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ADS7142: Strange power consumption with low AVDD

Harald Ilg
Genius 3060 points
Part Number: ADS7142
Other Parts Discussed in Thread: CC2640,

Hy,

in a battery driven application I need to run an ADS7142 with a CC2640 BLE controller. I need to measure resistance rather than voltage, so I want to use a processor port pin as AVDD to get independent of the battery voltage. Overall AD results look as expected, but there is a strange effect with power consumption:

To save power I only want to switch on AVDD ever once in a while and measure and switch it off immediately. I connect AVDD in the following way:

where pwr_adc is a processor port pin

So if switched on there is a AVDD voltage of around 1.4V (dependent on battery voltage).

When I measure the overall power consumption, I see a very strange effect:

Before I start the cyclical measurement, everything looks quite normal. Then I see the cyclical measurement (once a second)

But when I look into power consumption between measurements, there is a slowly increasing current to 500uA over 1s. In this time, the AVDD is switched off completely.

Any idea for the root cause of this?

May I set AVDD to 1.4V when DVDD is at 2.8V? Is there min voltage for AVDD?

Any help appreciated

Harald

  • 0
    TI__Expert 4405 points

    Harald,

     

    Thank you for your interest in the ADS7142.

    At what point are you measuring power consumption?

    The device is designed to operate with an AVDD in the 1.65V to 3.6V range, so you need to modify this in your design. And it is best to generate the required AVDD using an LDO or a low impedance power source, rather than a resistor divider.

    The ADS7142 is ideally suited for low power IOT applications such as the one you are targeting. You should consider using the Autonomous modes of operation of the ADS7142 where the current consumption is <1uA compared to the 100s of uA that you are currently seeing.

    Do let me know if you need any help configuring the device in this mode.

    Thanks.

     

    Regards,

    Sandeep

  • 0 in reply to Sandeep Kasargod
    Genius 3060 points

    Hy Sandeep,

    thanks for your reply

    >And it is best to generate the required AVDD using an LDO or a low impedance power source, rather than a resistor divider

    As I need to measure a resistance rather than a voltage I can not use an LDO as power source because I want to compensate both, battery voltage and voltage drop over the CPU IO port. And I need to switch on and off the analog power, which not only supplies the AVDD of the AD Converter but also the resistor connected to YF111 and YF112.

    > At what point are you measuring power consumption?

    We measure the overall power consumption of the whole board, but we use this board many times without the ADC, so we assume the current draw is from the ADS7142. (Usual power consumption is about 16uA)

    We did some more measurements. As long as the AVDD is higher than 100mV, there seems not to be a current. As soon as the AVDD drops below 100mV, we measure a current consumption of several 100s uA. I can't find any reference to this in the datasheet. Is it not allowed to have an AVDD of 0V? Can we even damage the part by a low voltage? On two boards, we measure a high current even with a switched on AVDD.

    Regards

    Harald

  • 0 in reply to Harald Ilg
    TI__Expert 4405 points

    Harald,

    If you are measuring the overall power consumption of the whole board, we need to localize this extra current to the ADS7142 before we dig further. Could you measure the drop across R104 to see if it is indeed being drawn by the ADS7142?

    Thanks.

    Regards,

    Sandeep

  • 0 in reply to Sandeep Kasargod
    Genius 3060 points

    >drop across R104

    As the current flows while we have switched off the AVDD there is no current over R104. We assume, that the current is drawn from DVDD.

    We did further tests and characterised the ADC without any other components. It seems, that there is a high current (400-500uA) into DVDD whenever DVDD is at 2.9V and AVDD is at 0V. As soon as AVDD raises over 100mV, the current stops.

    In our application we solved it by adding 50k from DVDD to AVDD, so the minumum AVDD is 100mV even when the PWR_ADC is switched off. Drawback is a constant current over the measure resistor connected from YF111 to YF112. For the first prototypes we can live with that but for a series solution we need to find something else.

    Regards

    Harald