“The ADS127L11 is a high-resolution high-sampling rate ΣΔ ADC released by Texas Instruments in 2021. The Histogram Analysis-based method discussed in this article can be used to evaluate its noise and effective resolution.
Author: Jerry Chen
The ADS127L11 is a high-resolution high-sampling rate ΣΔ ADC released by Texas Instruments in 2021. The Histogram Analysis-based method discussed in this article can be used to evaluate its noise and effective resolution.
First, input a stable DC voltage source to the ADS127L11EVM, which requires deep decoupling to reduce the noise effect of the input, and then resample it a lot. The sampling statistics will be displayed in the Histogram Analysis interface of the ADS127L11EVM-PDK GUI. As shown in Figure 1, 16384 sampling results are counted.
Figure 1. Histogram Analysis of ADS127L11EVM-PDK GUI
Code spread is the peak-to-peak value of the ADC output code, which is 307 in Figure 1.
In the statistical results, there is also a parameter called Sigma(σ), which is defined as the standard deviation of the statistical results.
where Code(i) is the result of each sampling, n is the sample size of 16384, and Mean is the average of all samples, which is 227.09 in Figure 1. We can use Sigma to calculate the effective resolution. If the input range of the ADC is selected to be ±2.5V, the full-scale range is FSR of 5V, and the value of 1LSB is,
The peak-to-peak (Vpp) and root-mean-square (Vrms) values of the noise can be calculated using Code spread and Sigma, respectively.
The effective resolution can be calculated using
The Noise Free Code Resoltuion can be calculated using the application. Output codes that exceed this number of bits are not explicitly resolved, so this indicator can be used to conservatively evaluate the resolution of the ADC.
It should be noted that there is a difference between the effective resolution and the effective number of bits ENOB calculated in this paper. The definition of ENOB is obtained by measuring the signal-to-noise ratio (SINAD) when a full-scale sine wave is input.
Also note that ADS127L11 can support high sampling rates, especially in High Speed Mode, you need to select appropriate buffers (such as THS4551) for the reference input and ADC input to provide enough driving capability, otherwise Histogram Analysis will introduce due to driving capability. errors caused by deficiencies. REF6225 integrates buffer, suitable for reference of ADS127L11.
The default input clock of the ADS127L11EVM is 24MHz. When using Low Speed Mode, you need to set the Clock Divider to Div by 8 to meet the requirement of the highest clock frequency of 3.2MHz in this mode.