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?
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.
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.
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.
ADC interface preferable is I2C.
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.