Cover story


Oscilloscopes and high resolution – no longer a contradiction


F


or many years, the 8-bit oscilloscope architecture has been – and still is – the optimal solution for most of the applications that run on it. However, this architecture can reach its limits with an increasing


number of power measurements or signal analyses with a high dynamic range. For applications that require a 12-bit architecture, other parameters are also important, such as noise performance of the front-end. The 12-bit architecture has certain advantages over the 8-bit structure, which, however, is sufficient for many oscilloscope applications. It is therefore important to consider what the advantages of a 12- bit structure are and whether they are really needed. Applications that require higher resolution largely concern the measurement of signals with a high dynamic range. This means that these can be signals with a very high amplitude, which also contain the smallest changes in amplitude, which also must be measured. With 8-bit oscilloscopes, a high-resolution mode with 12 bits can be activated for signal low-noise measurement, which is calculated mathematically in the oscilloscope after


the signal has been recorded. Oversampling is used here, which generates an average value from the finer amplitude values that are now occurring with a higher resolution. This averaging reduces noise components so that finer amplitude deviations can be displayed. However, the architecture remains 8 bit (= 28 = 256 values). The second disadvantage is that the bandwidth is significantly reduced depending on the time setting. In addition, the noise behaviour is adapted to an 8-bit structure. RIGOL’s DHO4000 series offers the possibility to switch


the device to a vertical scaling of 1mV/DIV and to scale it up to 100µV/DIV. The minimum noise performance (with 20MHz bandwidth limitation) in this range is less than 18µArms


(see


Figure 1, with 1mV/DIV). Even the smallest voltage signals or current values can be measured here (e.g., via a suitable impedance shunt). RIGOL’s DHO1000 and DHO4000 series use a 12-bit


Figure 1: DHO4000series – low noise performance below 18µVrms


architecture, which is realised with the new “Centaurus” ASIC chipset developed by RIGOL. The number of vertical values thus increases from 256 levels to 212 = 4096 values, which corresponds to 16 times finer vertical resolution compared to an 8-bit architecture. A compromise, e.g. related to the bandwidth reduction as with the high-res mode, is not necessary. In addition to the improved vertical resolution, the DHO4000 has a very high real-time sampling rate of up to 4GSa/s and a maximum memory of 500Mpts. This means that there is an optimal combination of very high vertical and horizontal resolution. In addition, due to the large memory, a large number of signals can be displayed with one acquisition. The chipset consists of two ASICs, of which the “θ-Centaurus” was developed for the analogue front end to enable the required bandwidth, the impedances of 1MΩ and 50Ω, a very fast overvoltage protection and very low noise. This ASIC has a very large dynamic range of up to 97dB and offers very high linearity with a total harmonic distortion (THD) of -55dBc. In addition, this chip has a very low noise performance to enable the 12-bit resolution, which is a very essential point in the 12- bit platform.


The second chip called “θ-Centaurus” is a high-precision


Figure 2: Measurement of a switching power supply with load voltage: yellow, current: pink


08 February 2023 www.electronicsworld.co.uk


digital signal processor chip, which has a 12-bit ADC, allowing sampling rates up to 4GSa/s. The chip is also characterised by its very high temperature stability of 3.6µV/°C and a very high synchronisation (multichip timing) of different signals with 33fs. Like the DS70000 series (5GHz/20 GSa/s), the DHO series uses the new UltraVision III platform. For many applications that require very high resolution, the bandwidth is in the lower range of the oscilloscope, where the effective bit value is the same as the ADC’s – 12 bits. However, it is sometimes necessary to know the resolution up to the maximum bandwidth. This value is defined in a frequency-dependent Effective Number of Bits (ENOB) value. With the DHO4000,


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