Reference voltage of single supply A-to-D converters

[SolarSunrise]

There's this very useful chip called ADS1299 which has 8 24 bit fully differential channel with a speed of 16ksps. I've worked with few ADC chips before but this chip is the first that I have seen that has 4.5v internal reference. Aren't the reference voltages usually 2.5v in a 5v single power supply? Maybe I've misinterpreted the data sheet but if it really has 4.5v internal reference, why? Isn't it more inefficient then having 2.5v as reference?

Datasheet: http://www.ti.com/lit/ds/symlink/ads1299.pdf

Thanks as always guys! I'm learning a lot from this forum right now 

[Nerull]

The ADCs I've seen use Vref as the top of the scale, either from 0V to Vref or -Vref to +Vref. I don't think I've ever seen one with a Vref in the middle of the scale.

[uwezi]

I would say that 4.5 V is a rather unusual reference voltage, but this has nothing to do whether it is efficient or inefficient.

This ADC is particularly designed for biomedical applications and I assume that in these fields the traditional signal level suits a 4.5 V reference voltage. This is possibly the range of the output signals which used to be fed into multi-track chart recorders..

[SolarSunrise]

I would say that 4.5 V is a rather unusual reference voltage, but this has nothing to do whether it is efficient or inefficient.

This ADC is particularly designed for biomedical applications and I assume that in these fields the traditional signal level suits a 4.5 V reference voltage. This is possibly the range of the output signals which used to be fed into multi-track chart recorders..

It could be inefficient because if the reference is 4.5v, the input voltage range could only be 0.5v above the reference before getting saturated. However, if the reference voltage is 2.5v, the input voltage is much wider: 2.5v above reference before getting saturated.

I just don't understand why the reference is so high. Is there a standard for these kind of devices? Hmm...

EDIT:
I just checked out the data sheet for ADS1298 (ADC specifically for EKG) and found out TI used 4v reference for a 5v power supply. Looks like these medical devices use very high references...

[uwezi]

It could be inefficient because if the reference is 4.5v, the input voltage range could only be 0.5v above the reference before getting saturated. However, if the reference voltage is 2.5v, the input voltage is much wider: 2.5v above reference before getting saturated.

The reference voltage normally defines the maximum input signal for the ADC. So the input signal should not be above the reference voltage anyway...