ADS124S08: Input protection against miswiring

Hi Mago-san,

If the device normal operating range (Recommended Operating Conditions) is exceeded there can be issues.  You are actually describing 2 possible conditions but they are both related to the Absolute Maximum Ratings.  One is related to the Absolute Maximum condition for current and the other relates to an operation less than the Absolute Maximum for current but still beyond the normal operating range for the analog input.  For example, the Absolute Maximum Ratings for the Analog input voltage is AVSS-0.3V and AVDD+0.3V and the Input current is rated at +/- 10mA.  For the ESD diodes to conduct, then the Analog input voltage has been exceeded.  In this case you may not be exceeding the input current rating but the analog input voltage rating is still exceeded.

So for the first question if current is flowing through the ESD diode of the ADS124S08 you must limit the current to less than 10mA.  Current will only flow through the ESD diodes if the input voltage has been exceeded by either AVSS-0.3V or AVDD+0.3V.  The best course of action to prevent the condition that causes the over voltage at the inputs.  This can be done in a variety of ways with some of them being TVS diodes and/or other diode arrays.

For question 2, you need to look at the Recommended Operating Conditions in section 7.3 of the datasheet and in particular the Absolute input voltage.  In the PGA bypassed case you are limited to 50mV from the supply as opposed to 300mV.  With the PGA enabled you are limited to AVSS+0.15 to AVDD-0.15 at a gain of 1.  As there is a considerable difference between the normal operating conditions and Absolute Maximum Ratings, even a small amount of current running through a tiny silicon chip can cause an error.  Consider mA of current flowing in areas and traces it was never meant to handle (where nA are normal).  As the condition of the input voltage is exceeding 300mV from supply to flow current within the ADS124S08, this will definitely exceed the Recommended Operating Conditions to which the ADS124S08 was tested and known to work. 

Best regards,

Bob B