Resolution and Accuracy-i.e., ResoluTIon and Accuracy-are two different parameters that are often mixed up. And the way ADC manufacturers define ADC performance in their datasheets is confusing and can lead people to make false inferences in application development. But the fact is that resolution does not represent accuracy, and vice versa.
Resolution and Accuracy
Resolution
Resolution (ResoluTIon) is the ability of an ADC to distinguish the smallest signal that can be quantized, expressed as a binary number of bits.
For example, the smallest quantization level that a 10-bit ADC can resolve is one tenth of 2 times the reference level (full scale). That is, the higher the resolution, the more copies of the level in the full range can be divided, the more accurate the result, and the closer the digital signal is to the original input analog value when converted back to DAC.
Therefore, for a given specific ADC device, the resolution value is fixed.
Precision
Precision refers to the closeness (what is the error value) between the actual digital output and the theoretical expected digital output for a given analog input. In other words, the precision of the converter determines how many bits of the digital output code represent useful information about the input signal.
Some ADC devices have a datasheet that indicates the precision value or precision range.
For a given specific ADC device, the accuracy value may vary depending on the external environment (temperature, interference, etc.).
Dynamic Range Accuracy and Resolution of ADCs
Dynamic range is defined as the ratio of the minimum and maximum signals that can be measured by the system.
The maximum signal can be an inter-peak value, a zero-to-peak (Zero-to-Peak) value, or a root mean square (RMS) full scale. Any one of these will give a different value. For example, for a 1V sine wave: Inter-peak (full scale) value = 2V Zero-to-peak = 1V
RMS full scale = 0.707 x peak amplitude = 0.707 x 1V = 0.707V
The minimum signal is usually the RMS noise, which is the root mean square value of the signal measured when no signal is applied. The RMS noise level obtained from the measurement will depend on the bandwidth used for the measurement. Each time the bandwidth is doubled, the recorded noise will increase by 1.41 or 3 dB.
Therefore, it is important to note that the dynamic range figure is always related to a certain bandwidth, which is usually not specified, making the recorded value meaningless. The signal-to-noise ratio (SNR) and dynamic range of a device are most of the time defined as the same value, i.e.: Dynamic range = SNR = RMS full scale/RMS noise and often using dB as the unit, i.e. dynamic range (dB) = SNR (dB) = 20*Log10 (RMS full scale/RMS noise)
In contrast to using RMS full scale, some manufacturers quote zero-to-peak or inter-peak values to make the graph look better, which increases the final dynamic range or SNR by 3dB or 9dB, so we need to study the specifications carefully to avoid misunderstandings.
ADC resolution is determined by the number of bits used when digitizing the input signal. For 16-bit devices, the total voltage range is expressed as (216 = 65536) individual digital values or output codes. Therefore, the absolute minimum level that the system can measure is expressed as 1 bit, or 1/65536 of the ADC voltage range.
As mentioned earlier, for 16-bit ADC resolutions, the actual accuracy may be much less than the resolution due to the presence of internal or external error sources. So, for example, a given 16-bit ADC may only provide 12 bits of accuracy. For this case, 4LSb (lowest significant bit) represents the random noise generated in the ADC. ADC dynamic range and ADC accuracy usually refer to the same content.
The ideal ADC generates a digital output code that is an equation about the analog signal voltage and the voltage reference input, where
Output code = Full-scale voltage × [VIN+ – VIN-] / [VREF+ – VREF-]
= Full-scale voltage × [VIN /VREF]
Each digital output code represents a fractional value of the reference voltage.
It is important to note that the ADC dynamic range should match the maximum amplitude of the signal to be converted in order to maximize the ADC conversion accuracy. Now assume that the signal to be converted varies between 0V and 2.5V and VREF is equal to 3.3V.
The 16-bit ADC will consist of 216 = 65536 steps or transitions and the lowest significant bit (LSB) = VREF/65536 = 3.3V/65536 = 50.35uV. For an ideal ADC, all codes have the same width of 1LSB.
If the ADC has a maximum signal value of 2.5V, that means a total of 49652 conversions (2.5V/1LSB). For this case, there will be 15884 conversions unused (65536-49652=15884). This reflects the loss of signal accuracy or loss of ENOB (effective number of bits) after conversion (0.4 bits lost). If the difference between the ADC reference (VREF) and the ADC maximum signal level increases, then the ENOB loss or accuracy loss will be exacerbated. For example, if the ADC maximum signal level is 1.2V and VREF = 3.3V, then the ENOB loss will be 1.5 bits. Therefore the ADC dynamic range must match the maximum signal amplitude to obtain the highest accuracy.
Analog-to-digital converters (ADCs) claim to have “n” bit resolution, which is often misunderstood as accuracy. Resolution and accuracy are two entirely different concepts and should not be confused. It is the specific application that should determine if missing codes are allowed and the ADC accuracy required.
In summary
Resolution and accuracy should not be confused together, where “accuracy” is used to describe the accuracy of a physical quantity, and “resolution” is used to describe the scale division.
In fact, for ADC, these two are very important parameters, and often determine the price of the chip, obviously, we are clear that the same series, 16-bit AD is generally more expensive than 12-bit AD, but the same 12-bit AD, different manufacturers and what parameters to distinguish between performance? Performance often determines the price, so what parameters have a greater impact on the price? At this point, we have to use the precision to measure.
