In this second chapter of “Best in test” series we will focus on the analog front-end (FE) which, as mentioned by Pierre-François the project manager in his interview, is one of the technology breakthrough of the signal analyzer winner, the M9703A.
For the R&D engineering team, the analog front-end (FE) is one of the most critical parts of the digitizer design. Every stage of the design process will influence the quality of the signal conditioning, and hence the signal integrity and data conversion quality. This is why you find in Agilent high speed digitizers very carefully designed analog FEs, from a well-defined and innovative architecture, to a meticulously implemented layout, followed by a rigorous qualification. But let us introduce you the analog FE engineer master, Neil, to speak about it. READ MORE
Neil, tell us a bit about your experience. How long have you been concentrating on analog FE design?
Neil: I’m embarrassed to tell you the number of years – I’ll just say it’s longer than most Special Reserve Scottish Single Malt Whiskies are left to mature! I started in FE design for scopes, then digitizer FEs for (at that time) Acqiris, who is now Agilent.
What is the purpose of the analog front-end and what type of functionalities does it offer?
Neil: The FE’s job is to scale and offset the Input Signal so that it is correctly positioned for optimal ADC performance. This will normally include single ended to differential conversion, and any other range of possible options such as choice of 50 Ω/1 MΩ input impedance, AC/DC coupling and switchable bandwidth filters. A trigger pickoff is often added for “Internal” trigger operation.
Why do not all of the FE’s offer all of these functionalities?
All engineering is a compromise – and FE design is one of the most challenging. Some functionalities are simply not compatible – for instance a 1 MΩ input impedance FE which also has a very high bandwidth with low noise and low distortion (Or if some genius out there knows how to do it – please let me in on the secret !). Also – as in all analogue design – less complexity in a FE is always better than more complexity from the point of view of preserving signal fidelity. The expertise lies in finding the right compromise.
Another constraint for us is that we have very tight physical space and power restrictions for our FEs – which is not generally a problem for oscilloscope manufacturers. This can also lead to a reduced number of options.
What are the other objectives and challenges behind the design of an analog front-end?
One challenge that is overlooked (but always expected by the customer) is having some form of Input Overload protection. It is of course possible to blow up anything if you try hard enough, but we must try and protect up to some reasonable level. For high bandwidth designs this input protection can create performance issues.
The required single ended to differential signal conversion is just in itself not an easy thing to do well. All previous designs used either a Balun ( transformer ) coupled input – which would then be automatically AC coupled. This gives optimal analogue performance – but a restricted range of usage scenarios. The second option is the use of an active single ended to differential amplifier. This gives true DC to HF functionality, but with an inevitable signal degradation of noise and distortion, especially at high frequency.
What is in your opinion the key thing that distinguishes a good design from a bad one?
As the Channel analogue performance is a combination of the FE and ADC behaviour, a good design is one that does NOT degrade the ADC performance too much, while offering a maximum of functionality. This has become much more difficult as ADC technology is progressing rapidly, and now 12 bit converters exist with very good analogue performance, but which also have high input bandwidths and sample rates. The module specification should show the channel performance over the whole useable frequency range, and these always come down to ENOB (effective number of bits), SNR (signal-to-noise ratio), SINAD (signal-to-noise and distortion ratio), SFDR (spurious-free dynamic range), DNL/INL (differential/integral non-linearity ), etc.
You have apparently succeeded to design a versatile FE on the M9703A which still guarantees extremely high performance over the full input frequency range (typically 58 dB SNR, more than 9 ENOB, 63 dBc SFDR). How did you achieve this?
The M9703A features an innovative DC-coupled input architecture for the single ended to differential conversion process. This combines the best analogue performance features of the AC type at high frequency, with the functionality of the DC type at low frequencies. This allows the very best noise and distortion performance over the whole Input Frequency range. It also means we can offer a very large range of Input Offset capability which drastically increases the general purpose functionality of the module. The available offset range is +/- 2 x the Full Scale input range, for both 1V and 2V Full Scale ranges.
As for Input Overload Protection – the M9703A has been evaluated by customers who frequently have large signal overvoltages and they have found the module to resist without being damaged – whereas other digitizers or oscilloscopes did not.
What is the future in analog FE design? Do you still see open space for technology improvements?
One essential area of IC technology that has not kept pace with ADC improvements is signal amplifier technology. Some IC manufacturers are addressing this problem right now. Over time, the FE design process will become more plug and play, but putting these components together will always require design expertise, which is what sets us apart from the competition !Tags: analog front end design m9703a share