I am trying to select and ADC with single ended inputs which can input positive and negative voltages (around +500mV to -500mV). Due to the nature of the system I am selecting it for, the voltages cannot be DC biased and also cannot be amplified (in other words the exact DC level is the information). I was wondering if it would be possible to set the reference voltages to REF+ = 500mV and REF- = -500mV, while the power supply is 2.5V to -2.5 V (the ADC allows for true bipolar operation). For instance the ADC I am looking at is an Analog Devices AD7176-2. I noticed in the data sheet that the limits for the reference is that their total voltage difference can be 1V, but the tests they have are only for +5 V to Ground supply, not bipolar operation. Hopefully someone has experience with operating an ADC in this mode. Thanks
Electronic – Ranges for ADC Reference Voltages
adcreferencesingle-ended
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It no longer makes sense to say Vref = 2V in a bipolar single-ended ADC. You now have Vref-, which is converted as full-scale low, and Vref+, which is converted to full-scale high. Inputs between your two reference inputs will be linear (within the linearity error), and inputs beyond either of the references will either be clipped or destroy the device (depending on whether you respect the maximum ratings). This is sometimes called pseudo-differential, because it is the same as subtracting the negative reference from all other analog inputs.
However, the ADS1282 would be better described as fully differential. In this case the inputs are not limited to the range determined by the references; a common-mode signal may exist which will be cancelled as long as the input range determined by the power supplies is not exceeded. And it does not make any sense to talk about an input of +1V, or -1V. You have two inputs per channel, and the difference and common-mode offset are both important.
In the context of the power supply, an ADC supporting bipolar supplies simply means that AVSS is not tied to DVSS.
A very simple reason might be that the differential pressure sensor might be wired backwards and this will cause the AD622 problems on the output without a negative supply: -
The output of the AD622 will swing somewhere between the neg and positive supplies but it won't get very close to them. See the box I've marked in red - it's saying that if your neg power supply is 0V, don't expect the output to get any lower than +1.1 volts in normal circumstances.
The smallest magnitude negative supply ought to be about -1.8 volts to be sure of being able to reach 0V cleanly on the output.
I suspect for your 2nd question that this is all about what happens when the pressure is too big and ch1 on the ADC cannot read it any more - at least ch2 can be relied upon for some readings even if they are subject to a few small extra errors due to resistor tolerances.
For question 3 - I'd like to see a circuit diagram.
Best Answer
The datasheet of the AD7176-2 is pretty careful to always refer to specific pin names, like AVSS or AVDD, rather than just generically talking about "ground".
For example, when they talk about the input voltage range (p. 4), under the line "Absolute AIN voltage limits", they give the min limit as "AVSS - 0.050" and the max as "AVDD1 + 0.05". Similarly when they give the "Absolute Reference Input Voltage Limits" (p. 5), they give the same limits. As long as you obey these limits, the chip should work as advertised.
In particular, if your AVSS is -2.5 V, then REF- of -0.5 V is within the limits.
That said, if you can get the attention of an Analog applications engineer (like if you are doing this for a commercial project or at a university), it wouldn't hurt to show them your final design before sending it out for fab.