Fundamentals of 8 versus 12-bit oscilloscopes and how to use modern 12-bit scopes
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.
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 16.8ms apart in time. Rather than capture the deadtime between these pulses, the Teledyne LeCroy WaveSurfer 4104HD 12- bit oscilloscope uses a sequence
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 982Hz. 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 10mV scale. The offset range scales with the vertical range of the oscilloscope. If greater offset range is required, Teledyne LeCroy has the RP40 6 0 rail probe with a 6 0 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).
mode acquisition which breaks the scope’s memory into a user- selected number of segments, five in this example. 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 21.8mV. 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
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: DigiKey
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
12 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). The upper grid shows the acquired signals in both 12-bit and 8-bit resolution overlaid. There is little observable difference between the
Figure 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: DigiKey
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: DigiKey
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