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ADS7041: ADS7041 vs ADS7142

cairdac_rd
Prodigy 30 points
Part Number: ADS7041
Other Parts Discussed in Thread: ADS7142

Hi,

I'm designing a sensor monitor solution with a sample rate of about 200 sps with the lowest power consumption. My target is a power consumption less than 500 nA for data acquisition.

My application uses a MCU (not defined yet) to store the ADC data in the RAM (FIFO format) and based on the user action, the last 5 minutes of the acquired data could be transfered to flash or sent by RF.

The ADS7041 datasheet claims a power consumption "Less than 1 μW at 1 kSPS with 3-V AVDD". what about power consumption at 200 sps ? linear relation ?

The ADS7142 datasheet claims "Autonomous monitoring at 900 nW". I suppose that this could be obtained when nCLK = 255. The sampling frequency is given in paragraph 7.3.5 as Fs = FCLK/nCLK. Where FCLK = 1/t_LPO and t_LPO is 95 us and 300 us max.

This means that the power consumption of 900 nW for ADS7142 is obtained for a sampling frequency of 41 Hz.. So I'm expecting 5 x 900 nW = 4.5 uW at 200 sps.

This means the ADS7041 will be consuming less power (if we don't take into account the power consumed to read the data by the MCU)  than the ADS7142.

Because Ti is promoting the ADS7142 as the lowest power ADC for sensor monitoring i was expecting less power consumption than the ADS7041.

How are compared these ADC with the internal ADC of MSP430 or MSP432 for this kind of slow data acquisition (200 sps)?

Kind regards

  • 0
    TI__Mastermind 24940 points

    Hello,

    The power consumption is linearly related to the sampling rate. Within the datasheet for ADS7041, figure 27 through figure 29 show data related to this.

    As for how the ADC's power consumption related to an internal microprocessor, we have short documentation that talks to this subject.

    Predictive Maintenance in Smart Meters Using Real-Time Monitoring compares an MCU to the ADS7142

    I would also like to point out Extend Battery Life and Simplify Calibration in Gas Sensing Applications, which talks about advantages to using an external ADC to an MCU ADC>

    Overall, and external ADC will provide better performance, and will consume less power.

     

    The two devices have comparable power consumptions, but you might be over looking specs that can be rather definitive in your application, aside from power consumption.

    Notice that ADS7142 is an I2C device, while ADS7041 is an SPI device. The communications protocol is likely to be decided based on the master or how expanse the system is.

    Also, you mention that you are looking FIFO, the ADS7142 can support this capability and relieve the MCU of this, it also has alert capabilities to wake up the MCU. In autonomous mode, output data is not available, not until a threshold has been triggered. The FIFO data is available, but is collected based on a threshold triggering.

    The ADS7041 does not provide any of the mentioned capabilities, but it can provide data after each sample, and fill the MCU FIFO

    Regards

    Cynthia

    The

     

  • 0 in reply to Cynthia
    Prodigy 30 points

    Hi Cynthia,

    Thank you for your feedback.

    The paper on Smart meters monitoring is very nice. It gives the answer to my question about MCU ADC vs external ADC.

    The power consumption of the 2 devices are not very similar for me:

     1-      The ADS7041 linear relation in Figure 28 does not show the low frequency range. A linear fit with the Figure 28 curve gives:

     I(uA)= 1e-4* Fs (Hz) +3.25 uA.

     So, there is an offset (static power consumption) whatever the sampling rate. For a sampling rate of 200 SPS the estimated current is 3.27 uA.

     

    2-      The ADS7142 power consumption specifications are confusing for me. In the paper you mentioned they talk about 450 nA @ 1.8V for 600 SPS. Which Figure in the ADS7142 datasheet show this? The highest sampling rate in the autonomous mode is about 10 ksps for nCLK = 1. The sampling rate of 600 SPS is obtained for nCLK = 18.

    According to Figure 30 the current consumption is about 4nA for NCLK = 18.

    But according to Figure 31 the current consumption is about 400 nA for nCLK=25 (Fs = 420 SPS).

    Can you explain "In autonomous mode, output data is not available, not until a threshold has been triggered" please? is it possible to read all the data acquired in autonomous mode without defining thresholds in order to get continuous monitoring?

    kind regards

  • 0 in reply to cairdac_rd
    Prodigy 30 points

    Hi again,

    According to my understanding of the "Autonomous Mode With Start Burst" and Figure 50 the ADC7142 will alert the MCU when 16 samples are filled.

    The MCU will read the 16 samples and set the SEQ_START bit gain to restart a new 16 samples acquisition.

    In this way the MCU can go to sleep state during the 16 samples acquisition and wakes-up every 16 samples to read that data.

    In this way we can achieve continuous mode acquisition without using the manual mode. But there will be some time delay for reading the 16 samples to be considered.  Is my understanding correct ?

    regards,

  • 0 in reply to cairdac_rd
    TI__Mastermind 24940 points

    Your understanding is correct, you can fill the FIFO up to 16 entries while the MCU is in sleep mode and can then read the FIFO values afterward.

    You are also correct on the delay for reading the FIFO data. During the time the MCU is reading the  FIFO values, new conversions will not take place

    Regards,

    Cynthia

  • 0 in reply to cairdac_rd
    TI__Mastermind 24940 points

    As for the ADS7041, you are correct that there is a static current, but this is more in the realm of 10nA. At low sampling rates, the digital power current consumption will likely dominate

    Also, I need to point out, that in your original post, when calculating the 900nW power consumption for the ADS7142, this was used with AVDD = 1.8V and nCLK = 18, which give a analog supply current of 0.45uA.