The ADS131M04-Q1 begins generating conversion data shortly after startup as soon as a valid CLKIN signal is provided to the ΔΣ modulators. The fast start-up feature is useful for applications such as circuit breakers powered from the mains that require a fast determination of the input voltage soon after power is applied to the device. Fast start up is accomplished via two mechanisms. First, the device internal power-supply circuitry is designed specifically to enable fast start up. Second, the digital decimation filter dynamically switches from a fast-settling filter to a sinc³ filter when the sinc³ filter has had time to settle.

After the supplies are ramped to 90% of their final values, the device requires t_POR for the internal circuitry to settle. The end of t_POR is indicated by a transition of DRDY from low to high. The transition of DRDY from low to high also indicates the SPI interface is ready to accept commands.

The ΔΣ modulators of the ADS131M04-Q1 require CLKIN to toggle after t_POR to begin working. The modulators begin sampling the input signal after an initial wait time delay of (256 + 44) × t_MOD when CLKIN begins toggling. Therefore, provide a valid clock signal on CLKIN as soon as possible after the supply ramp to achieve the fastest possible startup time.

The data generated by the ΔΣ modulators are fed to the digital filter blocks. The data are provided to both the fast-settling filter and the sinc³ filter paths. The fast-settling filter requires only one data rate period to provide settled data. Meanwhile, the sinc³ filter requires three data rate periods to settle. The fast-settling filter generates the output data for the two interim ADC output samples indicated by DRDY transitioning from high to low while the sinc³ filter is settling. The device disables the fast-settling filter and provides conversion data from the sinc³ filter path for the third and following samples. Figure 8-14 shows the behavior of the fast start-up feature when using an external clock that is provided to the device right after the supplies have ramped. Table 8-8 shows the values for the various start-up and settling times relevant to the device start up.

Figure 8-14 Fast Startup Behavior and Settling Times

The fast-settling filter provides conversion data that are significantly noisier than the data that comes from the sinc³ filter path, but allows the device to provide settled conversion data during the longer settling time of the more accurate sinc³ digital filter. If the level of precision provided by the fast-settling filter is insufficient even for the first samples immediately following start up, ignore the first two instances of DRDY toggling from high to low and begin collecting data on the third instance.

The start-up process following a RESET command or a pin reset using the SYNC/RESET pin is similar to what occurs after power up. However there is no t_POR in the case of a command or pin reset because the supplies are already ramped. After reset, the device waits for the initial wait time delay of (256 + 44) × t_MOD before providing modulator samples to the two digital filters. The fast-settling filter is enabled for the first two output samples.