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ADS1248: ~20uA higher supply current after reading internal DVDD versus AVDD

Part Number: ADS1248

I send the following bytes the ADC to read AVDD: 0x40, 0x06, 0x01, 0x00, 0x76, 0x02, 0xEC, 0xFF, 0xFF

Followed by 0x04, 0x04 to start the conversion.

The expected result is read by sending 3 X 0xFF.

The supply current remains nominal.

I send the following bytes the ADC to read DVDD: 0x40, 0x06, 0x01, 0x00, 0x77, 0x02, 0xF9, 0xFF, 0xFF

Followed by 0x04, 0x04 to start the conversion.

The expected result is read by sending 3 X 0xFF.

The supply current remains 20uA higher!?!?

If I read AVDD again, the supply current drops by 20uA...

  • Hi Richard,

    There are a couple of things that are not quite clear. Is the START pin high continuously? Also, when you are measuring the current, are you measuring the total device current? Or is it just AVDD or just DVDD? Have you tried measuring the current when not measuring AVDD or DVDD (i.e. connecting to the MUX)?

    I don't have the specific details regarding the internal voltage divider, but what I think might be happening is when DVDD is being measured there is a specific connection to DVDD that is not normally a part of the analog section, so when you connect the measurement to DVDD the current is being pulled through the DVDD section to the voltage divider circuit. Until you change the monitor to something else, DVDD current will be drawn from the digital supply to the analog section.

    Best regards,
    Bob B
  • Hi Bob,

    Thanks for the quick reply!  No, START is only high when I am writing the registers or commands.  It is low the rest of the time.  I can only measure my total unit current, a sum of AVDD and DVDD, and other logic.  I see a static DC shift after reading DVDD of ~20uA.  Yes, it could be something peculiar to a DVDD divider, but I am surprised it does not happen with AVDD.  I am worried now that I must reset the internal registers to their defaults after every reading to guarantee the lowest current consumption.

    Thanks,

    Richard

  • Hi Richard,

    Here is something to try. Leave the START pin high and measure the current and compare to AVDD connected, DVDD connected and MUX (normal operating mode) connected to the system monitor. I think what might be happening is when START pin is low and the conversion has completed, the DVDD is still connected to the analog portion even though the analog portion is powered down when the DVDD is connected to the system monitor. The current from DVDD will pass through a portion of the analog section which will actually attempt to turn some portions of the analog on. This would be a sort of back driving the AVDD supply. If I'm correct, this would only happen in the case when DVDD is connected to the analog. So when using the system monitor to measure DVDD, I would change this particular setting to something else prior to setting the START pin low. This should not affect the other connections on the analog side. The verification would be based on the measurements I suggested at the beginning.

    Best regards,
    Bob B
  • Hi Bob,

    What you say makes sense. I'll change the code and try it tomorrow and let you know...

    Thanks,
    Richard
  • Hi Bob,

    Yes, as you suggested, DVDD does seem to be back-feeding the powered down analog sections through the mux.  I wrote the mux register back to its default and the current went back to normal.  This write starts an unwanted conversion which wastes power for 50ms.  Can you think of any way around this?

    Thanks,

    Richard

  • Hi Richard,

    I spent some time in the lab today looking at currents.  I too have a similar issue as you as my board can separate the analog and digital currents, but not devices that are powered by these supplies.

    Looking at the AVDD supply, there is about 130uA greater current consumed when taking the DVDD measurement as compared to the AVDD measurement.  However the DVDD measurement consumes about 40-50uA less current than the normal mux input measurement (I think this is due to powering the PGA).  When the START pin is low I see no difference in the analog current.

    From the digital current perspective, I see about 20-30uA less current when taking the DVDD measurement as compared to the normal mux input measurement.  I see about 20-30uA less current from taking the AVDD measurement as compared to the DVDD measurement.  When the START pin is low, I see no difference in the current from the normal measurement to the AVDD measurement, but I do see the 20-30uA difference between the AVDD selection and DVDD selection.  This would confirm what you are seeing.

    My conclusion is the DVDD is still connected to the measurement string when the START pin is low as the currents remain pretty consistent with respect to the 20uA.

    I'm not sure how often you are checking DVDD voltage, but one thought would be not to check all that often.  Another would be to change the data rate to something faster at the same time you switch the system monitor away from DVDD.  This may seem like an extra step, but keep in mind that when you write to any of the the first four registers the conversion will automatically restart, so rewriting the register at the beginning of the next measurement may not take up more time than you are currently doing as the SYNC command which would no longer be necessary.

    Best regards,

    Bob B

  • Hi Bob,

    Thanks for your suggestions and your thorough investigation of this issue.  I will make the changes you suggested.  I also changed my code to just pulse START high instead of issuing the SYNC commands.

    Thanks again for solving this!

    Richard