ADC data sheets typically report noise with the inputs shorted (V_IN = 0 V). This configuration provides the purest measurement of ADC intrinsic noise, which also includes amplifier noise if the ADC has an integrated PGA. This measurement does not include voltage reference noise, which scales linearly with the input signal. However, this is generally not a concern for bridge measurement systems that use a ratiometric reference configuration where the voltage reference noise and drift tend to cancel out.

The actual values shown in an ADC noise table are comprised of several thousand data points or several seconds worth of data. Statistical analysis is performed on this data set to determine root-mean-square (RMS) and peak-to-peak values. For delta-sigma ADCs, this information is then reported for each combination of output data rate (ODR), filter type, and gain setting (if applicable).

For example, Table 5-1 shows a portion of the noise performance information from the ADS1235 data sheet. Each row in Table 5-1 is a different ODR and filter type combination while each column represents the available PGA gains.

The noise values in Table 5-1 are referred to the input (RTI). The RTI noise of the ADC measurement is the magnitude of the equivalent noise as seen at the input of the ADC after gain. For example, the noise in Table 5-1 is referred to a ±5-V range when the gain = 1 V/V. When gain = 128 V/V, the noise is referred to a significantly smaller ±39.06-mV range.

Table 5-1 also includes two figures of merit derived from the noise values: effective resolution and noise-free resolution. Effective resolution in an ADC data sheet is the dynamic range of the full-scale range (FSR) relative to the RMS noise in the measurement, V_N,RMS. Comparatively, noise-free resolution in an ADC data sheet is the dynamic range of the FSR relative to the peak-to-peak (PP) noise in the measurement, V_N,PP. These noise parameters are calculated using Equation 20 and Equation 21:

For example, at gain = 128 V/V and ODR = 20 SPS, Table 5-1 shows that the ADS1235 finite impulse response (FIR) digital filter offers noise performance of 0.029 µV_RMS or 0.16 µV_PP. Equation 22 and Equation 23, respectively, calculate the ADS1235 effective resolution and noise-free resolution at these settings:

Note that the results in Equation 22 and Equation 23 match the reported values in the last column of Table 5-1.

Bridge measurements often characterize performance using a third parameter called noise-free counts (NFC), which is derived from noise-free resolution. This is especially important for weigh scale applications where the design requires that the smallest digit displayed in the scale measurement is stable (or noise-free). Designing a weigh scale using effective resolution targets would likely result in the last digit on the scale constantly moving because effective resolution is based on the RMS noise.

NFC is defined by Equation 24 while Equation 25 calculates NFC for the given ADC parameters:

A weigh scale with level of performance described in Equation 25 might be acceptable, though it is important to consider how this parameter is defined. Specifically, noise-free resolution and NFC are calculated assuming that the ADC input uses the entire FSR. However, if the weigh scale system does not use the entire ADC FSR, the system NFC performance will be different than the values shown in the ADC noise table. This reduction in NFC performance is described in the following section.