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Home > News > Company News > Comment: ADCs can provide the .....

Comment: ADCs can provide the signal detail for everyday devices

  • Author:Ella Cai
  • Release on:2017-09-07
High vertical resolution analogue-to-digital converters provide the signal detail for everyday devices, writes Andreas Grimm of Rohde & Schwarz.

Today, even the most commonplace electronic designs involve highly integrated components and an array of once-specialist technologies.

To measure small parts of signals, in the presence of large amplitudes, oscilloscopes with high vertical resolution and wider bandwidth are needed.
Signal detail

The incorporation of 10-bit analogue‑to-digital converters (ADCs), as well as averaging and oversampling modes in new, entry-level digital oscilloscopes can help everyone see signal detail much more precisely.

When selecting digital oscilloscopes for electronic research, education and industry, inevitably users must square the technical requirements – bandwidth, sample rate and memory depth, for example – with the realities of productivity, usability and budget.

In the past they have had to compromise at the lower end of the performance spectrum, typically with a ceiling of 8-bit resolution. These days, that is often not enough.

In modern devices voltage levels are progressively falling and power-supply efficiency requirements continue to rise. For the test and measurement engineer, this means tolerances when measuring electronic circuits are becoming even tighter. With greater accuracy comes the need for higher resolution on measurements.

p28 table WEBMoving up from 8-bit to 10-bit ADCs makes a substantial step in the right direction, as shown in Table 1. The example calculation for a 1V signal yields a resolution of about 1mV for the 10-bit converter, compared with nearly 4mV for the 8-bit ADC.

Developers who want to characterise signals of power electronics or who work with high dynamic ranges, can benefit from four times better resolution.

The downside is that until recently, ADCs offering 10-bit were only available in instruments costing €5,000 ($5,500) and more.

We have the technology

Fortunately, the technology is now available for making higher-resolution ADCs with the necessary sample rate and low noise. State-of-the-art manufacturer-specific ADCs can be produced for the same cost as off‑the‑shelf models, bringing a four‑fold improvement in resolution to the value end of the market.

The game is changing, with the introduction of instruments like the new R&S RTB2000 oscilloscope. This uses a proprietary 10-bit ADC, which Rohde & Schwarz originally developed for its Scope Rider, an isolated portable, multifunctional oscilloscope that provides options such as MSO and serial triggering, and decoding of serial protocols.

The same ADC in the R&S RTB2000 makes it possible to achieve a higher vertical resolution compared with oscilloscopes that have an 8-bit ADC.

To make effective use of these smaller signal details, the oscilloscope must also reduce the front-end noise. A 10-bit ADC achieving 1mV resolution would offer no advantage in an oscilloscope with 4mV noise. The impact of noise is often reduced either by averaging the signal or employing high-resolution (hi-res) acquisition modes.

Reducing noise levels

One of the two most common methods for increasing resolution is the use of averaging. This is typically provided as a mathematical functionality on the oscilloscope, which relies on multiple consecutive acquisitions. The more acquisitions are averaged, the better the resolution.

A further advantage of averaging is that it also reduces wideband noise, giving a more accurate representation of the measured signal.

However, there are two important disadvantages of this method:

It takes time to acquire and process the data – several hundred times to get the required resolution

It only works with repetitive signals, such as clock signals.

Most signals do not repeat periodically, so the method of averaging over several acquisitions cannot be used. To overcome this drawback oscilloscopes increasingly support special acquisition (sometimes also called decimation) modes.

One example is the hi-res acquisition method which exploits oversampling: resulting from a higher ADC sampling rate than the minimum required for accurate signal reconstruction, keeping in line with the Nyquist criteria. Additional samples are averaged and the resulting waveform is displayed. Since averaging is done within a single acquisition, this method leads to higher vertical resolution whether for repetitive signals or nonrepeating events.

Oversampling – the theory

Theory states that the gain in vertical resolution is 0.5*ln(N)/ln(2), where N represents the oversampling factor. For example, an oversampling factor of 16 is required to gain two additional bits.

What this means is that for oscilloscopes based on an 8-bit ADC, having a maximum sampling rate of 1Gsample/s, will have a decimated sampling rate of 62.5Msample/s at 10‑bit vertical resolution in hi-res mode.

Test p 29 WEBFigure 1 shows how the vertical resolution changes for higher or lower decimated sample rates. Theory says that low digital bandwidth will be the main hindrance to hi-res mode.

In the real world, the efficacy of higher ADC resolution can be demonstrated by using an exponentially damped sine function as a test signal.

This is a highly dynamic signal: a relatively strong pulse with damped amplitude to both sides. A signal of this type is available as a predefined function in many waveform generators.

The signal is connected to two scopes enabling a comparison of how well they characterise small details (Figure 2). Conventional, older instruments are represented by the R&S HMO2024, which uses a good off-the-shelf 8-bit ADC. The same signal is fed in to the new R&S RTB2000, which has an integrated 10-bit ADC developed by Rohde & Schwarz.

The four times higher resolution of the R&S RTB2000 ADC combined with the low-noise front-end gives more detailed information and allows more accurate measurements. If the zoom is set so that vertically and horizontally the same signal segment can be seen in both oscilloscopes, the difference becomes even more visible. On the HMO, you can see the least significant bit of the 8-bit A/D converter, represented by the step-like display.

The value oscilloscope also boasts a 10.1-inch capacitive touchscreen – twice the display size and up to 10 times the number of pixels of comparable scopes in this price band. As well as providing a better view of the signals under test, capacitive touch technology allows the user to operate the instrument more quickly and efficiently.

Innovations like these are making it easier to measure small portions of signals in the presence of large amplitudes. That is just what is needed for characterising today’s highly integrated components and varied technologies.