ADS124S08: When AVDD short with GND , what could be the DRDY signal level

Hi Arumugam,

When you remove the analog power the device should reset.  The information is given on page 59 in section 9.4.1.1 Power-On Reset.  If a power-on reset takes place the flag FL_POR in the status register will be set.  This bit should be cleared and monitored for a power event.

A true black out event (supply completely goes to 0V) will trigger a full reset.  This event can be monitored and software can reconfigure the device to the proper settings.

However a brownout event could occur where the device may not reset given a condition where the voltage does not go low enough to trigger a reset.  This would cause a condition where the device will still try to convert, but the reference voltage is missing or be reduced and the analog supply is not high enough in voltage to operate the analog circuitry.

A third situation can arise when you remove the analog supply voltage, but sensor output can drive the analog supply through the ESD structure (often considered as diodes) and prevent a true blackout event from occurring.  This condition can damage the ADS124S08 and any potential input current must be limited to less than 10mA into the analog input pins.  This is similar to the brownout event in that the ADS124S08 will most likely not reset due to the external input voltage powering the analog supply through the ESD structure.

If you need to reduce power consumption it is better to use the POWERDOWN mode as discussed in section 9.4.2.

I noticed that in your schematic AGND and DGND are separate grounds.  These grounds should be connected together at or near the ADS124S08 with a low impedance connection (direct short with no ferrites used).  This could be another issue as when the analog supply is switched off/on there will be a current surge and any inductance can create a voltage potential.  This transient can affect the operation of the device which may be placing the ADS124S08 in an undesired state (such as the continuous conversions being seen).

I would suggest not powering off/on the analog supply unless absolutely necessary.  And if necessary, the analog inputs be prevented from accidentally powering the analog supply.  I would also suggest monitoring the status register for a reset event.

You will most likely benefit by adding capacitors to create low pass filters on the analog inputs.  This will help reduce noise and create an antialiasing filter.

Best regards,

Bob B