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ADS127L11: I encountered an issue with excessive current when using the ADC and couldn't find the cause

畅畅 赵
Prodigy 10 points
Part Number: ADS127L11
Other Parts Discussed in Thread: LM27762,

Tool/software:

Hello everyone,

Recently, I have been using this ADC chip to collect voltage in a transimpedance amplification setup. However, I encountered an issue with excessive current during use. When the PCB is first powered on, the operating current is below 100mA. Within 1 to 2 minutes, it steadily rises to 600mA. Since I'm using the LM27762 power supply chip, when the current rises to 200mA, the negative voltage output starts to drop slowly from -2.5V to -2.2V, causing zero drift in the front-end amplifier.

Currently, I can communicate with the ADC via SPI normally when the current is stable, but the high current affects the power supply voltage, leading to measurement accuracy problems on the PCB. I tried removing some components to determine if other components were causing the issue, but it didn't help. Only when the ADC is removed does the current drop to around 10mA. My circuit is essentially consistent with the reference design in the datasheet. Below is the schematic of my PCB design. I hope you can help me identify the problem. Thank you.

In practice, I replaced the 1.8V digital power supply chip with a 3.3V chip to communicate properly with the FPGA.

  • 0
    TI__Mastermind 46550 points

    Hello,

    If the ADS127L11 is the device consuming the current, then it has been damaged and needs replaced.  This could be caused by a power supply transient during power-up of your board.  The ADS127L11 can also be damaged when probing the pins, if the CAPA or CAPD pins are shorted to the AVDD (+2.5V) or AVSS (-2.5V) supplies.

    I also noticed you have an LC filter for your power supply input.  Although this filter is good to reduce power supply noise, it can also have large over-voltages if you connect your lab supply to this LC filter when the supply is on.  You should make sure that when you make connections, the power supply output is OFF and then enable the power supply output after all connections have been made.

    Below is a quick simulation of your input filter with a sudden 5V step input.  You can see that the output of the filter is more than 2x of the input voltage for several milliseconds.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 10 points

    Thank you for your reply. In my previous query, I mentioned that under these high current conditions, I am able to communicate with the ADC normally, and the values read out are consistent with the output of the front-end amplifier. Additionally, I currently have three PCBs, and each one exhibits the same behavior, so it doesn't seem to be due to a damaged ADC.

    Additionally, according to the issue you raised regarding the power supply filtering circuit, my power-up sequence meets the requirements. To determine if there is an unknown cause, I replaced the inductor with a 0-ohm resistor and removed the components around the ADC before powering on again. The situation remained the same. Finally, when I removed the ADC, the current dropped to only 10mA. Below are the test images.

    Regards.

  • 0 in reply to 畅畅 赵
    TI__Mastermind 46550 points

    Hello,

    I missed this the first time, but looking at your schematic more closely, it appears you have the thermal pad connected to ground.

    This pad must be connected to AVSS, or the VCC_/2V5 net in your schematic.  The thermal pad is not electrically isolated from the die, and must be at the most negative voltage connected to the device.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 10 points

    Thank you for your response. I carefully reviewed the chip's datasheet and found that there was indeed a connection error.