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ADS7945: Digital supply without analog supply, and power down mode

Adnan Ashraf
Prodigy 10 points
Part Number: ADS7945
Other Parts Discussed in Thread: ADS7954

Hi,

I have connected a separate digital supply and a separate analog supply to DVDD and AVDD respectively. The digital supply is always connected to ADS7945 while the analog supply is normally turned off and only turned on when ADS7945 needs to be operated. Is it safe and permissible to keep 3.3 V connected to DVDD supply of ADS7945 while its AVDD supply is 0 V? Besides, how will it affect the power consumption?

Can the PDEN pin of ADS7945 be tied with its CS pin in case of 16-clock burst mode? Moreover, if PDEN pin is set to logic high, would toggling CS pin after its first rising edge result in exiting the power down mode?

Best,

Adnan

  • +1
    TI__Guru**** 181021 points

    Hi Adnan,

    Welcome to our e2e forum!  Ideally the DVdd supply should not exceed the AVdd supply (see page 4).  What are you doing with the analog inputs while AVdd is turned off?  Are they disconnected?  By the ABS MAX specs on page 2, AINx cannot exceed AVdd by more than 300mV.  You can have DVdd without AVdd, but you can't have AINx without AVdd.

    PDEN can't be tied to /CS, it should be held static or controlled by a GPIO.  Please refer to Figure 83 and the POWER-DOWN section on page 30 of the datasheet.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Hi Tom,

    Thank you for your prompt response.

    The information on page 4 assumes that AVDD is within 2.7 V to 5.5 V range, but it doesn't specify the case when AVDD is 0 V since minimum DVDD mentioned is 1.65 V (and not 1.65 V or AVDD if AVDD < 1.65 V).

    The analog inputs will also be 0 V when AVDD is 0 V. In fact, the whole analog section will be off unless a mechanical event is detected but the digital section will always be on. This is the reason that DVDD will be supplied but AVDD will only be supplied when an event is detected.

    Thank you again!

    Best,

    Adnan

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Just to further confirm, the unused differential channel of ADC is connected to AGND. I hope, this is the recommended way to connect unused channels.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Regarding PDEN pin, my case is more relevant to Figures 84 and 85. It is mentioned in the datasheet that The devices remain in a power-down state as long as CS is low. A CS rising edge with PDEN = 0 brings the devices out of the power-down state. But it doesn't specifically mention the case when PDEN = 1 but CS is toggling. I hope, it will keep the device in power down mode even if the CS is toggled when PDEN = 1 after the device has entered into power down mode internally (i.e., once the last frame is read).

  • 0 in reply to Adnan Ashraf
    TI__Guru**** 181021 points

    The information on page 4 is actually telling you that DVdd should not exceed AVdd in order for the specifications of the supply currents to be within the limits shown in the table. 

    Regarding PDEN, If you plan on shutting off AVdd and the AINx channels, PDEN should not really even be a concern for you.  The power down function only applies to the analog section of the ADS7954.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Thanks a lot! In fact, once the analog supply is turned on, the ADC will still be sitting idle most of the time, so power down mode would be helpful in reducing the average current consumption.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Hi Tom,

    Apologies for bothering you again but just want to confirm that will the following connections for ADS7945 work for the device which will go into production without any risk of failure due to these connections?

    Signals when analog supply is off:

    DVDD = 3.3 V

    CS, SCLK, SDO = 0 V (no communication) (can be set to logic high if required)

    CH SEL, PDEN = 0 V

    AVDD, GND, REF, REFGND, AIN0P, AIN0N, AIN1P, AIN1N = 0 V

    Signals when analog supply is turned on:

    DVDD = 3.3 V

    CS, SCLK, SDO = digital signals

    CH SEL = 0 V

    PDEN = digital signal

    AVDD = 3.3 V (not connected to DVDD)

    REF = 3 V

    AIN0P, AIN0N = analog signals

    GND, REFGND, AIN1P, AIN1N = 0 V

    Best,

    Adnan

  • 0 in reply to Adnan Ashraf
    TI__Guru**** 181021 points

    Hi Adnan,

    Unfortunately, I can't tell you that there is 'no risk'.  Technically you are operating the device outside the published limits for DVdd.  Limiting the DVdd current will minimize risk of damaging the device though.