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Digital-to-Analog Converters (DACs): Why INL and DNL Are Important Parameters

The performance of a Digital-to-Analog Converter (DAC) is characterized by various parameters, including Integral Non-Linearity (INL) and Differential Non-Linearity (DNL). These parameters describe the linearity of a DAC's output and are related to each other but have different definitions. Here, we will consider the INL and DNL, the difference between the two and why they are important to the performance of a DAC.

Exploring DACs Beyond Audio: Key Parameters and Applications

Frequently when Digital-to-Analog Converters (DACs) are mentioned in a conversation, the first responses are, “Ahh, so you are really involved in audio and sound systems!” or “You must be really into music!” While it is true that DACs are a key component in many audio applications, they are also present in many other electronics, systems and markets. The use of DACs for signal conditioning is critical in communications, medical, consumer, automotive and other markets. These signal conditioning ICs have some key parameters that help get a good clean output signal conversion which we will discuss below.

DACs are used to convert a digital signal into an analog signal and are essential components in many modern electronic devices and systems, such as audio, video, control and measurement systems. The process of converting digital signals to analog signals involves the use of a reference voltage and an output voltage that varies in response to the digital input. The performance of a DAC is characterized by various parameters, including resolution, channels, Integral Non-Linearity (INL) and Differential Non-Linearity (DNL). The accuracy and precision of the analog output voltage depend on the resolution and linearity of the DAC. INL and DNL are parameters that describe the linearity of a DAC's output, which is its ability to produce a proportional and consistent analog output in response to a digital input. These linearity parameters are related to each other but have different definitions and units. Why is a consistently accurate and precise linearity so important? If the output of the DAC has poor linearity, it signals that there are errors in the conversion process from digital to analog. This will result in deviations from the ideal output voltage, resulting in various consequences depending on the specific application and the severity of the linearity errors. Here we will consider the INL and DNL, the difference between the two, the effects of poor linearity and why they are important to the performance of a DAC.

What Is Integral Non-Linearity (INL)?

INL is a critical parameter in the performance of DACs, especially in applications where high accuracy and precision are required. It is calculated by summing up all the differences between the ideal output and the actual output for each digital input and expressing the result as a fraction of the Least Significant Bit (LSB) of the DAC. The lower the INL error is, the higher the accuracy and precision of the analog conversion. INL errors can cause significant distortions in the output signal, leading to errors in the system. This type of error is highly critical in applications within measurement and control systems. One example of INL error effects is called error accumulation, which affects the accuracy of the output voltage over the full range of the input. These errors can accumulate and result in a cumulative offset or gain error as the digital input values change. Reliability and performance of the system are impacted if the device is used over a long period of time. Other error types will also be discussed with DNL linearity and in general.

What Is Differential Non-Linearity (DNL)?

Differential Non-Linearity (DNL) is a measure of the deviation of the actual output from the ideal output between adjacent digital inputs. The DNL is calculated by measuring the difference between the ideal output voltage and the actual output voltage at each digital input code and then calculating the difference between adjacent codes. DNL errors can also cause significant distortions, noise and other artifacts in the output signal, leading to errors in similar applications as mentioned above. Non-uniform steps are attributed specifically to DNL linearity and impact the uniformity of the voltage steps between adjacent digital input codes. If the steps are not consistent, it can introduce unevenness in the output signal. Results would be irregularities in waveforms, inaccurate amplitude, noise and jitter introduced in the system.

How Do INL and DNL Errors Affect DAC Performance?

In practical applications, INL errors are more critical than DNL errors. This is because INL errors affect the accuracy of the output voltage over the full range of the input, while DNL errors affect the accuracy of the output voltage only between adjacent digital input codes. For this reason, DACs are frequently identified or rated by their INL and not DNL. Here are a few additional possible outcomes when the linearity of a DAC is compromised:

Output drift: Some linearity errors can cause the ideal output voltage to drift over time. Over the lifetime of the device, the compounding effect of the drift on the output signal can lead to significant inaccuracy and can even compromise the integrity of the reliability of the system. This drift can also occur with changes to the input signal, and both would have serious implications in measurement and control systems.

Signal distortion: Distortion of the shape of the output signal introduces unwanted artifacts and harmonics that are particularly impactful on audio and video signals. Linearity errors in these cases can lead to audible distortion, loss of fidelity and reduced dynamic range.

Loss of resolution: Large INL and NDL errors can reduce the effective resolution of the DAC, so much so that adjacent digital codes can produce similar or overlapping output voltages. This reduces the ability to accurately represent and distinguish between different input values, resulting in a loss of resolution.

It's important to note that the impact of linearity errors will depend on the specific application requirements. While some applications may be more sensitive to linearity errors and require higher accuracy, others may tolerate certain levels of nonlinearity without significant consequences. Selecting a DAC with appropriate linearity specifications for the desired application is crucial to ensure optimal performance and avoid unwanted distortions or inaccuracies in the output signal.

Want More?

Microchip’s portfolio of DACs offers a great range of INL performance across a variety of resolutions, channel counts (1, 2, 4, 8) and internal memory options. Our best-performing DAC offers INL ratings at .1, .25 and 1 LSb at 8-bit, 10-bit and 12-bit resolutions, respectively. If an application does not require such high accuracy, we also have several DACs ranging from 1.5 to 14.5 LSb. Many of our DACs use the same packages and pinouts so that customers can test out multiple accuracies without having to redesign boards. For more information on DACs, please visit our product family page, or for a quick look at a table of all our DACs, please see the parametric chart on our website.

Dustin Tenney, Jan 16, 2024

Tags/Keywords: Industrial and IoT

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