Why is Oscilloscope Vertical Accuracy Important?

Why is Oscilloscope Vertical Accuracy Important?

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What Does “Vertical Accuracy” Mean?  

The horizontal axis of an oscilloscope is the time base (seconds per division or s / div), and the vertical axis shows us the voltage (volts per division, V / div). Vertical accuracy refers to the accuracy of the voltage we see on screen, both visually and in measurements. How close is the voltage you are reading on the oscilloscope screen to the actual voltage of your signal? It depends on the vertical accuracy.

 

Highest ADC bits + Lowest noise floor = Highest vertical accuracy

 

Two key specifications define vertical accuracy:

  • The number of analog-to-digital converter (ADC) bits.
  • The front-end noise floor of the oscilloscope.

 

The higher the number of ADC bits, the more vertical resolution you have. The more vertical resolution you have, the more accurate a signal you see. Furthermore, the lower your front-end noise floor is, the less your oscilloscope impacts the signal you are measuring. All oscilloscopes have some intrinsic noise, just like every electronic device has some noise. Any noise present in the oscilloscope is going to ride on top of your signal and skew your measurements. You want an oscilloscope with the least amount of noise possible so that it does not affect your measurements. This consideration is important with any type of signal but even more critical when measuring very small voltages.

 

Using an oscilloscope with a low ADC and high noise floor can cause inaccurate measurements and lead to redesigns, resourcing components, and, ultimately, wasting valuable time. To minimize the time you spend validating and redesigning, you should evaluate an oscilloscope’s vertical accuracy to ensure reliable measurements.

 

ADC Bits and Minimum Resolution

The ADC is crucial for vertical signal accuracy. The higher the number of ADC bits, the more resolution the oscilloscope has. An oscilloscope with a 14-bit ADC should provide 64 times the resolution of a scope with an 8-bit ADC.

 

Resolution refers to the smallest quantization level determined by the ADC in the oscilloscope. An oscilloscope’s ADC with a resolution of 8 bits can encode an analog input into one in 256 levels since  28 = 256. We will refer to these as quantization levels (Q levels).

 

The ADC operates on the oscilloscope’s full-scale vertical value. For both current and voltage measurements, the Q-level steps are associated with the full-scale vertical scope setting. If the user adjusts the vertical setting to 100 mV per division, full screen equals 800 mV (8 divisions * 100 mV / div), and Q-level resolution is equal to 800 mV divided by 256 levels, or 3.125 mV.

 

For example, two scopes are scaled to 800 mV full screen. A scope with an 8-bit ADC has a resolution of 800 mV / (28 = 256 Q levels), or 3.125 mV. A scope with a 14-bit ADC, like the Keysight InfiniiVision HD3 Series, has a resolution of 800 mV / (214 = 4,096 Q levels), or 48.8 microvolts. Each scope can only resolve signals down to the smallest Q level. 

 

Many oscilloscopes also offer high-resolution mode. Oversampling techniques combined with digital signal processer (DSP) filters can increase vertical resolution. Vendors often refer to this increase in terms of bits of resolution. In the case of the InfiniiVision HD3 Series, high resolution enhances the bit depth from the intrinsic 14-bit ADC resolution to a 16-bit resolution. This technique requires an ADC architected with an excess sample rate relative to the hardware bandwidth needed for a particular measurement.

 

A high number of ADC bits will theoretically increase resolution. However, that is not always the case. Vertical resolution depends on not only the ADC, as we learned above, but also the front-end noise of the oscilloscope. The effective number of bits (ENOB) specification takes the noise of the system into account and tells you how many of those bits are actually effective in making measurements. Not only does the HD3 Series have a 14 bit ADC and low noise (50 mV), but it also has high ENOB. Learn more about ENOB in the following section and the Understanding ADC Bits and ENOB white paper. 

 

Additional topics include:

  • Effective Number of Bits (ENOB)
  • Scaling’s Impact on Resolution
  • Oscilloscope Noise
  • Frequency Responses
  • Correction Filters