Why ENOB Matters

Mon, 09/27/2010 - 11:10am
Chris Eriksen, Rohde & Schwarz
Why ENOB Matters


Why ENOB Matters

There are two kinds of oscilloscope users: Those who know why ENOB matters and those who should. Before we get into the meaning and importance of ENOB (Effective Number of Bits), we should spend a few words on "NOB" itself. One important spec of any Analog to Digital Converter (ADC) is its resolution - or the number of bits used to describe the analog signal values.

During signal acquisition, the ADC assigns a binary, numerical voltage value at the sampling interval. There are a finite number of discreet voltage levels available based on the resolution of the ADC. For an 8-bit system, there are 2^8, or 256 possible levels. In reality, system noise and distortion reduces the "effective" number of bits to something that is less than the ideal. Since the converter is able to represent signal levels below the system noise floor, the less significant bits can be in the noise and therefore not contain useful information.

Effective Number of Bits, or ENOB, is a composite specification that helps us quantify all the sources of noise and distortion in an oscilloscope. The higher the ENOB, the better the overall accuracy and signal fidelity of the system. As you look at an oscilloscope’s total signal path, there are many possible sources of noise, error, non-linearity, and distortion. Performance starts with the quality of the front-end. Many of today's realtime oscilloscopes are not capable of maintaining high bandwidth performance at low volts/division settings.

To show these small signals with accuracy, it is difficult and expensive to develop amplifiers that maintain performance as we get below 10mv/div. In many cases, scope manufacturers employ software zoom or bandwidth limiting to get around this problem. Another significant source of noise and distortion in realtime scopes is sample interleaving. This is something that I will address in more detail in a future blog, but it is appropriate to cover the topic briefly as it relates to ENOB. Interleaving is a widely used technique to increase the sampling rate by using multiple ADC's in parallel.

The sample clocks of adjacent ADCs are delayed and interleaved to produce a high-resolution waveform. Errors in the phase delay and mismatch of the multiple ADCs result in phase and amplitude errors, and ultimately reduced ENOB. This adversely affects a variety of measurements, including risetime, jitter, and FFT. The new RTO oscilloscopes from Rohde & Schwarz have taken many steps to reduce noise and distortion - resulting in the industry's best ENOB. This starts with a high-performance frontend that delivers industry-best noise performance at full bandwidth down to 1mv/div. This provides improved overall accuracy, better test margins, higher dynamic range and increased sensitivity.

Chris Eriksen is the Business Development Manager for Oscilloscope Products and Broad Market Initiatives for Rohde & Schwarz Inc.


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