8.1 Noise Performance

Delta-sigma (ΔΣ) analog-to-digital converters (ADCs) are based on the principle of oversampling. The input signal of a ΔΣ ADC is sampled at a high frequency (modulator frequency) and subsequently filtered and decimated in the digital domain to yield a conversion result at the respective output data rate. The ratio between modulator frequency and output data rate is called oversampling ratio (OSR). By increasing the OSR, and thus reducing the output data rate, the noise performance of the ADC can be optimized. In other words, the input-referred noise drops when reducing the output data rate because more samples of the internal modulator are averaged to yield one conversion result. Increasing the gain also reduces the input-referred noise, which is particularly useful when measuring low-level signals.

Table 1 and Table 2 summarize the ADS111x noise performance. Data are representative of typical noise performance at T_A = 25°C with the inputs shorted together externally. Table 1 shows the input-referred noise in units of μV_RMS for the conditions shown. Note that µV_PP values are shown in parenthesis. Table 2 shows the effective resolution calculated from μV_RMS values using Equation 1. The noise-free resolution calculated from peak-to-peak noise values using Equation 2 are shown in parenthesis.