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High-Speed Event and Defect Detection with Real-Time Response

Posted: 03 Oct 2003     Print Version  Bookmark and Share

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Application Note 177

National InstrumentsTM and ni.comTM are trademarks of National Instruments Corporation. Product and company names mentioned herein are trademarks or trade

names of their respective companies. For patents covering National Instruments products, refer to the appropriate location: Help;Patents in your software, the

patents.txt file on your CD, or ni.com/patents.

342179A-01 ) 2002 National Instruments Corporation. All rights reserved. January 2002

High-Speed Event and Defect Detection with

Real-Time Response

Overview

Event and defect detection (transient signal capture) is by nature an unpredictable practice requiring fast, accurate

detection. A wide array of applications, including semiconductor reliability testing, disk drive manufacture, neurology,

physics, meteorology, seismology, nondestructive testing (NDT), material characterization, and many others, require

measurements of rapidly occurring events and/or transient periodic signals. A few high-frequency transient signals

from these disciplines that require fast, well-resolved measurements are:

7 Electrostatic discharge (ESD)

7 Neurological action potentials

7 LIDAR and RADAR signals

7 NDT ultrasonic echo signals and eddy currents

7 Reflected laser signals from disk drive surfaces for defect detection

7 Breakdown (quasi and full) events in semiconductor oxides

NI 5112

The NI 5112 two-channel high-speed digitizers from National Instruments come with unique event detection

capabilities that can detect events or transient signals continuously over months and timestamp these events with

nanosecond accuracy. With a maximum sampling rate of 100 MS/s, an analog input bandwidth of 100 MHz and deep

memory of up to 32 MS/channel, an NI 5112 can digitize thousands of transients with resolution high enough to extract

detailed information from the signal.

Event Detection from Seconds to Months

Suppose you need to capture the following signal output from a transducer (Figure 1). Note that the signal spans an

hour or so. During this period, a total of six `spikes' occurred, indicating significant change in the application or

experiment. The main objectives are to determine the times these spikes occurred relative to each other and to look at

the spikes in detail. The signal between the spikes is not relevant.

The measurement device, in this case a digitizer, should be capable of three tasks:

7 Continuously monitor the signal for spikes during the duration of the application or experiment

7 Determine the times the spikes occur at relative to each other

7 Digitize at a high rate to resolve the details of the spikes

Application Note 177 2 ni.com

For example, in NDT applications a microsecond impulse stimulus is applied at a 15 kHz rate, typically, and the

material response is measured. Depending on the application, you might be interested in events such as threshold

violations or the whole response (so-called A, B, C, and D-images or scans) to the impulse stimulus. Essentially there

are two measurements that can be made:

7 To determine stimulus propagation in the material, the whole response to the impulse stimulus is acquired and

images of the propagation are constructed

7 To detect structure flaws, threshold violations can be detected and measured

Figure 1. Transient Signal Illustrating Spikes Occurring at Random Intervals

A common approach to the task is to digitize the entire signal from start to finish. Although this approach yields more

complete information, it has two major drawbacks:

1. In acquiring the entire signal, the measurement system cannot react to event detection in real-time. In numerous

applications, the detection of events and defects has to trigger something else and the response time has to be

extremely fast.

2. You need to supply a large high-speed memory buffer to store the digitized signal. If it is not critical to resolve the

finer details of the signal, you can digitize the signal at a slower rate and thus capture the signal for a longer period.

In most applications, however, both the times at which the spikes occur and the details of the spikes are important.

In NDT applications, for example, shapes of responses plays an important role. You could have many spikes (or

hills and valleys) in a signal. These indicate reflections, which can be translated into the geometry of objects or

flaws. A quick calculation of sampling at 100 MS/s for one minute indicates the need of 100 MB of data storage

at 8-bit resolution. Sampling at MHz rates for minutes imposes demands for memory buffer sizes similar to

common desktop PC Zip drive memory sizes, which escalates the cost of measurement systems.

So how do you resolve these issues?

) National Instruments Corporation 3 Application Note 177

Multirecord Capability and Deep Memory with

Real-Time Response to Events

An intelligent approach is to make the digitizer smarter by making it selective in what it digitizes. Because the NI 5112

performs multirecord acquisitions, one can capture multiple triggered waveforms without software intervention.

Multirecord acquisition is a practical and efficient way of capturing the relevant portions of the signal. Each record is

stored in a separate buffer in the onboard memory of the digitizer. Thus, with accurate analog triggering circuitry, one

can capture the signal at the correct times. Furthermore, when each record is triggered, the NI 5112 can output a

TTL-level signal to trigger other devices in real time as well for real-time response to event detection.

For example, in semiconductor reliability testing, gate oxide breakdown events are of interest. When a full breakdown

event occurs, the voltage stress applied to the failing part needs to be shutdown soon thereafter. In most reliability

testing systems, many parts are tested either in parallel or in series. If they are tested in parallel, a failing part must be

switched out of the circuit to prevent it from drawing a current large enough to alter the voltage stress on the other parts.

If they are tested in series, then a special shorting circuit must be switched in to continue testing the rest of the parts.

A real-time response to a full breakdown event is critical in either parallel or series reliability testing.

Timestamping over Months with Nanosecond Accuracy

Users doing transient capture over any period of time also need to know exactly when the transient occurred. You can

use multirecord acquisition to easily capture all six spikes shown in Figure 1 by setting up the digitizer to acquire six

records and to trigger on a rising slope and signal level of +0.3 V. To determine when these spikes occurred with respect

to each other, you use the "timestamping on events" feature of the NI 5112 digitizer. The NI 5112 uses a clock to

accurately timestamp the trigger event to within 2 ns. On the NI 5112, this clock is a high-precision 48-bit counter

clock. Using timestamps, you can correlate multiple records or even multiple acquisitions. You can, for instance,

determine the time between acquisitions, or between multiple records for up to 130 days and have each acquisition

temporally correlated to within 2 ns accuracy.

In meteorology studies, for instance, lightning strike detection is important. Here the randomness of the lightning strike

makes it imperative that your measurement system be capable of monitoring the thunderstorm for all lightning strikes

that occur. The storm may span minutes or hours and the lightning strikes can occur seconds or minutes apart. Using

the timestamping abilities of the NI 5112, you can monitor the entire storm, detect the lightning strikes, and know when

they occurred.

Application Note 177 4 ni.com

Illustration of Multirecord Acquisition

Consider the signal output in Figure 2. The signal reflects events occurring at 7 kHz intervals. If we sample at 100 MS/s

and acquire a single record of 500 kS, we capture 31 events for a period of 5 ms.

Figure 2. Single-Record Capture of High-Speed Phenomena

Figure 3 shows the signal from Figure 2 digitized in multirecord acquisition mode. With a memory buffer of a total of

200 kS (4 kS/record), you can capture 50 events spanning a period of more than 8 ms with each event timestamped. In

multirecord acquisition mode, one can capture a larger number of events with optimal use of the onboard memory. As

pointed out earlier, when each event is detected, the NI 5112 can also output a TTL signal as a real-time response.

) National Instruments Corporation 5 Application Note 177

Figure 3. Multirecord Acquisition of the Signal in Figure 2

Conclusions

Using the following features of the NI 5112, you can address applications calling for high-speed event and defect

detection:

7 Maximum sampling rate of 100 MS/s

7 Input analog bandwidth of 100 MHz

7 Deep memory with multirecord acquisition mode of up to 65,536 records

7 Timestamping of records to nanosecond accuracy

7 Real-time response output to event or defect detection

For more information, go to ni.com/info and enter the info code exqmei to download example programs illustrating

multirecord acquisition with the NI 5112, the NI 5112 User Manual, a customer solutions article from Seagate

Technologies using the NI 5112 for defect detection on disk drives surfaces, and other related information.

Application Note 177 6 ni.com

Appendix

The following section is an excerpt from the NI-SCOPE Software User Manual which details the available NI 5112

triggering modes available for event, defect, and transient signal detection.

Triggering Modes of the NI 5112

There are several triggering methods for the NI 5112. The trigger source can be one of three modes.

1. Analog trigger - the trigger can be an analog level that is compared to the input signal or an auxiliary analog signal.

2. Digital trigger - the digital triggers are TTL-level signals with a minimum pulse-width requirement of 10 ns.

3. Software trigger - you can call a software function to trigger the device.

The main trigger modes useful for event, defect, and transient signal detection are analog and digital triggering, with

analog triggering being the prevalent trigger mode for this class of applications.

Figure A-1 shows the different trigger sources.

Figure A-1. Different Trigger Sources

For further information on RTSI and PFI lines refer to the NI 5112 User Manual.

a. Analog Trigger Circuit

+

COMP

COMP

-

Analog

Trigger

Circuit

High Level

Low Level

GAIN

Analog

Input

ATC_OUT

7

2

Software

ATC_OUT

RTSI <0..6>

PFI1, PFI2

Trigger

b. Trigger Sources

) National Instruments Corporation 7 Application Note 177

Analog Triggering Modes

Analog Trigger Circuit

The analog trigger on the NI 5112 operates by comparing the analog input to an onboard threshold voltage. This

threshold voltage is the trigger value, and can be set to any voltage within the input range. A hysteresis value associated

with the trigger is used to create a trigger window the signal must pass through before the trigger is accepted. Triggers

can be generated on a rising-edge or falling-edge condition as illustrated in Figures A-2 and A-3.

Positive-Slope Hysteresis Analog Triggering Mode

A positive-slope hysteresis trigger is generated when the signal crosses below the voltage specified by the trigger level

parameter minus the hysteresis parameter and then crosses the trigger level.

Figure A-2. Positive-Slope Hysteresis Analog Triggering Mode

Negative-Slope Hysteresis Analog Triggering Mode

A negative slope hysteresis trigger is generated when a signal crosses above the voltage specified by the trigger level

parameter plus the hysteresis value and then crosses the trigger level.

Figure A-3. Negative-Slope Hysteresis Analog Triggering Mode

Hysteresis

Value

Trigger

Value

Trigger

Trigger Events ----

Trigger

Value

Hysteresis

Value

Trigger

Trigger Events ----

Application Note 177 8 ni.com

Edge Triggering

An edge trigger occurs when a signal crosses a trigger threshold you specify. The slope can be specified as either

positive (on the rising edge) or negative (on the falling edge) to the trigger. Figure A-4 shows edge triggers.

Figure A-4. Positive and Negative and Edge Trigger Modes

Window Triggering

A window trigger occurs when a signal either enters or leaves a window you specify.

Figure A-5. Entering Window Trigger Mode

Figure A-6. Leaving Window Trigger Mode

Trigger Level

Negative Edge Trigger

Positive Edge Trigger

High Level

Low Level

Trigger Trigger

High Level

Low Level

Trigger Trigger

) National Instruments Corporation 9 Application Note 177

Digital Triggering

Digital triggering is useful for applications where another device initiates operation of the application and the digitizer

is required to commence sampling. NDT applications where ultrasound impulse stimuli are applied to materials at high

repetition rates require digital triggering.

A digital trigger occurs on either a rising edge or falling edge of a digital signal. Digital triggering is available on the

RTSI lines, PFI lines, and the PXI Star Trigger line. For more information on these lines refer to the NI 5112 User

Manual.

342179A-01 Jan02

*342179A-01*





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