Test and measurement equipment often provides multiple scales so users can examine signals across different ranges. Although a large scale can accommodate a larger signal, it is not automatically the best setting for every measurement. When the signal is small compared with the selected range, the instrument may not use its available digital levels effectively.
This issue is especially important when measuring small voltage changes with an oscilloscope. A trace may look reasonable on the display, yet the selected scale can limit the instrument’s ability to distinguish fine differences in amplitude. Understanding the relationship between scale and analog-to-digital converter resolution helps users make more informed measurement choices.
Why measurement scale selection matters
A common approach is to leave an instrument at its default scale or select a large range so that both small and large signals remain within view. This can prevent a signal from exceeding the displayed range, but it also spreads the converter’s available levels across a wider voltage span.
The result is a larger voltage interval between adjacent digital levels. If the signal changes by less than that interval, a single ADC sample cannot clearly represent the change. Therefore, a setting that provides broad coverage may sacrifice useful detail when the actual signal occupies only a small portion of the range.
How ADC quantization affects the result
An analog-to-digital converter represents an analog input using a finite number of digital levels. In the example provided, a 9-bit ADC has 29, or 512, valid levels. Those levels must represent the full voltage span associated with the selected scale.
Consider a 1000 V scale example with a maximum input range of ±1000 V. The total span from negative to positive is 2000 V. Dividing this span among 512 levels gives an interval of approximately 3.9 V per level:
2000 V ÷ 512 ≈ 3.9 V
This interval illustrates the quantization limitation at that setting. If the same scale is used to examine a signal around 11 V, a change of only 1 V is smaller than the approximately 3.9 V interval. Under these conditions, an individual ADC sample cannot resolve that 1 V change.
The important point is not simply whether the complete signal fits within the selected range. Users should also consider whether the available ADC levels provide enough detail for the voltage changes they need to observe.
Higher-resolution ADCs provide finer voltage steps
Increasing ADC bit depth creates more valid digital levels across the same input span. As a result, the voltage represented by each level becomes smaller. The source example compares 9-bit conversion with the 12-bit and 14-bit ADC options in the OWON XDS oscilloscope series.
Using the same 1000 V scale example, a 12-bit ADC provides a resolution of about 0.5 V. A 14-bit ADC provides a resolution of about 0.125 V. Compared with the approximately 3.9 V interval in the 9-bit example, these smaller steps allow finer signal changes to be represented.
Higher vertical resolution is particularly relevant when detailed amplitude information is important. However, ADC bit depth does not remove the need to select an appropriate scale. Even when more digital levels are available, matching the scale to the signal remains an important part of using those levels effectively.
A practical approach to selecting the scale
When a large range is not required, avoid using it to measure a small signal. Instead, choose a scale that more closely matches the expected signal level while keeping the signal within the usable measurement range.
A practical selection process can include the following steps:
- Consider the approximate amplitude of the signal before selecting the scale.
- Choose a range that can contain the signal without being unnecessarily large.
- Check whether the detail of interest is greater than the effective voltage interval represented by the ADC at that scale.
- If small changes are difficult to distinguish, review the scale rather than relying only on the default setting.
- Use the available scale options to make the signal occupy a more useful portion of the measurement range.
This approach helps balance range coverage with the need to observe smaller variations. It also reduces the risk of accepting a visually plausible result without considering the underlying quantization limitation.
Scale and resolution should be considered together
Accurate small-signal measurement depends on more than selecting the widest possible range. The measurement scale determines how the instrument distributes its ADC levels across the input span, while ADC bit depth determines how many levels are available.
The OWON XDS series includes 12-bit and 14-bit oscilloscope options, providing more measurement levels than the 9-bit example. These options can support finer voltage resolution, while the available scale choices help users select a setting suited to the signal being measured.
For routine oscilloscope work, the central guideline is straightforward: do not use a large scale for a small signal unless the wider range is necessary. Select a scale that more closely matches the signal level, and consider ADC resolution when evaluating whether the instrument can represent the changes you need to measure.
