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ADS114S06: Larger than expected ADC reading

Part Number: ADS114S06

Basic problem description:

For a given voltage input the ADC code generated is greater than expected.

Detailed problem Description:

10uA Burn-out current was selected on AIN3 and AIN4 pins via the System Control register. These pins were connected to a 1500ohm resistor. As expected 15mV was developed across the resistor and between pins AIN3 and AIN4. However, the resultant ADC code was greater than what 15mv should give. After some investigation it appears as if there is roughly an additional fixed 850ohm within the ADC that the burn-out current also develops a voltage across resulting in a larger ADC code than expected. If I take this suspected internal 850ohm resistance into account, then my ADC codes are correct for whatever external resistance I use.
Does this ADC have internal resistance that the burn-out current develops a voltage across or is there some other reason for the larger ADC code?
If there is internal resistance, then what is the range of this resistance?



  • Hi Nasir,

    Any resistance encountered in the path to include internal mux resistance or analog filter resistors will affect the measurement.  The internal mux resistance will vary over temperature (as most likely the BOCS will as well).  There is the following statement in the ADS114S06 datasheet in section

    Distinguishing a shorted sensor condition from a normal reading can be difficult, especially if an RC filter is used at the inputs. The voltage drop across the external filter resistance and the residual resistance of the multiplexer can cause the output to read a value higher than zero.  

    Obviously the same is true when checking for sensor degradation.  The use of the BOCS is primarily to check for opens and therefore the name 'burn-out'.  However the need is often there to check for degradation or shorts as well.  If there are open analog pins, the IDAC current sources could be used outside of any filters which would be a better reflection of the resistance value.

    To give an idea how this may be accomplished check the RTD wire break detection application note.

    As to the min/max values of the internal mux resistance or the BOCS output current this is not information I can give as it is not a guaranteed specification as shown in the electrical characteristics table.  The value you determined would not be unreasonable, but could vary over temperature and from device to device.

    Best regards,

    Bob B