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ADS1120-Q1: can't reset or respond after power-up in low frequency

Ting Song
Prodigy 20 points
Part Number: ADS1120-Q1
Other Parts Discussed in Thread: ADS1120

Hi Experts,

I use ADS1120-Q1 with an ARM-M0 MCU's SPI. It's the only device on the SPI bus with din dout sclk and cs. /DRDY is connected but not used. ADC's clk pin is tight to GND. ADC and MCU use same 5V LDO(TPS70950). The LDO is powered by 12V. The LDO has an 2.2uF output capacitor and ADS1120-Q1 has an 0.1uF input capacitor.

When the 12V power connector of PCB board is connected, and then take on/off the 12V power supply . The ADC can start normally and work well.

But, when the 12V power supply is keeping on, and take the connector plug and unplug, (maybe, the action takes power jitter) there is about 1/50 PCB board have 20% chance to get problem.

The problem shows: ADS1120-Q1 don't respond any SPI command including RESET. Reset MCU's SPI bus and full MCU don't get any help. When /CS is low, the DOUT pin is high. When /CS is high, the DOUT pin is Hi-Z. The only way to let the ADC working again is power of and re-power it.

Add an 10uF capacitor to the Vin of  ADC, it can help to reduce the problem chance.

Could you tell me how it happened and is there any way to solve the problem?

  • 0
    TI__Guru** 112785 points

    Hi Ting,

    Welcome to the E2E forum!  Most likely the issue is related to the POR circuit for the digital supply on the ADS1120-Q1.  What has been seen is that if the POR does not properly reset the device, the SPI is in a blocked state.  Please refer to the Power-Supply Ramp Rate discussion in section 10.2 of the ADS1120-Q1 datasheet.  The internal reset mechanism in the ADS1120-Q1 must stay low long enough or the internal POR may not complete as expected.  This internal low state is directly related to the DVDD ramp rate.

    I'm not sure what plug you are referring to, but most likely there is a transient on the power supply that is causing the device to reset.  If there are connections that go to the ADC, then you must make sure that any transient does not exceed the Absolute Maximum Ratings for the ADS1120-Q1.  Transients on the analog inputs could be driving back into the supply through the ESD protection structures.

    So you must make sure that any transient from external connections does not effect the ground or supply to the ADS1120-Q1.  This may require protection devices such as TVS diodes on the signal lines.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,

    Thank you for your reply and great help. It's seems to be the POR issue. when I used a relay, a MOS stich or directly plug the Vin connector, the issue is shown frequently. Power-up from a slow source, such as 200ms/5V, the issue doesn't happen.

    When I add an 4.7uF cap to 5V of ADS1120, the issue is solved.

    It shows the power-supply ramp rate is the cause.

    Is there any way to make sure the PCB design to match the 50us/V requirement? A series resistor may cause voltage fluctuations。

  • 0 in reply to Ting Song
    TI__Guru** 112785 points

    Hi Ting,

    Usually on a normal powerup there is enough inductance and board capacitance to maintain a slower ramp rate.  When this issue was first discovered it was due to fast powering of the device through a switch.  The fast start transient on DVDD may effect some devices to miss the hold time required to keep the ADS1120-Q1 internals at a reset state.  The most important transition region is 0 to about 1.4V.  So if the ramp rate is held at 50us/V in this region you should be ok.  Normally this can be done with just using a cap load near the DVDD pin and apparently this worked for you.  You could also add a small value of resistance in the range of 1 to 10 Ohms on the DVDD line which should not impact the operation of the device.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,

    Thank you for your quick response.

    I still have 2 questions:

    1. For a normal IC, The power ramp-rate will not to be care, there will be a low-voltage-detection block. When the voltage gets to lowest limit, the reset function will be executed. Why the ADS1120-Q1 have a special reset requirement?

    2. The LDO for ADC is TPS70950, the power ramp rate will match the 1V/50us when the capacitor is larger than 22uF without series resistor. It's too big to the PCB area and will takes cost issue. Could you help to provide a fit value for the ADC power RC circuit's value?

    Thank you again. 

  • 0 in reply to Ting Song
    TI__Guru** 112785 points

    This discussion has been take offline.

    Best regards,

    Bob B