Top 10 Things to Consider When Selecting a Digitizer

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Top 10 Things to Consider When Selecting a

Digitizer/Oscilloscope

Publish Date: Dec 06, 2011 | 53 Ratings | 2.72 out of 5 | PDF

Overview

The modern day digital storage oscilloscope is dramatically different from the cathode

ray oscilloscope German scientist Karl Ferdinand Braun invented in 1897. Technology

advances continue to provide new features that make the oscilloscope more useful to

engineers, but one of the most significant transformations of the oscilloscope was its

transition into the digital domain, which enabled powerful features such as digital signal

processing and waveform analysis. Digital oscilloscopes today include a high-speed,

low-resolution (typically 8 bits) analog-to-digital converter (ADC), defined controls and

display, and a built-in processor to run software algorithms for common measurements.

Digitizers, on the other hand, leverage the latest processing power and high-resolution

display available from a PC, while providing all the other features that comprise an

oscilloscope. Since digitizers are PC-based, you have the advantage of being able to

define your instrument functionality in software. As a result, you can use a digitizer notjust for oscilloscope measurements, but also for custom measurements, and even as a

spectrum analyzer, frequency counter, ultrasonic receiver, or other instrument. With

their open architecture and flexible software, digitizers provide several advantages over

traditional stand-alone oscilloscopes. However, digitizers and oscilloscopes have many

similarities and share a common set of considerations for selection.

This paper discusses the top 10 things you should keep in mind if you are considering a

new digitizer/oscilloscope.

Table of Contents

1. Bandwidth

2. Sampling Rate
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3. Sampling Modes

4. Resolution and Dynamic Range

5. Triggering

6. Onboard Memory

7. Channel Density

8. Multiple Instrument Synchronization

9. Mixed Signal Capability

10.Software, Analysis Capability, and Customizability

11.Conclusion

1. Bandwidth

Bandwidth describes the frequency range of an input signal that can pass through the

analog front end with minimal amplitude loss - from the tip of the probe or test fixture to

the input of the ADC. Bandwidth is specified as the frequency at which a sinusoidal

input signal is attenuated to 70.7 percent of its original amplitude, also known as the -3

dB point.

In general, it is recommended that you use a digitizer with bandwidth at least two times

the highest frequency component in your signal.

Oscilloscopes and digitizers are commonly used for measuring rise time of signals such

as digital pulses or other signals with sharp edges. These signals are composed of

high-frequency content. To capture the true shape of the signal, you need a high-

bandwidth digitizer. For instance, a 10 MHz square wave is composed of a 10 MHz sine

wave and an infinite number of its harmonics. To capture the true shape of this signal,

you must use a digitizer with bandwidth large enough to capture several of these

harmonics. Otherwise, the signal is distorted and your measurements incorrect.
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Figure 1: A high-bandwidth digitizer is important when capturing a waveform with

high-frequency components

As a rule of thumb, use the following formula to figure out the bandwidth of your signal

based on its rise time (defined as the time taken to transition from 10 to 90 percent of

signal amplitude).

Figure 2: Rise time defines the time a signal takes to go from 10 to 90 percent of

its full-scale value. Rise time and bandwidth are directly related, and one can be

calculated from the other using the equation above.


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Ideally, you should use a digitizer with three to five times the bandwidth of your signal

as calculated in the equation above. In other words, your digitizers rise time should be

1/5 to 1/3 of your signals rise time to acquire your signal with minimal error. You can

always backtrack to determine your signals real bandwidth based on the following

formula:

= measured rise time, = actual signal rise time, = digitizers rise time

Back to Top

2. Sampling Rate

In the previous section, you learned about bandwidth, which is one of the most

important specifications of a digitizer or oscilloscope. However, high bandwidth can be

much less useful if the sample rate is insufficient.

While bandwidth describes the highest frequency sine wave that can be digitized with

minimal attenuation, sample rate is simply the rate at which the analog-to-digital

converter (ADC) in the digitizer or oscilloscope is clocked to digitize the incoming signal.

Bear in mind that sample rate and bandwidth are not directly related. However, there is

a rule of thumb for the desired relationship between these two important specifications:

Digitizers real-time sample rate = 3 to 4 times digitizers bandwidth

Nyquist theorem states that to avoid aliasing, the sample rate of a digitizer needs to be

at least twice as fast as the highest frequency component in the signal being measured.

However, sampling at just twice the highest frequency component is not enough to

accurately reproduce time-domain signals. To accurately digitize the incoming signal,

the digitizers real-time sample rate should be at least three to four times the digitizers

bandwidth. To understand why, look at the figure below and think about which digitized

signal you would rather see on your oscilloscope.
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Figure 3: The figure on the right shows a digitizer with a sufficiently high sample

rate to accurately reconstruct the signal, which will result in more accurate

measurements.

Although the actual signal passed through the front-end analog circuitry is the same in

both cases, the image on the left is under sampled, which distorts the digitized signal.

On the other hand, the image on the right has enough sample points to accurately

reconstruct the signal, which will result in a more accurate measurement. Since a cleanrepresentation of the signal is important for time domain applications such as rise time,

overshoot, or other pulse measurements, a digitizer with a higher sample is beneficial

for these applications.

Back to Top

3. Sampling Modes

There are two main sampling modes real-time sampling and equivalent-time sampling

(ETS).

Real-time sample rate is the one discussed above, which describes the clock rate of the

ADC and indicates the maximum rate an incoming signal can be acquired in a single-

shot acquisition. On the other hand, equivalent-time sampling is a method of


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reconstructing a signal based on a series of triggered waveforms that are each acquired

in single-shot mode. The advantage of ETS is that it offers a higher effective sample

rate. The downside, however, is that it takes more time and is applicable only for

repetitive signals. Note that ETS does not increase the digitizers analog bandwidth, and

instead is only useful when you need to reconstruct the signal at a higher sample rate. A

common implementation of ETS is random-interleaved sampling (RIS), which is

available on most NI digitizers as listed in the table below.

Digitizer

Model

Channels Real-Time

Sample

Rate

Equivalent-

Time

Sample

Rate

Bandwidth Resolution

NI 5152 2 2 GS/s 20 GS/s 300 MHz 8 Bits

NI 5114 2 250 MS/s 5 GS/s 125 MHz 8 Bits



NI 5105 8 60 MS/s 60 MHz 12 Bits

NI 5922 2 500 kS/s

to 15 MS/s

6 MHz 16 to 24 Bits

User-

Defined

Back to Top

4. Resolution and Dynamic Range

As described above, digital oscilloscopes and digitizers both have ADCs that convert

the signal from analog to digital. The number of bits returned by the ADC is the

digitizers resolution. For any given input range, the number of possible discrete levels
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used to represent the signal digitally is 2b, where b is the digitizers resolution. The input

range is divided into 2b steps and the smallest possible voltage that is detectable by the

digitizer is denoted by (Input Range/2b). For example, an 8-bit digitizer divides a 10 Vpp

input range into 28 = 256 levels of 39 mV each, while a 24-bit digitizer divides the same

10 Vpp input range into 224 = 16,777,216 levels of 596 nV (approximately 65,000 times

smaller than in the 8-bit case).

One of the reasons for using a high-resolution digitizer is to measure small signals. The

question is sometimes asked, why not just use a lower resolution instrument and a

smaller range to zoom in on the signal to measure small voltages? However, many

signals have both a small signal and a large signal component. Using a large range, you

could measure the large signal but the tiny signal would be in the noise of the large

signal. On the other hand, if you use a small range, then youd clip the large signal and

your measurement would be distorted and invalid. Thus, for applications that involve

dynamic signals (signals with large and small voltage components), you need a high-

resolution instrument, which has a large dynamic range (the ability of the digitizer to

measure small signals in the presence of large ones).

Traditional oscilloscopes typically use ADCs with 8-bit resolution, which is not enough

for many applications involving spectral analysis or dynamic signals such as modulated

waveforms. Such applications may benefit from one of the several high-resolution

digitizers highlighted in the table below. These include the NI PXI-5922 flexible-

resolution digitizer, which was awarded 2006 Test Product of the Year by Test and

Measurement World. This module uses linearization techniques to provide the industrys

highest dynamic range of any digitizer or oscilloscope.

Digitize

r

Model

Resolutio

n

Channels Real-Time

Sample Rate

Bandwidth


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NI

5922

16 to 24

Bits

(User-

Defined)

2 500 kS/s to 15

MS/s

6 MHz

NI

5122

14 Bits 2 100 MS/s 100 MHz

NI

5124

12 Bits 2 200 MS/s 150 MHz

NI

5105

12 Bits 8 60 MS/s 60 MHz

Back to Top

5. Triggering

Typically, oscilloscopes and digitizers are used to acquire a signal based on a certain

event. The instruments triggering capability allows you to isolate this event and capture

the signal before and after the event. Most digitizers and oscilloscopes include analog

edge, digital, and software triggering. Other triggering options include window,

hysteresis, and video triggering (featured on theNI 5122,NI 5124andNI 5114).

High-end digitizers feature fast rearm times between triggers, which enables a multi-

record capture mode, where the digitizer captures the specified number of points upon a

given trigger, quickly rearms and waits for the next trigger. A fast rearm time ensures

that the digitizer does not miss the event or trigger. Multi-record mode is very useful in

capturing and storing only the data that you need, thereby optimizing the use of the

onboard memory as well as limiting the activity of the PC bus.

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6. Onboard Memory
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Often times, data is transferred from the digitizer or oscilloscope to the PC for

measurements and analysis. Although these instruments can sample at their maximum

rate, which can be in the several GS/s range, the rate which the data can be transferred

to the PC is limited by bandwidth of the connecting bus such as PCI, LAN, GPIB, etc.

While today none of these buses are able to sustain multi-GS/s rates, this may become

a non-issue as PCI Express and PXI Express evolve to allow several GB/s data rates.

If the interface bus can not sustain continuous data transfer at the sample rate of the

acquisition, onboard memory on the instrument provides the ability to acquire the

signals at the maximum rate and later fetch the data to the PC for processing.

Deep memory not only increases acquisition time, but also provides frequency-domain

benefits. The most common frequency-domain measurement is the fast Fourier

transform (FFT), which shows a signals frequency content. If an FFT has finer

frequency resolution, discrete frequencies are more easily detected.

In the equation above, there are two ways to improve the frequency resolution reduce

the sample rate or increase the number of points in the FFT. Reducing the sample rate

often is not the ideal solution because this will also reduce your frequency span. In this

case, the only solution is to acquire more points for the FFT, which requires deeper

onboard memory.


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Figure 4: More onboard memory lets you sample at a high sample rate for a

longer period of time to capture more points. Using more points when

calculating an FFT results in greater frequency resolution.

Digitizer

Model

Channels Real-Time

Sample

Rate

Equivalent-

Time

Sample

Rate

Bandwidth Memory

Options

NI 5152 2 2 GS/s 20 GS/s 300 MHz 16 MB,

128 MB,

512 MB,

1 GB

NI 5114 2 250 MS/s 5 GS/s 125 MHz 16 MB,

128 MB,

512 MB
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NI 5124 2 200 MS/s 4 GS/s 150 MHz 16 MB,

64 MB,

512 MB,

1 GB

NI 5122 2 100 MS/s 2 GS/s 100 MHz 16 MB,

64 MB,

512 MB,

1 GB

NI 5105 8 60 MS/s

60 MHz 16 MB,

128 MB,

512 MB

NI 5922 2 500 kS/s to

15 MS/s

6 MHz 16 MB,

64 MB,

512 MB,1 GB

Back to Top

7. Channel Density

An important factor in an oscilloscope or digitizer purchasing decision is the number of

channels on the instrument or the ability to add channels by synchronizing multiple

instruments. Most oscilloscopes have two to four channels, each simultaneously

sampled at a certain rate. It is important to be wary of how sample rate is affected when

using all the digitizer channels. This is because of a commonly used technique called

time-interleaved sampling, which interleaves multiple channels to achieve a higher

sample rate. If the digitizer or oscilloscope uses this method and you are using all the

channels, you may not be able to acquire at the maximum acquisition rate.
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The number of channels required entirely depends on your particular application.

Frequently the traditional two to four channels may not be sufficient for a given

application, in which case there are two options. The first one is to use a higher channel

density product such as the eight-channel (simultaneous)NI 510512-bit, 60 MS/s, 60

MHz digitizer. If you are unable to find an instrument that matches your resolution,

speed, and bandwidth requirements, you should consider using a platform that lets you

scale your test system by providing tight synchronization and allows triggers and clocks

to be shared. While its practically impossible to synchronize multiple boxed

oscilloscopes over GPIB or LAN due to high latency, limited throughput and need for

external cabling, PXI provides a superior solution. PXI is an industry standard that adds

world-class synchronization technology to existing higher speed busses such as PCI

and PCI Express.

Figure 5: Using synchronization technology, you can create high-channel-count

digitizers. The picture above shows a system that can acquire up to 136 phase

coherent channels. Multiple chassis can be synchronized for even higher

channel counts.

NI digitizers including theNI PXI-5105andNI PXI-5152provide a technology called T-

Clock, which provides synchronization accuracy in the tens of picoseconds. For

instance, using this technology, you can build a 34-channel (simultaneous), 1 GS/s

oscilloscope usingNI PXI-5152digitizers in a single 18-slot chassis. Likewise,

multipleNI PXI-5105digitizers can be synchronized to provide a system with 136

synchronized channels, each running at 60 MS/s with 12-bit resolution (see figure
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above). For higher channel count, PXI also provides timing modules to scale to multiple

chassis for up to 5000 channel count systems.

Back to Top

8. Multiple Instrument Synchronization

Almost all automated test and many benchtop applications involve multiple types of

instruments such as digitizers, signal generators, digital waveform analyzers, digital

waveform generators, and switches.

The inherent timing and synchronization capability of PXI and NI modular instruments

allows you to synchronize all these types of instruments without the need for external

cabling. For instance, you can integrate a digitizer (such as theNI PXI-5122) and anarbitrary waveform generator (such as theNI PXI-5421) for performing parameter

sweeps, which is useful for characterizing the frequency and phase response of the

device under test. The entire sweep can be automated, which obviates the need for

manual setting of parameters on the scope and generator followed by offline analysis. A

modular approach with PXI results in orders of magnitude improvement in speed and

improves your efficiency by letting you focus on the results rather than the cumbersome

steps needed to get those results.

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9. Mixed Signal Capability

The same T-Clock technology that enables creating systems with up to 136

synchronized channels in a single PXI chassis or up to 5000 channels using multiple

chassis (as described in the section above) also allows for synchronization of

instruments of different types. For instance, an NI digitizer can be T-Clock synchronized

with signal generators, digital waveform generators, and digital waveform analyzers forbuilding mixed signal systems.
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Figure 6: The VI above demonstrates an application that has been configured for

mixed signal oscilloscope (analog and digital input) functionality. In addition,

digital or analog output functionality could be added to the application and all

instruments could still be synchronized.

Rather than settle for a mixed-signal oscilloscope with limited digital functionality, you

can use a modular PXI digitizer with arbitrary waveform generators and digital waveform

generator/analyzers to build a complete mixed-signal application with the benefits of

both an oscilloscope and a logic analyzer.

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10. Software, Analysis Capability, and Customizability

Determining software and analysis capabilities is very important when choosing a

modular digitizer or a stand-alone oscilloscope for your application, and this factor may

help you choose between the two instruments.

Stand-alone oscilloscopes are vendor-defined while digitizers are user-defined and


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flexible in the applications they can solve. A boxed oscilloscope provides many of the

standard functions that are common to the needs of many engineers. As you can

imagine, these standard functions will not solve every application, especially for

automated test applications. If you need to define the measurements your oscilloscope

makes, you might select a modular digitizer, which leverages the PC architecture while

letting you customize an application to your requirements, instead of the fixed

functionality of a stand-alone oscilloscope.

NI digitizers are all programmed using the free NI-SCOPE driver software. This driver

comes with more than 50 prewritten example programs that highlight the full

functionality of any NI digitizer, and the included NI-SCOPE Soft Front Panel provides a

familiar interface similar to an oscilloscope. The same hardware can also be

programmed for both common and custom measurements in a broad range of

applications using programming languages including NI LabVIEW, LabWindows/CVI,

Visual Basic, and .NET. The driver also supports express configuration-based functions

within LabVIEW.

Figure 7: Using preconfigured Express blocks lets you quickly set up your

digitizer to quickly acquire data. NI LabVIEW SignalExpress is an interactive


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environment that lets you acquire, analyze, and log your data with no

programming required.

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11. Conclusion

Although modular digitizers and stand-alone oscilloscopes are both used to acquire

voltages, the instruments offer different benefits. However, the considerations discussed

above are important when purchasing either instrument. Thinking ahead about

application requirements, cost constraints, performance, and future expandability can

help you choose the instrument that best meets all your needs.

Documents

Top 10 Things to Consider When Selecting a Digitizer