ADS1299: Single Shot mode - RDATA and START pin

Hi Tom,

Thanks for your questions.

RDATA Mode and Single-Shot Mode refer to two totally different aspects of the ADS1299. Essentially, one has to do with when you choose to read the data and the other has to do with when the ADC makes a conversion. I've explained this in more detail in previous posts, such as this one.

The ADS1299 conversions can be controlled via the START pin or via the START SPI command for both Continuous and Single-Shot conversion modes. When START is high or the START command is sent, the modulator will begin converting and a new sample will be available to read whenever you see /DRDY pulse low. You can send the START command to multiple devices or tie their START pins together to synchronize their conversions.

In Continuous Mode, a new sample is available at the set data rate, following the initial settling time of the digital filter. In Single-shot Mode, the modulator automatically stops after a single conversion is completed and you must wait for the digital filter to settle after each conversion. This effectively reduces your output data rate by 4x. Delta-sigma ADCs require a history in their digital filters, so it is highly recommended to leave the ADC converting continuously.

Now, RDATA and RDATAC modes refer to when the output shift register is loaded with new data for you to read. In RDATAC mode, the output shift register is updated as soon as the next sample is ready to read. If you are still reading the old data at that time, the old data will be overwritten with new data. Conversely, RDATA mode only updates the output shift register with the most recent data when you send the RDATA command. If you do not wish to read every sample, you can use this mode to read the data on-demand.

Lastly - Daisy-Chain configuration allows you to read data from multiple ADS1299 devices without occupying additional pins on your uP. To use multiple devices in a daisy-chain configuration with RDATAC mode, your interface clock (SCLK) must be fast enough to shift out all 16 channels worth of data (384 bits) plus two 24-bit STATUS words before the next sample is ready. In RDATA mode, the data read transaction could overlap the next sample without data corruption, as I mentioned above. Alternatively, if you have extra GPIO pins available on your uP, you could use a separate /CS signal for each ADS1299 device. This allows multiple devices to share the same SPI bus in what is known as a Cascaded configuration. This may be useful if you want the devices to have different register settings. Please see the Multiple Device Configuration section on page 64 (datasheet Rev C, January 2017).

Best Regards,