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I've been reading up on differential ADCs but haven't quite wrapped my head around them. I'm a hobbyist with no real training, you see.

I'm wondering, is it possible to use a differential ADC with +5/0 supply and an input signal where the negative input is below 0V? I'm wondering if I just have to keep the signal difference within 5V (like +/-2.5V) or if the signal must be between the supply rails of the ADC?

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  • \$\begingroup\$ There are devices that work from 5V and 0V and the input can go below 0V - do you have a specific device in mind? \$\endgroup\$
    – Andy aka
    Jul 25, 2014 at 12:57
  • \$\begingroup\$ I would be inclined to say that if the input signal have a differential that is positive then it should work. I say this because internally the signals are likely first subtracted from one another via an analog circuit before any sample and hold measurements take place. As such, the ADC never converts negative voltages so it should work? I posted a comment because I am not entirely sure. \$\endgroup\$
    – sherrellbc
    Jul 25, 2014 at 12:58
  • \$\begingroup\$ If you can't find the type of ADC you are looking for, you may find a current sense amplifier (CSA) that can convert the voltage to a range you can sample with a normal ADC. Or maybe you can find an integrated ADC and current sense amplifier that can digitize negative voltages. For sure I have seen CSA's that can tolerate at least one input lower than the negative rail. \$\endgroup\$
    – user57037
    Dec 29, 2017 at 18:34

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First, that question should be clearly answered in the datasheet for the device. The absolute maximum section will tell you what voltage range the inputs can tolerate without damage to the part, and the operational spec will tell you what input voltage range it can sense.

Second, it is very unlikely that the A/D can sense voltages outside its power supply range. Again though, read the datasheet for the definative answer.

Most likely you will need to put a resistor divider in front of your A/D. That attenuates signals, so the signal level into the resistor divider can be larger than the native signal range the A/D can convert. Note that the second resistor of the divider (the one your input signal is not attached to) doesn't have to go to ground. In your case, you may want to connect it to the positive supply, or some other fixed voltage source.

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  • \$\begingroup\$ As Sherrellbc said, I was thinking the inputs might be subtracted first which would yield a positive voltage but couldn't figure out if that voltage would still be negative compared to the - power rail. I don't have a specific device in mind and in fact I'm planning on biasing the sensor to make its output fall within the adc power rails. Just wanted to get a better idea of how the differential adcs work. Thanks very much! \$\endgroup\$
    – user3758018
    Jul 25, 2014 at 14:04

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