What the graphs are for
Datasheet tables list a few numbers at stated conditions. Graphs fill in the picture between them: how a parameter changes with temperature, supply voltage, current, frequency or time. They are the best place to see trends, such as how much a leakage current grows when the part gets hot, or where a gain starts to fall with frequency.
They are also easy to misread. Most are drawn from measurements on a small number of sample parts, under particular conditions, and are labelled typical. They illustrate behaviour; they do not guarantee it. The guaranteed numbers are the minimum and maximum columns in the tables, as the article on typical versus worst-case values explains.
Read the title and conditions first
Before looking at a curve, read what is printed around it. A title often carries the conditions, such as a supply voltage and a temperature, or a note under the plot. Without those, the shape is not interpretable. Typical details to find:
- the supply voltage or bias point;
- the ambient or junction temperature;
- the load, source impedance or test circuit;
- whether several curves are shown for different values of a parameter, and how they are labelled.
If your own conditions differ, the curve may not apply directly. Look for another curve that is closer, or treat the plot as a guide to direction and approximate size only.
Check the axes
Look at what each axis shows, its units and its range.
Units and prefixes. Microamps versus milliamps, nanoseconds versus microseconds. A missed prefix changes a result by a factor of a thousand, and SI prefixes are a frequent trap on axis labels.
Start of the range. An axis that does not start at zero can make small changes look dramatic. A curve that appears to fall steeply may change only a few percent when you read the actual numbers.
Linear or logarithmic. If equally spaced gridlines represent multiplication, as in 1, 10, 100, 1000, the axis is logarithmic. Equal distances on a log axis mean equal ratios, not equal differences. This changes how to read values, covered below.
Reading values off a linear axis
On a linear axis, read the nearest gridlines on either side of your point and estimate the position between them. If the point lies halfway between 2 and 3, the value is about 2.5. When the curve is nearly straight between two known points, linear interpolation gives a reasonable estimate; when it bends sharply, estimate more cautiously or read a finer plot.
It also helps to remember that reading a printed graph is limited by its resolution. A value read from a plot has perhaps two significant figures at best, so avoid quoting more digits than the graph can support.
Reading values on a log axis
On a logarithmic axis the spacing between 1 and 10 equals the spacing between 10 and 100. Between 1 and 10 the gridlines get closer together: the midpoint of the distance between 1 and 10 on the axis is about 3.16 (the square root of 10), not 5.5. A point visually halfway between two decade lines is therefore roughly three times the lower one, not five.
A common use is frequency response, where frequency runs along a log axis from, say, 10 Hz to 1 MHz. Gain is often shown in decibels. The decibel scale is itself logarithmic, so a change of 20 dB corresponds to a tenfold change in voltage ratio, and 6 dB corresponds to roughly a factor of two. Because the decibel is a ratio, these relations are worth knowing before reading amplifier or filter plots.
Straight lines on log-log plots usually represent power-law relationships, and the slope tells you how fast the quantity changes per decade. Read the slope from the plot only if you take care with units.
Common types of plot
- Parameter versus temperature. Shows drift. Check whether the curve is for typical parts or has min and max bands. If bands are shown, those are more useful for worst-case design.
- Parameter versus supply voltage or current. Often reveals where a part leaves its comfortable region.
- Output characteristics, such as current versus voltage. The shape of a family of curves shows how the device behaves at different bias points. Find your operating point on the family before reading off a value.
- Frequency response. Shows gain or impedance versus frequency, often with a log frequency axis.
- Derating curves. Show how the allowed power or current falls as temperature rises. These are limits, not typical behaviour, so read them with design margins in mind.
- Transient and timing plots. Show how a signal changes over time. Pay attention to scale: nanoseconds per division changes the meaning completely.
Areas and slopes
Sometimes the quantity you want is not a point but a slope or an area. The slope of a curve gives a rate of change, such as the gain of a device at an operating point. The area under a curve can represent accumulated quantity, such as charge from a current-versus-time plot. Reading either from a graph is approximate; the ideas behind rates of change and areas under curves explain what the slope and area mean in physical terms, and a rough estimate is often enough to see if a design is in the right range.
Typical curves are not limits
A typical plot may show a part behaving better than its guaranteed minimum would suggest. Designing against the typical curve means that parts at the edge of the specification may not behave as drawn. Where it matters, use the specification tables for guaranteed numbers and use the graph to understand how a value changes with conditions.
Some plots do show worst-case bands or limits, and the labelling will say so. If you cannot tell whether a curve is typical or guaranteed, assume typical, or ask the manufacturer.
A short checklist
- Read the title, conditions and footnotes.
- Identify each axis: units, prefixes, start, linear or log.
- Find your operating point, not just the headline one.
- Read values with the right interpolation for the axis type.
- Decide whether the curve is typical or a limit.
- Cross-check the number against the specification table.
- Where a decision depends on it, verify on real parts.
Graphs are a guide to behaviour, not a promise. For regulated or safety-related designs, follow the standards that apply in your country and industry and rely on qualified engineers for critical judgments.