Designers often have questions about a typical specification of an amplifier to design a more robust circuit. Due to natural variation in process technology and manufacturing procedures, every specification of an amplifier exhibits some amount of deviation from the ideal value, like the input offset voltage of an amplifier. These deviations often follow Gaussian (bell curve), or normal distributions, and circuit designers leverage this information to guard band systems, even when there are no minimum or maximum specifications in the Electrical Characteristics table.

Figure 6-7 Ideal Gaussian Distribution

Figure 6-7 shows an example distribution, where µ, or mu, is the mean of the distribution, and σ, or sigma, is the standard deviation of a system. For a specification that exhibits this kind of distribution, approximately two-thirds (68.26%) of all units can be expected to have a value within one standard deviation, or one sigma, of the mean (from µ – σ to µ + σ).

Depending on the specification, values listed in the typical column of the Electrical Characteristics table are represented in different ways. As a general rule, if a specification naturally has a nonzero mean (for example, like gain bandwidth), then the typical value is equal to the mean (µ). However, if a specification naturally has a mean near zero (like input offset voltage), then the typical value is equal to the mean plus one standard deviation (µ + σ) to most accurately represent the typical value.

Use this chart to calculate approximate probability of a specification in a unit; for example, for TLV936x-Q1, the typical input voltage offset is 400µV, so 68.2% of all TLV936x-Q1 devices are expected to have an offset from
–400µV to 400µV. At 4σ (±1600µV), 99.9937% of the distribution has an offset voltage less than ±1600µV, which means 0.0063% of the population is outside of these limits, which corresponds to about 1 in 15,873 units.

Specifications with a value in the minimum or maximum column are specified by TI, and units outside these limits are removed from production material. For example, the TLV936x-Q1 family has a maximum offset voltage of 1.7mV at 125°C, and even though this corresponds to about 4.25σ (≈2 in 100,000 units), which is unlikely, TI verifies that any unit with larger offset than 1.7mV is removed from production material.

For specifications with no value in the minimum or maximum column, consider selecting a sigma value of sufficient guardband for your application, and design worst-case conditions using this value. For example, the 6σ value corresponds to about 1 in 500 million units, which is an extremely unlikely chance, and can be an option as a wide guardband to design a system around. In this case, the TLV936x-Q1 family does not have a maximum or minimum for offset voltage drift, but based on the typical value of 1.25µV/°C in the Electrical Characteristics table, the 6σ value is calculated for offset voltage drift is about 7.5µV/°C. When designing for worst-case system conditions, use this value to estimate the worst possible offset across temperature without having an actual minimum or maximum value.

However, process variation and adjustments over time can shift typical means and standard deviations, and unless there is a value in the minimum or maximum specification column, TI cannot verify the performance of a device. This information must be used only to estimate the performance of a device.