements. Review PXI RF and wireless instruments when calibrated RF conversion, modulation analysis or vector signal functionality is required.

Software and Automated Waveform Analysis

NI PXI oscilloscopes and digitizers commonly use NI-SCOPE for instrument configuration, triggering, waveform acquisition, synchronization and data transfer. InstrumentStudio provides an interactive environment for supported devices, while LabVIEW and other supported programming environments can be used to build automated applications.

A typical software stack may include:

• NI-SCOPE: Instrument driver and programmatic acquisition API
• InstrumentStudio: Interactive configuration and waveform viewing for supported instruments
• NI Measurement & Automation Explorer: Device recognition, self-test and configuration
• LabVIEW: Automated acquisition, processing and user-interface development
• TestStand: Production-test sequencing and result management
• NI-TClk: Synchronization of compatible modular instruments

Depending on the installed NI-SCOPE version, supported APIs may also be available for C/C++, .NET and Python. Confirm the exact model, operating system, driver version and programming environment before changing an established system.

Waveform Processing and Measurements

Digitizer data can be used for automated time-domain and frequency-domain analysis. Common measurements include:

• Peak, average and RMS voltage
• Frequency and period
• Rise time and fall time
• Pulse width and duty cycle
• Overshoot and undershoot
• Propagation delay and channel skew
• FFT and spectral analysis
• Total harmonic distortion
• Signal-to-noise ratio
• Custom waveform-mask and pass/fail testing

The selected sample rate, record length, windowing, averaging and filtering methods affect calculated results. Automated measurements should be validated against the required engineering method and test limits.

Calibration and Measurement Uncertainty

Digitizer accuracy can depend on gain, offset, input range, temperature, timebase, noise, distortion, probe accuracy and cable performance. Self-calibration can correct selected internal errors but does not replace scheduled external calibration where traceability is required.

A complete uncertainty review may include:

• Digitizer gain and offset accuracy
• Timebase accuracy and jitter
• Probe ratio accuracy
• Cable and adapter loss
• Input loading
• Noise and distortion
• Temperature drift
• External calibration uncertainty

Specify calibration documentation and required calibration interval before quotation when the hardware will be used in production, regulated or accredited laboratory environments.

Typical Applications

• Electronic design validation and debugging
• Semiconductor characterization and production test
• Power converter, inverter and motor-drive testing
• Power-rail and startup-sequence analysis
• Automotive ECU and power-electronics validation
• Pulse, timing and transient measurements
• Radar, avionics and defense signal acquisition
• IF and wideband signal recording
• Communications waveform analysis
• Multichannel scientific experiments
• Automated production test equipment
• Long-duration fault and event capture

Legacy PXI Digitizer Replacement

Replacing a legacy digitizer requires more than selecting a newer instrument with a higher sample rate. The original module may be tightly integrated with software, test fixtures, waveform limits and historical production data.

Compare the following before approving a replacement:

• PXI or PXIe slot compatibility
• Analog bandwidth and frequency response
• Sample rate and sampling behavior
• Vertical resolution and noise performance
• Input range, impedance and coupling
• Connector type and cable requirements
• Trigger modes and trigger routing
• Record length and onboard memory
• NI-SCOPE and operating-system support
• NI-TClk synchronization behavior
• Calibration procedure and measurement correlation

A newer model may provide better specifications but still require code changes, fixture modifications and system requalification. In validated production systems, maintaining the original model can sometimes present less engineering risk than changing the instrument architecture.

PXI Oscilloscope and Digitizer Procurement Guide

For pre-owned digitizers, request device-recognition, self-test and functional acquisition results. Verify the input connectors, trigger connector, backplane connector and module cooling surfaces for damage or contamination.

FAQ

What is the most important PXI digitizer specification?

There is no single most important specification. Analog bandwidth, sample rate, resolution, input range, channel count and acquisition duration must be evaluated together for the signal being measured.

Is a higher sample rate always better?

No. The sample rate must be sufficient for the signal bandwidth and required timing detail. Excessive sample rates increase memory, transfer, storage and processing requirements without necessarily improving the final measurement.

How much oscilloscope bandwidth do I need?

Bandwidth should be based on the highest-frequency signal content and required waveform fidelity. Fast pulses can require substantially more bandwidth than their repetition frequency suggests.

What is the difference between the PXI-5922 and PXIe-5164?

The PXI-5922 emphasizes flexible resolution and precision measurements, providing resolution up to 24 bits at lower bandwidth and sampling rates. The PXIe-5164 emphasizes faster wideband acquisition with two 14-bit channels, 400 MHz bandwidth and rates up to 1 GS/s.

When should I choose the PXIe-5105?

The PXIe-5105 is suitable when eight simultaneously sampled channels and moderate waveform bandwidth are more important than the highest possible sample rate.

Can multiple PXI digitizers be synchronized?

Yes, when the selected modules, chassis and software support the required clock and trigger architecture. NI-TClk can synchronize compatible modular instruments, but the complete configuration and achievable skew must be verified.

Can a PXI digitizer stream continuously at its maximum sample rate?

Not automatically. Continuous performance depends on the module interface, chassis topology, controller, system memory, storage and software. Maximum finite-record acquisition rate and maximum sustained streaming rate are different specifications.

Can I use a PXI digitizer for RF measurements?

A sufficiently fast digitizer can acquire compatible IF or wideband electrical signals. Calibrated RF measurements, frequency conversion, vector signal analysis and modulation testing may require dedicated PXI RF hardware.

Can a traditional PXI digitizer be installed in a PXIe chassis?

Only when the chassis provides a compatible hybrid peripheral slot. PXIe digitizers require PXIe-compatible slots. Check the exact module and chassis slot map before ordering.

Recommended PXI Oscilloscopes and Digitizers

Related NI PXI Solutions

Browse additional PXI and PXIe modules for complete automated test systems. Combine digitizers with PXI signal generators for stimulus-response testing or review the complete NI PXI systems category for chassis and controller options.

NI PXI Oscilloscopes and Digitizers from PXISOURCE

PXISOURCE supports engineers, system integrators and procurement teams with PXI digitizer selection, bandwidth and sample-rate review, chassis and controller compatibility, probe and cable matching, software verification and legacy-system replacement. Send the signal requirements, channel count, acquisition duration, chassis model and software version for a technically matched quotation.