Measure upto +-40V using MCU

Discussion in 'Embedded Systems and Microcontrollers' started by Vindhyachal Takniki, May 8, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    349
    6
    1. I have to make a multimeter (very low cost basic) , measuring 0V-40V & in reverse also.
    2. I have a ckt which is attached.
    3. MCU is powered by +5V , 10 bit adc.
    4. First I have level shifted the signal to +2.5V i.e mid supply. Then 40V is divided into 2V.
    So that max signal can swing 0.5 to 4.5V at adc input of MCU.
    5. Any comments/suggestions on it?
    6. In simulation it works fine.
    7. Also I have added two back to back diode for protection in case of overvoltage.
     
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  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Your circuit assumes the 40V is completely isolated from your device, make sure that is so.

    The accuracy of the measurement is dependent on the accuracy of the 5V power source, and also all those resistor tollerances too.

    Protection diodes as you added are a good thing. Using schottky diodes is even better, and recommended for some devices as the internal ESD diodes inside the ADC may conduct and cause problems. This is covered in a recent app note from Microchip on inputs with analog capabilities; pure digital inputs seem OK.
     
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