Normally, the dynamic range of the ADC matches the amplitude of the maximum input signal, which is very important for obtaining the maximum ADC conversion accuracy.
What does it mean that the dynamic range of the ADC does not match the maximum input signal amplitude seriously? Let us give an example.
Assuming that the signal to be converted varies from 0V to 2.5V, and VREF+ is equal to 3.3V, and the resolution of the ADC is 12 bits, the ADC conversion value of the maximum signal of 2.5V is 3102, as shown in the figure below. In this way, there are 993 unused conversion values ​​(4095-3102 = 993), that is, the accuracy of the conversion signal is lost.
It might as well be another way of saying it.
When the maximum input is 2.5v and the reference voltage is 3.3V, the actual resolution corresponding to the input signal is 2.5v/3102.
If the ADC reference voltage is adjusted to match the input, and the reference voltage is also changed to 2.5V, then the ADC value corresponding to the 2.5V input is 4095 full scale, and the resolution at this time is increased to 2.5v/4095.
In actual applications, someone may deliberately select an ADC chip or MCU with ADC with a satisfactory resolution. However, in the specific design, the maximum input signal of the ADC is still quite different from the reference voltage of the ADC module. ADC resolution accuracy is not fully utilized at all. If this is the case, it is better to choose an ADC chip that has a low resolution, but the reference voltage matches the amplitude of the input signal. This may save some cost.
How to match the dynamic range of the ADC with the amplitude of the maximum input signal?
Generally, the maximum possible resolution can be obtained by selecting the reference voltage reasonably or using a pre-amplifier and using the full-scale output of the ADC to improve the conversion accuracy.
1. Choose a suitable reference voltage
Match the reference voltage with the amplitude of the signal to be measured, and use the full range and full-scale output of the ADC module to obtain the maximum resolution.
2. Use a preamplifier
If the signal to be measured is too small compared to the measurement range of the ADC, an external pre-amplifier can be used. For example: if the range of the signal to be measured is between 0V and 1V, and VDDA is 3V, this signal can be amplified so that its peak-to-peak amplitude is the same as the value of VDDA, and the gain is 3. This amplifier can convert the range of the input signal to the range of the ADC module. However, at this time, pay attention not to introduce other additional errors when designing this amplifier.
Therefore, when doing ADC applications, pay attention to the input signal amplitude to match the reference voltage of the ADC module as much as possible, and make full use of the full scale to achieve the best conversion accuracy.
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