Reading negative voltage with ADUC7060


             Anyone knows if it´s possible to inject negative voltage in the ADUC7060 A/Ds? This part is not much clear in datasheet.

             I need to read the voltage of a sensor that have an output of + or - 414mV, and I don´t want to use ampops to treat the signal to a positive voltage.

             I´m afraid of inject a negative voltage in my ADUC7060 kit and burn it.

thank you!

  • 0
    •  Analog Employees 
    on Dec 24, 2010 6:05 AM

    The ADuC706x can measure bipolar signals, but as with any kind of single supply silicon (2.5V for the ADuC706x) negative input voltages are not allowed and clamped below -300mV at least. But in the electrical specifications page 6 of the data sheet REV. B the absolute input ranges where the correct operation of the part is guaranteed is given as follows:

    Main Channel          0.1V to AVDD - 0.7V for both inputs -VIN and +VIN

    Auxiliary Channel    0.1V to AVDD - 0.1V for buffer enabled

                                 AGND to AVDD         for buffer disabled !!! higher input current !!!

    With buffer disabled on the auxiliary channel you may be able to measure directly negative voltages, but they are clamped at -300mV at least and no performance is guaranteed.

    For a correct measurement with best performance I recommend to bias the sensor output, as i.e. shown on page 54 in figure 19 for a thermocouple. You may not need to use a external reference for this. You can also use a simple voltage divider from AVDD, the DAC output or a filtered PWM output.

    What kind of sensors are you using in your application?

  • I´m using a pH sensor (-414mV to 414mV)  and an ORP (Oxidation Reduction Potential) sensor (-2V to 2V).

  • 0
    •  Analog Employees 
    on Dec 28, 2010 9:30 AM

    Can you please provide some more information on how the output circuit of the sensor is looking in detail ?

  • The pH Sensor has two terminals, and works as a pH dependent battery   (+ and -) with high impedance (gigaohms at 0 oC, hundreds of megohms at  25 oC).

    I Don´t know the impedance of the ORP sensor, but I believe that is the same order of the pH. The ORP sensor has two terminals, works as a ORP* dependent battery, and has an output from -2V to 2V.

  • 0
    •  Analog Employees 
    on Feb 7, 2011 9:22 AM

    So the best recommendation I can provide is to use a IN-AMP to level-shift the signal to the appropriate input-range of the ADuC706x.

    You may find a example on our PLC-Demo-System in the Users Guide on page 11 figure 21 - in this case isolated.

    In this application the IN-AMP AD8226 is used in front of a AD7793 24-Bit Sigma-Delta-ADC, which is similar to the ADC on the ADuC706x. There is a complete standard input circuit shown with the required robustness for real world applications.