On Wed, Aug 5, 2009 at 7:50 PM, Xiaofan Chen wrote: >> Hi Mike, they are all within the supply voltage. But the 4-20mA are >> independently powered. >> > > If you really want to use differential mode, then you'd better use > a differential amplifier, something like AD629 which can withstand > high common mode voltage > > As Mike mentioned, you have to be careful of the maximum common mode > voltage, which is often 1.5V below Vdd with the internal buffer is on. By the way, another way is to use isolated channels, but the cost is in general much higher (individual ADC for each channel and isolated power supply) and much more difficult to get right. Sometimes customers like to have isolated channel for easier handling. -- Xiaofan http://mcuee.blogspot.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist