Oscilloscopes and ENOB

When oscilloscope users choose which oscilloscope to use for critical measurements, knowing the quality of the scope’s measurement system is paramount.  While banner specs like bandwidth, sample rate, and memory depth provide a basis of comparison, these specifications alone don’t adequately describe oscilloscope measurement quality.   Seasoned scope users will also compare a scopes update rate, intrinsic jitter, and noise floor, all of which enable better measurements.  For scopes with bandwidth in the GHz range, another quality metric involves characterizing a scope’s analog-to-digital converter (ADC) using effective number of bits (ENOB).  When selecting which scope to use, how important is ENOB and how effective is ENOB at predicting a scope’s measurement accuracy?

Designing oscilloscope architectures for measurement accuracy involves both front-end and ADC technology blocks.  A scope’s front end conditions a sampled signal so that the ADC can properly digitize the signal.  The front end consists of attenuator, pre-amplifier, and path routing.

Engineers who design scopes spend significant effort designing front-ends that have flat frequency responses, low noise, and desired frequency roll-offs.  Due to unique requirements for ADC technology, each scope vendor designs their own ADCs.  Development of a new front end or ADC requires significant investment.  Therefore, the resulting technology blocks will typically be used across multiple scope families and generations.  Scope design teams maximize a scope’s accuracy when these technology blocks induce the least change to the measurement of sampled signals.    

While users can characterize the combination of the ADC and front-end, users can’t easier characterize the technology blocks individually.  There are many ways to measure an oscilloscope’s front end measurement quality.   Oscilloscope vendors typically will use noise measurements and ENOB as useful characteristic for determining how well a scope’s front end and ADC are designed.  It is often beneficial to consider the entire oscilloscope performance, instead of evaluating just ENOB or noise floor in isolation.

Characterizing an oscilloscopes noise floor at different vertical settings and offset provides an excellent criterion in determining a scope’s measurement quality.  These measurements tell the user how effective the scope’s design team was in designing a quiet front-end and ADC converter.  Oscilloscope noise adds unwanted jitter and erodes design margins.  Typically the higher the bandwidth of the oscilloscope the more internal noise the oscilloscope produces as the scopes are accepting cumulative noise from higher frequencies that are rejected by the lower frequency roll-off of lower-bandwidth scope.  A straightforward method of characterizing a scope’s noise is to disconnect all inputs and see measurement the RMS voltage readings while varying both vertical sensitivity and offset.  

IEEE defined a method for determining the goodness of ADCs using ENOB. Today’s oscilloscopes typically will use to ADC architectures, pipelined or flash.  Pipelined ADCs use two or more steps of subranging to achieve higher sample rate, for instance the 90000A Series oscilloscope has a 20GSa/s ADC, which combines 80 subranges of 256MSa/s to achieve the high sample rate.  Interestingly and contrary to common wisdom, some scopes provide more accurate measurements when not running at fastest sample rate, due to additional interleaving distortion that can occur at the fastest sample rates and the addition of high frequency noise.  Flash ADCs have a bank of comparators sampling the input signal in parallel, each firing for their decoded voltage range. The comparator bank feeds a logic circuit that generates a code for each voltage range*  Each ADC technology has its own inherent limitations, for instance flash ADCs are more prone to linearity errors, while pipelined ADCs typically will have more interleaving error.  IEEE created the ENOB standard to help users determine the goodness of various ADCs.  

Scope vendors will internally characterize standalone ADCs.  They also characterize overall ENOB of a scope system.  The resulting system ENOB will be lower than the ENOB of a standalone ADC.  As a scope’s ADC is part of an overall system, and can’t be used independently, only ENOB results from the overall system are useful.  

Users will generally use less than the full 8-bits of a scope’s ADC.  For example, to take advantage of the entire 8-bit vertical range, users would have to scale waveforms to consume the entire vertical range.   This makes reading a signal more difficult, and the user runs the risk of driving the ADC into saturation, which causes undesired effects.  For a signals that is scales to take 90% of the vertical range, the user reduces the scope’s 8-bit converter to 7.2 bits (90%*8 bits).  Front-end noise, harmonic distortion, and interleaving distortion will further reduce the effectiveness of the scope’s ADC.