Project help;4 channel voltmeter application

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Sep 3, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    I have to make a 4 channel voltmeter. This voltage is to be read by MCU & display on graphical lcd.
    Three with +-20V range (differnetial) & one with 0-3.3V(singl ended).
    Need to take 10sps of each.

    I was looking at ADS1248 which has 8 channel SPI interface.

    Some of problems i think are:
    1. Usually in voltmeter/multimeter the black probe is not shorted with ground of its 9V battery.
    So if I use ADS1248 then its ground will be differnet then voltage at differnetial pins.
    Then I don't know how Vcm range equation is satisfied here.
    Can I apply voltage at differnetial inputs of adc which have no direct relation with ground of adc?

    2. Even if I short black probe with gnd for measurement purpose & provide +-2.5V to adc for negative voltage measurement,
    then it will short one channel of all channels, which I want to avoid. i want to keep all channel isolated from each other.

    3. Also planning to place LTC1966, a rms to dc converter in front of each channel so that i can measure exact dc value even if ac voltage is applied.

    4. Currenly I am searching for some switch or mux (single pole 4 pole), so that I can switch between channels for isolation in case its not possible to isolate each channel otherwise. Also in this case i will have to place only one LTC1966.

    5. ADC interface preferable is I2C.

    6. Can provide system upto +-3.3V only. This is one rule. For higher voltage measurement i have to step down the signal.

    Any suggestions how to do it.