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ADC with differential inputs.

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David_

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Hello.

I'm sorry, this is my second thread on this subject but I'm puzzled about the concept of differential inputs.
I understand what it is and how it is implemented with two opposite signals that are summed together...?

What I find screwing up my thoughts is that in a ADC which power supply is a single supply between VCC and GND and has a 2.048V voltage reference has an input range of ±2,048V, how does it handle negative voltages without a negative supply?

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barry

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With differential inputs the two inputs are SUBTRACTED (the word "differential" is from the root "difference"), NOT summed.

When the plus input is 2.048 volts HIGHER than the minus input, that's +2.048. When the plus input is 2.048 volts LOWER than the minus input, that's -2.048. So, you could have the plus input at 3.048, and the minus input at 1.0, or reversed, and you'd get the +/- input range without exceeding either supply rail. Your COMMON MODE voltage (look it up) is usually set midway between the supplies, so that for, say, a 5V VCC your differential range could be +/- 2.5V
 
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