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ADC Power Issue

Hi,

We are using six ADCs - ADS61B29IRGZR(ADC-14-/12-Bit, 250-MSPS ADCs With Integrated Analog Buffer from TI)in our design . Among them 5 are operating at 50 MHz clock and the other one at 200MHz.

The analog 3V3 power supply for ADC  is provided by TPS7A4700 (1A Low-Drop-out Linear Regulator from TI) and the digital 1.8V power for ADC by TPS71718DCKT (Low-Noise, High-Bandwidth PSRR, Low-Dropout,150-mA fixed Linear Regulator from TI). Separate 1.8 and 3.3 regulators are present for all the ADCs and the 1.8 is derived from 3.3V of respective ADCs. All the 3.3V power regulators gets the input from external 5V input power .

All the 3.3V power is coming properly. For one of the 50 MHz ADC both 1.8V  and 3.3V  powers are coming proper. But for other ADC we are facing some issues for the 1.8V output. The voltage is rising to 1.96,2.01 and 3.1 for different regulators . when we are soldering similar circuit  for TPS71718DCKT  in a general purpose board(without load)  we are observing the requires  1.8V output. But when the same circuit is coming to the PCB board and connected to ADC it is rising high as mentioned above.

Best Rgds,

Mahesh

  • Mahesh:

    The symptoms are characteristic of the LDO "loosing" its regulation due to insufficient voltage or too much current demand.  The 1.8V LDO is a 150 mA device which does not sound like too much.  The ADC nominally draws 70 mA but can vary depending on the LVDS load presented to the device.  Also, the max current may only be available with very low drop-out.  Usually there is a deration curve with the LDO based on current load and drop-out voltage.

    I would check a couple of things.  Is the 3.3V input voltage accurate at each of the 1.8V regulator input.  Perhaps there is a voltage drop across the board that is impacting the actual voltage at the input.  If that is good, measure the current delivered from each of the 1.8V regulators to verify that it is not over-range.  You may not be able to do this too accurately if the voltage is not set right.  Alternatively, pipe in an external 1.8V from a bench supply and measure the current to ensure the load is not too excessive.

    If you are able, you may opt to shotgun in a higher current 1.8 LDO.  Often there are compatible families so that no new layout is required.

    --RJH