Knowing the quality of the scope’s measurement system is paramount when you need to have accurate measurement results. 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 scope’s update rate, intrinsic jitter, and noise floor, all of which enable better measurements. For scopes with bandwidths in the GHz range, another quality metric involves characterizing a scope’s ENOB.
What is ENOB in the first place? It stands for Effective Number of Bits and is really the measure of how well your oscilloscope accurately represents the captured waveform.
The higher the ENOB, the better the oscilloscope sees the signal the way the components in your design see the signal.
Bits of Resolution and Effective Number of Bits
The ADC is the most recognized component on the oscilloscope. It converts the analog data to digital data. It drives the oscilloscope’s bits of resolution. It is defined by its sample rate and its signal to noise ratio. Typically, scopes have 8 bits of resolution, although recently oscilloscopes have added 10 and 12 bit ADCs.
Effective number of bits (ENOB) is a measure of the dynamic performance primarily associated with signal quantization levels of your oscilloscope.
While some oscilloscope vendors may give the ENOB value of the oscilloscope’s ADC by itself, this figure has no value. ENOB of the entire system is what is important.
While the ADC could have a great ENOB, poor oscilloscope front-end noise would dramatically lower the ENOB of the entire measurement system.
Oscilloscope ENOB isn’t a specific number, but rather a series of curves. I am often asked, “what is the ENOB of a specific Keysight oscilloscope?” Many vendors simply state a specific single number for ENOB, for example, an ENOB of 5.5. The reality of the situation is this is just not how effective number of bits work. They are frequency dependent. So, it may be 5.5 at one specific frequency setting but is probably not 5.5 across the entire bandwidth of the oscilloscope.
ENOB was established by IEEE in 1993 as a measurement of an oscilloscope’s signal integrity and measurement accuracy.
It directly correlates to an oscilloscope’s signal to noise ratio. A higher ENOB will provide better oscilloscope measurements for Jitter, eye height and width, and amplitude. ENOB is a metric, and does not indicate what is causing signal integrity issues.
Effective number of bits is directly related to the ADC within an oscilloscope. In general, the bits of resolution within the ADC determines the quantizing levels for your oscilloscope as shown in Figure 1.
Bits of Resolution
At 1 Volt, Full Scale
1 LSB =
At 16 mV, Full Scale
1 LSB =
Figure 2: Oscilloscope specification comparison
Increasing the number of ADC bits makes each quantizing step size smaller, so the maximum error is minimized.
ENOB is measured as a fixed amplitude sine wave at varying frequencies. Each curve is created at a specific vertical setting while frequency is varied. ENOB calculations are easy to make.
- First, input a perfect sine wave, capture it on a scope and measure the deviation from the result vs the input. For example, input a sine wave from a PSG at 1 GHz into the scope and measure the 1 GHz sine wave.
- Then fit it against a perfect 1 GHz sine wave.
The difference between the data record and best fit sine wave is assumed to be signal error. ENOB considers noise, ADC non-linearities, interleaving errors, and other error sources.
What erodes the bits of resolution?
ENOB is primarily impacted by noise and distortion. Noise of course effects your signal-to-noise ratio and distortion impacts the total harmonic distortion. If the base noise of an oscilloscope is greater than the quantizing levels of the ADC, then there is no way for the scope to accurately represent the digital signal level to the least significant bit.
ENOB values will always be lower than the oscilloscope’s ADC bits. In general, a higher ENOB is better. However, a couple cautions need to accompany engineers who look exclusively at ENOB to gauge signal integrity. ENOB doesn’t consider offset errors or phase distortion that the scope may inject. So, it is also important to look at the base noise of an oscilloscope as well as its frequency response (amplitude flatness), phase linearity and gain accuracy to get a complete picture of the accuracy of an oscilloscope.
In general, by choosing an oscilloscope with superior ENOB, you are choosing a scope with better signal integrity.
You not only impress your colleagues but you also get more accurate waveform shapes, more accurate and repeatable measurements, wider eye diagrams and less jitter.
Figure 3: ENOB of the S-Series DSOS104A 1 GHz real-time oscilloscope from 100 MHz to 1 GHz.
For more information on determining measurement quality, check out the Scopes University S1E4 video, Determining Oscilloscope Measurement Quality.