ADS124S06: Acquisition Problem

Hi Hendrickx Olivier,

One thing I noticed in the schematic snippet is you show two different ground symbols where the AGND differs from digital ground symbol.  The grounds should be at the same reference point.  Generally a common ground is preferred, but a star ground configuration would also work.  The key consideration here is the AGND and DGND should not vary by more than 300mV apart when using unipolar power supplies (AVSS is connected to AGND).  Any trace or device inductance can cause issues if transients are present.

I also see that there is no antialiasing or input protection to the ADC.  Connecting a bench supply directly to the ADC can be an issue that if the current through the device exceeds 10mA the ADC can be damaged as per Absolute Maximum Ratings.

As to the issue you are seeing, you are testing the circuit differently than what was intended by your design.  You are also assuming that the bench supply you are using is within the input range of the ADC.  I have often seen AC powered bench supplies differ relative to earth ground by 40V or more.  So this means you really need to have a common ground point.  If the output voltage of the bench supplies relative to AGND is outside the input range of the ADC, then you again you are exceeding the Absolute Maximum Ratings of the ADC and can damage the ADC.

So I would suggest using an equal value voltage divider (10k for example) for AINCOM where the resistors are connected between 3.3V and AGND and the junction of the resistors connected to AINCOM.  The bench supply can be connected with the - terminal connected to AGND and the + terminal connected to AIN1.  This is called a pseudo-differential measurement where you will see positive codes above 1.65V (3.3V AVDD/Ref) and negative codes below 1.65V when the voltage is connected to AIN1.

Best regards,

Bob B

Hi Olivier,

As far as input protection there are two primary concerns but they are both related.  The first concern relates to input voltage.  The Absolute Maximum Rating for the analog input voltage is 300mV from the analog supply rails.  Beyond 300mV the ESD diodes will begin to conduct.  The second concern relates to the Absolute Maximum Rating for the input current.  It the input voltage exceeds the input range, the ESD diodes will conduct and current will flow through the input pins.  This current must be less than 10mA maximum.

As to the amount of protection required will depend on the system.  For example, if the sensor has wiring connected to the ADC, the wiring can pick up EMI/RFI which can produce a transient that may exceed the supply voltage and current could potentially flow through the input pins.  The amount of protection required in this use case would be much different than an onboard sensor that is PCB trace connected and not exposed to an outside the system environment.  You can see an example for prevention of electrical overstress in this application note.

In your circuit, the current can be limited as a part of the antialiasing filter.  There is a discussion on the input filtering in section 10.1.2 on page 87 of the ADS124S06 datasheet.

Best regards,

Bob B