Designers often have questions about a typical specification of an amplifier when designing a more robust circuit. Because of natural variation in process technology and manufacturing procedures, every specification of an amplifier exhibits some amount of deviation from the ideal value. The offset voltage of an amplifier is one such example. These deviations often follow Gaussian (bell curve), or normal distributions. Thus, allowing circuit designers to leverage this information to guard band the system, even when a minimum or maximum specification is not in the Electrical Characteristics table.

Figure 7-4 shows an example distribution. In this figure, µ (or mu), is the mean of the distribution, and σ (or sigma), is the standard deviation of a system. For specifications exhibiting this kind of distribution, approximately two-thirds (68.26%) of all units are expected to have a value within one standard deviation of the mean. One standard deviation of the mean is µ – σ to µ + σ.

Figure 7-4 Ideal Gaussian Distribution

Depending on the specification, values listed in the typical column of the Electrical Characteristics table are represented in different ways. Generally, if a specification naturally has a nonzero mean (for example, gain bandwidth), then the typical value is equal to the mean (µ). Some specifications (such as offset voltage) have a mean near zero. In this case, the typical value equals the mean plus one standard deviation (µ + σ) to most accurately represent the typical value.

Use this chart to calculate the approximate probability of a specification in a unit. For example, the INA600A typical offset voltage is 1100µV. Thus, 68.2% of all INA600A devices are expected to have an offset from –1100µV to +1100µV. At 4σ (±4400µV), 99.9937% of the distribution has an offset voltage less than ±4400µV. Therefore, 0.0063% of the population is outside of these limits, which corresponds to approximately 1 in 15,873 units.

Specifications with a value in the minimum or maximum column are verified by TI, and units outside these limits are removed from production material. For example, the INA600A family has a maximum gain error of ±0.05% at 25°C. Even though this error corresponds to 5σ, TI verifies that any units with gain error larger than ±0.05% are removed from production material. This error equals approximately 1 in 3.5 million units, which is extremely unlikely.

For specifications with no value in the minimum or maximum column, consider selecting a sigma value of sufficient guard band for the application. Design worst-case conditions using this value. A 6σ value corresponds to approximately 1 in 500 million units, which is extremely unlikely. This 6σ value is an option as a wide guard band for designing a system around. Histograms for some of the important specifications (such as offset, offset drift, CMRR, and gain error) are shown in the Typical Characteristics section.

For gain error drift, the INA600 family does not specify a maximum value based on final test. Instead, this value is based on characterization, as mentioned in the Electrical Characteristics table. The corresponding distribution mentioned in Figure 6-4 has a mean of –0.00216% and sigma of 0.0047%. Thus, the mean plus 6σ value for gain error is calculated to be approximately 0.025%. When designing for system conditions with 6σ guard band, use this method and value to estimate the worst possible gain error. However, process variation and adjustments over time potentially shift typical means and standard deviations. Unless there is a value in the minimum or maximum specification column that is specified based on final test, TI cannot verify device performance.

Thus, the maximum gain error specification in the Electrical Characteristics table is relaxed beyond 6σ guard band to be ±0.05%.