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Fundamentals of 8-Bit Versus 12-Bit Oscilloscopes and How to Use Modern 12-Bit Scopes

By Art Pini

Contributed By Digi-Key's North American Editors

There are many applications where designers and test and measurement engineers need to make wide dynamic range measurements to look at very small signals in the presence of large signal amplitudes. Power integrity assurance, echo location and ranging systems like radar and sonar, medical imaging systems such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), as well as non-destructive testing using ultrasound, are among these types of applications.

Oscilloscopes are of course the go-to tool for making these measurements in the system development and prototyping stages, but these are primarily limited by the vertical resolution of the scope’s front-end. For example, an 8-bit oscilloscope has a dynamic range of 256:1, so on a 1 volt range, the theoretical minimum signal is 3.9 millivolts (mV). When trying to view millivolt level ripple signals on a 3.3 volt bus, higher sensitivity and offset range are needed. Also, when using high attenuation probes to prevent circuit loading, signal levels will be attenuated at the scope input and so will be hard to measure unless the instrument has a high resolution.

The problem is that higher sensitivity in the presence of a larger signal or offset requires higher resolution scopes, and these typically are costly, especially for a quality scope with low noise inputs. Higher resolution without a lower noise floor is useless.

What designers and developers need is a reasonably priced 12-bit scope with a low front-end noise floor. One solution to this need for high resolution with front-end low noise at low cost is Teledyne LeCroy's WaveSurfer 4000HD series of high definition oscilloscopes. This article will discuss the difficulty of high dynamic range measurements, the role of high definition oscilloscopes, and how they can be used effectively for high dynamic range measurements.

Oscilloscope vertical resolution

Oscilloscope vertical resolution refers to the ratio of the highest input signal the oscilloscope can handle to the smallest signal amplitude it can detect. Resolution is generally quantified by the number of bits in the analog-to-digital converter (ADC). The resolution is equal to 2 raised to the power of the number of bits. As such, an 8-bit converter has a resolution of 28 or 256:1. A 12-bit converter has a resolution of 4096:1, which is 16 times greater than an 8-bit converter.

For years, digital oscilloscopes offered 8-bit resolution in higher bandwidth oscilloscopes. This is because of an engineering tradeoff in ADCs that makes resolution, measured by the number of bits, inversely proportional to the ADC’s maximum sampling rate. About eight years ago, Teledyne LeCroy pioneered 12-bit oscilloscopes termed high definition or ‘HD’ oscilloscopes. They have recently added the WaveSurfer 4000HD series to the HD product line. The series includes four oscilloscopes with bandwidths of 200, 350, 500, and 1000 megahertz (MHz). They all sample at 5 Gigasamples per second (GS/s) which is very respectable for a 12-bit oscilloscope. Internal mixed-signal digital inputs, DVM, function generator and frequency counter are available to round out this multi-instrument offering. The family offers all of this along with 12-bit resolution at a reasonable price point.

Of course, increasing the resolution of an oscilloscope requires more than simply changing the ADC. It also requires improving the signal-to-noise ratio (SNR) of the oscilloscope’s front-end so that the sensitive ADC is not filled with noise. A 12-bit scope with an 8-bit front-end is still an 8-bit scope. The WaveSurfer 4000HD oscilloscope family, however, has successfully implemented the HD concept. Its 12-bit vertical resolution, coupled with a low noise front-end, delivers 12-bit performance that, in the real world, actually is 16 times more sensitive on any given amplitude range than an 8-bit scope.

12-bit vs 8-bit measurements

HD oscilloscopes are intended for measurement applications that have waveforms exhibiting high dynamic range. These are measurements that simultaneously include a high amplitude signal component along with low signal levels. Consider an application such as an ultrasound range finder. It transmits a high amplitude pulse, then waits for a low amplitude echo from the target. The high amplitude signal determines the voltage range of the scope’s vertical amplifier that is required. The resolution and system noise determine the smallest echo signal that can be measured (Figure 1).

Image of ultrasonic signal rendered with both 12-bit and 8-bit vertical resolutionFigure 1: The same ultrasonic signal rendered with both 12-bit and 8-bit vertical resolution. The upper trace comprises both versions of the full signal overlaid on each other. The lower traces show a zoomed portion of the waveform. There is little difference looking at the high amplitude signal components, but the lower level signals show a clear advantage for the 12-bit rendering. (Image source: Digi-Key Electronics)

The upper grid shows the acquired signals in both 12-bit and 8-bit resolution overlaid. There is little observable difference between the overlaid waveforms. The center grid shows the 12-bit waveform expanded both horizontally and vertically. The bottom grid is the same portion of the 8-bit waveform. The loss in detail for the low-level signals in the 8-bit version is quite apparent. Note also that the signal peaks in the 12-bit rendering show obvious differences which are lost in the 8-bit version.

High dynamic range measurement applications

High dynamic range measurements include all echo location and ranging applications like radar, sonar, and LiDAR. Many medical imaging technologies like NMR and MRI are based on similar techniques: bouncing a high-level transmitted pulse off the body and acquiring and analyzing echoes or stimulated emissions due to the transmitted signal. Similarly, ultrasonic-based technology like non-destructive testing (NDT) uses reflected ultrasonic pulses to discover cracks and faults in solid materials.

Power integrity measurements, where small, millivolt, signals like noise and ripple are measured on bus voltages of between 1 and 48 volts, or greater, also need high dynamic range scopes.

Consider measuring signals from even a simple ultrasonic range finder or electronic tape measure (Figure 2). The ultrasonic range finder emits five pulses for each measurement spaced about 16.8 ms apart in time. Rather than capture the deadtime between these pulses, the Teledyne LeCroy WaveSurfer 4104HD 12-bit oscilloscope uses a sequence mode acquisition which breaks the scope’s memory into a user-selected number of segments, five in this example.

Image of Teledyne LeCroy WaveSurfer 4104HD oscilloscope (click to enlarge)Figure 2: A Teledyne LeCroy WaveSurfer 4104HD oscilloscope used in the acquisition of a 40 kilohertz (kHz) ultrasonic range finder signal. At top it shows five pulses for each measurement spaced about 16.8 milliseconds (ms) apart. (Image source: Digi-Key Electronics)

Each segment acquires one transmitted pulse and time stamps the trigger point. The upper trace is the acquired waveform with each segment marked. A zoom trace (bottom grid) shows a selected segment, in this case the first one. The table at the bottom of the screen shows the time stamps marking the time of each trigger, the time since segment 1, and the time between segments. The transmitted pulse has a peak to peak amplitude of 362 mV, while the reflected echo has a peak to peak amplitude of only 21.8 mV. It is this difference in amplitude that makes this a high dynamic range measurement. The figure uses an echo amplitude that can be seen on the screen, but 12-bit resolution captures this signal at amplitudes lower than the pixel rendering of the scope, as seen in Figure 1.

Power integrity measurements also require scopes with high dynamic range. Ripple voltage measurements require being able to measure millivolt signals riding on power buses. In the Figure 3 example, the upper trace measures ripple on a 5 volt bus. The ripple voltage is 45 mVpeak-to-peak riding on a bus voltage of 4.98 volts as directly read using the WaveSurfer 4104HD’s measurement parameters P2 and P1, respectively. The lower trace is the fast Fourier transform (FFT) of the ripple voltage showing a harmonic rich spectrum with a fundamental component of 982 Hz.

Image of power integrity measurement on a 5 volt bus for a daughter card (click to enlarge)Figure 3: A power integrity measurement on a 5 volt bus for a daughter card shows the ripple voltage and the FFT of the ripple. (Image source: Digi-Key Electronics)

In addition to high resolution, this application requires an oscilloscope with a good offset range. In this example, the scope has a ±8 volt offset range on the 10 mV scale. The offset range scales with the vertical range of the oscilloscope. If greater offset range is required, Teledyne LeCroy has the RP4030 rail probe with a 30 volt offset range. Rail probes are specifically designed for probing of low impedance power rails. They feature large built-in offset, high input impedance, and low attenuation and noise. This particular probe has a bandwidth of 4 gigahertz (GHz), an attenuation of 1.2, and an input impedance of 50 kilohms (kΩ).

HD oscilloscopes can also handle higher voltage measurements like those encountered in switched mode power converters (SMPCs). SMPCs include power supplies, inverters, and industrial controllers. They control power by adjusting the duty cycle or frequency of a switched waveform. The main measurements involve the voltage across and the current through the power switching device(s), usually a field effect transistor (FET). To help developers with SMPC measurements, Teledyne LeCroy provides application-specific software and voltage and current probes. A typical measurement is shown in Figure 4.

Image of characterizing an SMPC’s losses (click to enlarge)Figure 4: Characterizing an SMPC’s losses involves measuring the voltage and current of the power switch devices and then calculating power loss in each phase of the power switching cycle. (Image source: Digi-Key Electronics)

The current, the pink trace, is measured with a Teledyne LeCroy model CP030A current probe. This clamp-on probe has a maximum current input of 30 amperes (A) and a bandwidth of 50 MHz. The voltage waveform, shown as a beige trace, is measured using a Teledyne LeCroy HVP1306 high voltage differential probe. This probe is rated for a maximum CATIII voltage of 1000 volts at a bandwidth of 120 MHz. Both probes are recognized by the WaveSurfer scope, which automatically scales the measured waveforms to account for the probe’s gains and units of measure.

The power measurement software automates the most common SMPC measurements. Figure 4 shows the calculation of device power dissipation as the yellow trace. This is calculated from the current and voltage waveforms for the whole switching cycle. Measurement parameters isolate and display turn-on, conduction, turnoff, and off-state losses based on the acquired waveforms, with each zone clearly delimited by a color overlay. It also shows the total loss from all zones as well as the switching frequency. Other available measurements, in addition to device measurements shown in the Figure, help characterize control loop dynamics, line power, and performance characteristics such as efficiency.

The 12-bit resolution is also useful in power measurements when calculating the drain source resistance (Rds) of the power FET. This requires measuring a voltage on the order of one or two volts on a waveform with a peak-to-peak swing on the order of 400 volts. The WaveSurfer 4000HD series is compatible with all Teledyne LeCroy probes compatible with the scope’s bandwidth range (Figure 5).

Image of Teledyne LeCroy WaveSurfer 4000HD oscilloscopeFigure 5: The Teledyne LeCroy WaveSurfer 4000HD oscilloscopes are compatible with the company’s extensive line of probes, including the power measurement related probes shown here. (Image source: Teledyne LeCroy)

Wide range of applications set higher standard for “workhorse” scope

The WaveSurfer 4000HD series is not limited to only high dynamic range applications. It is an excellent scope in its own right and may set a higher standard for “workhorse” scopes. It is a good choice for low-speed serial data troubleshooting, offering analysis packages and probes to support serial buses like SPI, I2C, UART-based links, as well as automotive buses like LIN, CAN, and FLEXRAY.

Serial bus analysis requires the ability to acquire and decode the bus protocol and to read the data content (Figure 6). The color-coded overlay shows each packet. The red overlay indicates the address data while the blue overlays mark the data packets. The address and data content appear within the overlay. Decode information is available in binary, hex, or ASCII format. The table at the bottom of the display summarizes acquired transactions showing time relative to the trigger point, address length, address, direction (read or write), the number of packets, and the data content. Triggering can be based on activity, address, data content, or a combination of address and data.

The Teledyne LeCroy ZD200 active differential probe is a good choice for measuring serial data. This 10:1 probe has an input impedance of 1 Megaohm, has a bandwidth of 200 MHz, and can handle differential voltages of up to 20 volts and common mode voltages of up to 50 volts. It is especially well matched to differential buses such as CAN.

Image of low-speed serial trigger and decode of the I2C bus (click to enlarge)Figure 6: Low-speed serial trigger and decode of the I2C bus includes the ability to read the data content of the bus. Shown is the acquisition and decoding of an I2C bus signal for both a read and a write operation. (Image source: Digi-Key Electronics)

Conclusion

While 8-bit oscilloscopes will always have a place, there are many applications that could use the HD and wide dynamic range of a true 12-bit oscilloscope, but their relatively high cost has kept them out of reach of many designers and test engineers. The Teledyne LeCroy WaveSurfer 4000HD series oscilloscopes go a long way toward addressing that problem with a much lower cost entry point.

It provides HD measurements based on 12-bit vertical resolution, a 5 GS/s maximum sampling rate, and a low noise floor. It is also compatible with Teledyne LeCroy probes and analysis software packages. As such, the scopes open the door to cost effective high-dynamic-range measurements and move its availability from the research lab to the engineer’s bench or factory floor.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Art Pini

Arthur (Art) Pini is a contributing author at Digi-Key Electronics. He has a Bachelor of Electrical Engineering degree from City College of New York and a Master of Electrical Engineering degree from the City University of New York. He has over 50 years experience in electronics and has worked in key engineering and marketing roles at Teledyne LeCroy, Summation, Wavetek, and Nicolet Scientific. He has interests in measurement technology and extensive experience with oscilloscopes, spectrum analyzers, arbitrary waveform generators, digitizers, and power meters.

About this publisher

Digi-Key's North American Editors