Project help;4 channel voltmeter application

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?

  1. 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.

  2. 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.

  3. 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.

  4. ADC interface preferable is I2C.

  5. 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.

Bench or lab-style voltmeters/multimeters are usually isolated from ground. And of course, handheld battery-powered multimeters have no outside ground connection, just a "black" or "common" connection (technically a reference connection).

Where I work we have a product with multiple isolated DACs & ADCs.

Each separate DAC/ADC circuit gets its power from it's own isolated DC-DC converter.

The data connections to the CPU circuitry are optically isolated (in both directions).

There's no common ground, power, or direct electrical data connection between the main CPU circuitry or between the DAC/ADC circuits.

The CPU section does have a ground connection to the outside world.