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 to Table 4 summarize the device noise performance. Table 1 and Table 2 list the ADC measurement noise using the sinc³ digital filter at different data rates and different PGA settings, and Table 3 and Table 4 list the ADC measurement noise using the low-latency digital filter. 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 and Table 3 list the input-referred root mean square noise in units of μV_RMS for the conditions shown. Note that peak-to-peak (µV_PP) values are shown in parentheses. Table 2 and Table 4 list 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 and Table 3) 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:

Table 5 to Table 8 repeat the measurements of Table 1 to Table 4 but use the global chop feature of the device. The global chop feature averages two measurement of the ADC with the inputs swapped. This feature significantly reduces the input offset of the device, and reduces noise in the measurement.

Noise performance with the PGA bypassed are identical to the noise performance of the device with gain = 1 in Table 1 to Table 8.