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ADS1299: Power consumption in Continuous Conversion mode vs. Single-Shot mode

Saeed Alighaleh
Prodigy 20 points
Part Number: ADS1299

Tool/software:

Hello TI Team,

I have designed a medical device that utilizes four ADS1299 chips in daisy chain mode with continuous reading mode. The current device operates at a sampling rate of 250 Hz (the lowest available value), and in this configuration, each chip consumes approximately 13mA. In our application, we can reduce the sampling rate to 40 Hz, but this is not possible with the ADS1299 in continuous reading mode.

I am considering using single-shot mode to manually sample at 40 Hz to save power. According to the ADS1299 datasheet (section 9.4.6), data conversion should be stopped between readings in single-shot mode: “Single-shot mode is enabled by setting the SINGLE_SHOT bit in the CONFIG4 register to ‘1’. In single-shot mode, the ADS1299-x performs a single conversion when the START pin is taken high or when the START command is sent. As shown in Figure 44, when a conversion is complete, DRDY goes low and further conversions are stopped.”

Based on this, I expected the power consumption to be reduced in single-shot mode. However, in a quick test, I did not observe any differences. I also tested with a sampling rate of 1 kHz in continuous mode, but there were no significant changes in power consumption.

Could you please help me understand if changing the reading mode (continuous/single-shot) or altering the sampling rate affects power consumption?

Thank you.

  • 0
    TI__Guru 64236 points

    Hello Saeed,

    Thank you for your post.

    Can you please clarify on which supply rails you are measuring the 13 mA per AD1299 device? That seems quite high as typical AVDD current is only 7 mA and typical DVDD current is about 0.5 mA.

    It's possible that other circuitry is dominating the total current consumption, even when conversions are running. I believe the delta-sigma modulators for all 8 channels should stop converting when START is taken low or when a single conversion completes in single-shot mode, so there should be some difference, but I'm not sure how much difference should be expected. Can you try leaving the devices in Continuous Conversion mode and measuring the AVDD current when START is high vs. when START is low?

    Note that by reducing power, you will likely see an increase in overall noise performance as these are generally direct trade-offs in analog signal chain.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hello Ryan, 

    Thanks for your quick reply. 

    Re: Current Measurement
    You are right. In the previous measurement, I used a board with 4 ADS and some components that drained 52mA. Now, I have set up another test with a board including one ADS1299 and a few regulators (+2.5V, -2.5V, 3.3V, and -3.7V). The new measurement confirms the values mentioned by you and the datasheet, totaling about 9.3mA for the whole board. I used the Nordic Power Profiler Kit (PPK2) and noted that the VDD is 3.3V.

    Re: Start Pin High/Low
    I did exactly what you suggested. I configured the ADS for continuous conversion, started recording by sending the RDATAC command, and set the START pin high. After 4 seconds, I set the START pin low. There was no change in the current. Then, I manually set the START pin high and low using a jumper wire—still no change. I also tried sending the START/STOP conversion command and using both command and GPIO methods. None of these methods changed the current consumption. Finally, I sent the STANDBY command after 4 seconds of conversion start, and the current dropped from 9.3mA to 1.2mA (see attached). But, I want to save power by single-shot mode, not Standby/wakeup. 

    I hope this information helps. I am happy to do more test to solve this issue. 

    Thanks,

    Saeed

  • 0 in reply to Saeed Alighaleh
    TI__Guru 64236 points

    Hello Saeed,

    Thank you for the update. Based on the results you shared, I believe this confirms that the overall power consumption during normal operation is dominated by other internal circuitry, such as the internal bandgap voltage (used for the ADC reference voltage), reference and BIAS amplifiers, etc. The ADS1299 does not scale power proportionally to its data rate as drastically as a SAR ADC. This remains true whether the device is converting continuously or on-demand. The power can only be reduced further by placing the device into Standby mode.

    Regards,

    Ryan