The ADC provides two operating speed modes (high speed and low speed) that allow trade-offs between power consumption and signal bandwidth. Low-speed mode operates the modulator at 1/8th speed for decreased device power consumption and, as a result, the output data rates are reduced by 1/8th. The programmable oversampling ratio (OSR) determines the output data rate and associated signal bandwidth, and therefore also determines noise performance at the sub-LSB level.

The wideband filter provides data rates up to 400 kSPS in high-speed mode and 50 kSPS in low-speed mode. The low-latency sinc4 filter provides data rates up to 1.067 MSPS in high-speed mode and up to 133 kSPS in low-speed mode. The low-latency filter provides the options of sinc4, sinc4 + sinc1, sinc3, and sinc3 + sinc1 configurations.

For dc-signal measurements, the ADC noise can result in code flicker between adjacent code values. The probability of code flicker depends on the location of the signal input relative to the next code transition and the amplitude of the ADC noise combined with noise present within the signal. The ADC peak-to-peak noise value is typically 6.6 × the RMS noise value. Code flicker results when the amplitude of noise is large enough to trigger a code transition.

The quantization error of any ADC is ±0.5 LSB. When an ac signal is applied, the quantization error becomes a quantization noise. Figure 7-1 shows the quantization error converting to noise (LSB error plot) as the signal changes. For non-coherent sampling, this quantization noise is approximated as white noise, spread evenly across the frequency band. For any N-bit ADC, the signal-to-quantization noise ratio (SQNR) is as follows: SQNR (dB) = 6.02 × N + 1.76. For a 16-bit ADC, the SQNR is 98.1 dB.

The SNR of an ADC is determined by the sum of the device thermal noise and the device quantization noise. For most data rates of the ADS117L11, the quantization noise is larger than the thermal noise, therefore the SNR is equal to SQNR.

Table 7-1 through Table 7-5 summarize the noise and SNR performance and signal bandwidth of the digital filter modes. Noise and SNR performance is specified with the 1x input range and a 4.096-V reference voltage. SNR performance when operating with a 2.5-V reference voltage (1x or 2x input range) is approximately –1 dB lower at OSR = 32, increasing to the same SNR performance at OSR = 128 and above.