How does a differential analog to digital converter differ from a regular ADC?
A differential ADC will measure the voltage difference between two pins (the plus and minus input). A single-ended ("regular") ADC will measure the voltage difference between one pin and ground.
A lot of differential ADCs can be configured to give you twice the channels in single-ended mode. For example the AD7265 has 6 differential channels and 12 single ended channels.
A regular ADC samples it's inputs in the range 0V to AVcc, where AVcc is often configurable (5V, 2.56V, user input etc).
A differential ADC shifts the lower reference from 0V to some other value - either a user input on a second analog input, or a internal reference. This is helpful for measuring small signals that have a large DC offset - eg measuring changes of 100mV in the range 2.5-2.6V.
Readings for voltages lower than the offset are hardware dependent - can give negative readings, absolute values, or zero.
A typical application is in a load cell which has a small voltage change at some DC offset.
As others said, it has two inputs for each signal, one of which is subtracted from the other.
This gives you more signal-to-noise ratio because
- The maximum input level is 6 dB higher
- Uncorrelated noise of the two inputs combines to be only 3 dB higher
- It cancels out any common-mode noise. (If the ADC's ground voltage is fluctuating relative to the ground of the thing being measured, for instance, both inputs will move up and down together, and this will be cancelled out. If the two inputs are both driven from the same op-amp and some of its power supply noise is getting into both outputs, that will be canceled out. etc.)
It's hard to say exactly what you're talking about without a reference, but I'm guessing that you're talking about an ADC that has a differential pair input.
Differential pairs are nifty things that allow you to double the perceived voltage swing without raising the supply and inducing additional noise. Essentially what goes on is that instead of having a signal referenced to ground, the two wires are total opposites; when one line is at +1.3V, the other is at -1.3V. The voltage of either line to ground is only 1.3V, but since the ADC is converting the difference of the voltage on these signals, you have 2.6V.
I assume you're talking about ADCs which sample differential signals.
Differential pairs are used where ever you want to limit induced voltages. Ethernet and USB are both differentially signaled. Lots of RF is differentially signaled. If you do some hunting on Google you'll find LOTS of more information.
Another point not yet mentioned is that a typical ADC which is designed to resolve 0-3 volt signals to one millivolt precision (12 bits) might not have much better than one-millivolt precision when trying to resolve a 0.1 volt differential signal riding on a two-volt common-mode signal (e.g. it might have 8 bits of useful precision), whereas an ADC which is designed to resolve small differential signals would be able to perform much better; a 12-bit ADC could be designed for such purposes to provide 12 bits of useful precision with a 0.1-volt signal without having to be designed to provide 16 bits of precision on a larger signal).
A Differential ADC is a two terminal device. In principle it takes the difference between the voltages on the two terminals and converts that into a 2's complement binary number. I would say it's common to see this type of ADC used for signals that vary around GND, since in principle negative conversions have meaning in this context. A single-ended ADC is a one-terminal device, where the voltage is converted to a binary number by comparing it to an internal reference (say ground). Typically these are used for sensors that have output a linear voltage in proportion to the phenomenon they are sensing.