SBAS949A February 2020 – February 2021 ADS131M06
The ADS131M06 begins generating conversion data shortly after startup as soon as a valid CLKIN signal is provided to the ΔΣ modulators. The fast startup 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 startup is accomplished via two mechanisms. First, the device internal power-supply circuitry is designed specifically to enable fast startup. 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 tPOR for the internal circuitry to settle. The end of tPOR 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 ADS131M06 require CLKIN to toggle after tPOR to begin working. The modulators begin sampling the input signal after an initial wait time delay of (256 + 44) × tMOD 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-startup 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 startup and settling times relevant to the device startup.
fCLKIN = 8.192 MHz (ms)
|tDATA = 1/fDATA||1024||1024||0.250|
|tSETTLE1||256 + 44 + 1024||1324||0.323|
|tSETTLE3||256 + 44 + 3 x 1024||3372||0.823|
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 startup, ignore the first two instances of DRDY toggling from high to low and begin collecting data on the third instance.
The startup 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 tPOR 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) × tMOD before providing modulator samples to the two digital filters. The fast-settling filter is enabled for the first two output samples.