Have you evaluated the noise performance of an ADC and found that the measured performance is different from the rated performance given in the device data sheet? Achieving high resolution in high-precision data acquisition systems requires some knowledge and understanding of analog-to-digital converter (ADC) noise. It is important to understand how the data sheet specifies noise performance and how external noise sources affect overall system performance. An example of one of the noise sources is my colleague Ryan Andrews in his blog post, "Be careful! Your ADC's performance may only be similar to its power supply performance." In this blog post, I will look at how the reference noise affects the DC noise performance in an incremental-accumulated ADC.
As shown in Figure 1, you can specify and measure the DC noise performance of an ADC with a positive and negative input that is shorted to the medium supply voltage. By measuring the noise under this condition, the noise in the ADC output code is hardly affected by changes in the reference voltage, reference noise, or input signal noise. Although this test condition is an ideal situation for practical applications, it does give a good ADC noise performance that is not affected by some external noise sources.
Figure 1: ADC noise performance test (and debug) configuration
Tip: When debugging, start the evaluation of the noise performance of the system with a short-circuit input test that evaluates the noise performance of the isolated ADC before starting other system noise performance tests.
How the reference noise affects the ADC DC noise performance
This effect is related to the basic task of the ADC; the basic task of the ADC is to provide an output code that represents the ratio of the input signal voltage to the reference voltage. Both the input and the reference voltage add a noise term to this ratio, as shown in Equation 1:
(1)
Input signal noise, 
The impact on the ADC conversion results is very straightforward. The ADC will capture any noise that has not been filtered out—using an external resistor-capacitor (RC) filter, or an incremental-accumulated ADC signal filter. due to For a direct effect on the ratio in Equation 1, you can observe it in the output code.
Tip: When evaluating ADC noise performance, since the noise of the input signal directly affects the output of the ADC, make sure the input signal is a low noise source.
However, the reference noise, 
, the impact on the ADC conversion results is not straightforward, because Appears in the denominator. When the numerator is zero (as is the case with the ADC input being shorted), this ratio is always zero, and Items will not affect the ratio. When the numerator is roughly equal to the denominator, Will have a big impact on the ratio. When the ratio is between 0 and 1, The impact is measured by the ratio value. Figure 2 shows the resulting behavioral mode of operation.
Figure 2: Relationship between ADC and reference noise and input voltage
When the reference noise is added to the noise of the ADC by using a rms-increasing method, this combined noise is a function of the input voltage, which increases as the positive or negative input voltage becomes larger. On the curve in Figure 2, there are a few points to note:
Point A, which is the ADC noise measured with the shorted input given in the ADC datasheet.
Point B, which is the total bandwidth limit reference noise, is usually limited by the bandwidth of the ADC digital filter.
If you know the noise spectral density and noise bandwidth for the noise source, you can calculate the reference noise (point B); otherwise, apply a full-scale voltage input to the ADC and measure the noise performance so that you can usually get A better reference noise measurement.
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