Learning Library · Oscilloscope specs, explained · 2
ENOB: effective number of bits
The bit count on an oscilloscope's datasheet describes its analog-to-digital converter (ADC). The bits you can actually use depend on everything in front of it as well: amplifiers, attenuators, and the noise they add. Effective number of bits, or ENOB, measures what is left.
Top: the captured signal. Middle: the error left after subtracting a perfect sine, magnified, with the ideal ADC's ±½ step band in green. Bottom: label bits vs. effective bits.
From noise to bits
A perfect N-bit converter digitizing a full-scale sine wave still has an error, because every sample is rounded to the nearest step. That rounding noise sets the best possible signal-to-noise ratio.
A real instrument adds its own noise and distortion. Measure the signal to noise and distortion ratio (SINAD) of the whole system, then run the formula backwards. The answer is the resolution of a perfect ADC that would perform the same.
When the test signal is below full scale, the standard method adds back the difference in dB before converting, so the result still describes the full-scale range. That correction is built into the readout here.
Try this
- Turn the noise to its minimum. ENOB climbs toward the ADC's bit count and stops just below it, because rounding noise never goes away.
- Raise the noise. Once it is larger than one step, the error trace spills outside the green band and every extra bit of ADC resolution stops helping. Switch between 12-bit and 14-bit to see how little changes.
- Keep the noise fixed and change the vertical scale. At sensitive settings the same noise is a bigger share of the screen, so ENOB falls.
Reading an ENOB specification
ENOB is only meaningful with its test conditions, because it changes with every one of them: the input frequency, the bandwidth, the input path, the vertical scale, and the signal level. Higher bandwidth lets in more noise, so the same scope shows a lower ENOB at full bandwidth than with a bandwidth limit on.
The UNI-T MSO8000HD datasheet, for example, states an ENOB above 7 bits at full system bandwidth, with a 50 Ω input, 50 mV/div, and a −3 dBFS signal. For the MSO8804HD, full system bandwidth is 8 GHz. Compare ENOB figures across instruments only when those conditions match.
On a repetitive signal, averaging and high-resolution acquisition modes lower the noise and raise the effective bits on screen. They trade away bandwidth or the ability to capture single events, so ENOB specifications are quoted in normal acquisition.
Low noise is what makes 12 bits count. 12-bit HD oscilloscopes from 70 MHz to 8 GHz.
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