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 the 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 device noise performance. Data are representative of typical noise performance at T_A = 25°C using the internal 2.5-V reference. Data shown are based on 512 consecutive samples from a single device with inputs internally shorted. Table 1 lists the input-referred root mean square noise in units of μV_RMS for the conditions shown. Peak-to-peak (µV_PP) values are shown in parentheses. Table 2 lists the corresponding data in effective resolution calculated from μV_RMS values using Equation 1. Noise-free resolution is calculated from µV_PP values using Equation 2.

The input-referred noise (Table 1) only changes marginally when using an external low-noise reference, such as the REF5025. To calculate effective resolution and noise-free resolution when using a reference voltage other than 2.5 V, use Equation 1 and Equation 2:

Noise performance with the PGA bypassed are identical to the noise performance of the device with gain = 1.