From a Cost Perspective, Accuracy, Speed, and Dynamic Range Must Be Considered
Low SNR and Voltage Reference Must Be Considered…ADC Front-End Design is Critical

Is an ADC a good product just because it has high resolution and fast sampling speed? And if the price is also cheap, can we call it the most ideal ADC?

Park Sun-hwa, Manager of Field Application Engineering at Analog Devices (ADI), says "not necessarily." "Rather than pinpointing just one condition as ideal, the best ADC is the one that fits the application you're designing."

For example, it works like this. Depending on what signal you detect—light, sound, video, etc.—the sensor differs, and according to the type of signal, the sensor stage used before the ADC also changes. What's important in a sensor is sensitivity and dynamic range. If the sound you're trying to detect is sometimes loud and sometimes quiet, the detection range must be wide.

Accordingly, if the signal sent from the sensor doesn't match the acceptable range of the ADC, you've made the wrong choice. But does that mean having a wider acceptable range is always better? On this point, Manager Park says definitively, "that's not the case."

From a cost perspective, three factors must be considered: how precise it needs to be, how fast it needs to be, and what dynamic range is required. According to the Nyquist sampling theorem, which states "for signals with limited bandwidth frequency, if sampling is performed at more than twice the maximum frequency of the analog signal, the reconstructed digital signal can faithfully reproduce the original signal," speed need only be more than twice the required rate, not necessarily as fast as possible. Generally, speeds are divided into high-speed and low-speed based on a 10 Mbps benchmark.

While the input dynamic range of the converter itself varies by application, if the waveform is large, there are limits to processing speed. Converters that handle high-speed applications have a fixed peak-to-peak voltage difference of 2V.

To evaluate precision, bit resolution must be considered. This is a matter of how many codes represent a single point in one cycle of a sine wave of an analog signal, and as the number of bits increases, resolution improves. This means the output is closer to the original signal.

While it varies by application, when asked what conditions are basically ideal for an ADC, Manager Park states, "if we exclude speed and input dynamic range since these can be selected, it's best to have ideal static performance. ADC is fundamentally a device whose performance changes depending on noise," and "from a system noise perspective, the ADC's signal-to-noise ratio (SNR) must be lower than the noise in the system without exception."

He added that voltage reference must be considered for proper ADC operation. Voltage reference is the maximum range value of all signals expressed by the converter. For example, if you want to improve bit resolution, you must lower the voltage reference. However, if you lower the voltage reference indefinitely, the converter's acceptable signal range itself becomes narrower, so caution is required.

Manager Park stated, "the capabilities of an ADC are limited. From a cost perspective, investing heavily in the ADC is not necessarily a good approach. Efficiency varies depending on how you design the front-end stage before the ADC."