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ADS124S08: Using the VBIAS register

Jack Scherlag
Prodigy 20 points
Part Number: ADS124S08

Hi everyone,

For measuring voltages, I have setup four channels that I will elaborate on below.

Channel # + Input - Input
1 AIN5 AINCOM
2 AIN3 AIN4
3 AIN2 AINCOM
4 AIN1 AINCOM

Before I setup the VBIAS register, floating inputs tended to cause very noticeable fluctuations in the internal temp sensor and the voltage readings. I have tried setting VBIAS on all inputs except the channel I want to read from, setting VBIAS on all the positive inputs (AIN5, AIN3, AIN2, AIN1), and setting VBIAS on all the negative inputs (AINCOM, AIN4). Right now, it seems that setting VBIAS on all negative inputs tends to perform the best. I can tell that VBIAS is set up properly if the voltages I am reading don't change if I connect other inputs or disconnect other inputs. I'm not sure if there is a correct way to set up the VBIAS register according to the inputs you intend to utilize. All I can really find on the data sheet about unused inputs is on the bottom of page 31 where it says not to overdrive an unselected input. Even though my query is pretty specific, I would appreciate any answer on dealing with floating or unused inputs and making sure they don't interfere with conversions.

            

  • +1
    TI__Guru** 112705 points

    Hi Jack,

    There is section 10.1.5 on page 89 of the ADS124S08 datasheet titled Unused  Inputs and Outputs.  Floating inputs should not effect the channel being measured.  Inputs that are connected in such a way as to be outside of the input range of the ADS124S08 might affect the measurement.  The analog input range is bounded by the analog supply (AVDD/AVSS).  If the input voltage is outside of the input range, there could be an issue with the measurement.  This can happen if your input voltage is connected to an ac powered external supply that outputs a dc voltage at a different potential relative to AVSS.  In this case VBIAS helps to adjust the bias to voltage within the input range.

    However, the use of VBIAS may sink/source current in this type of configuration that will add error into the measurement..  The purpose behind VBIAS is to set a bias point for sensors such as thermocouples.

    If you still have questions, please send me your schematic and the input voltage(s) you are applying.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,

    I have read through the section on unused inputs and outputs, and I still have some confusion on the use of the VBIAS register. There is an isolated thermocouple I am measuring, and I have two questions regarding it:

    1) What happens if you apply the bias voltage to both the positive and the negative inputs?
    2) Is it better to connect the VBIAS to the positive versus negative input?
    Thank you for your time,
    Jack S
  • +1 in reply to Jack Scherlag
    TI__Guru** 112705 points

    Hi Jack,

    There are may possible configuration for biasing the TC.  A good resource is A Basic Guide to Thermocouple Measurements.  The ADC measurement is a differential measurement between AINP and AINN.  If you have the same voltage at both inputs, then the result is 0.  However, the actual outcome may differ depending on the specific mux resistance relative to the VBIAS path and the influence relative to the TC, but in any case you only want to set the VBIAS to either the AINP input or the AINN input, but not both.

    As to whether AINP or AINN should be used for VBIAS, there should not be a measurable difference.  The only thing that would change slightly is the common-mode for the PGA and this will not affect the TC measurement as the TC voltage is small.

    Best regards, 

    Bob B