I want ADS1246 to measure a voltage start from 0, for example, VINN=0, VINP from 0 to 2V. Datasheet gives the balanced bipolar supplies as VDD=2.5V and VSS=-2.5V, it is good. But because I have to measure another differential voltage like VINN=2V, VINP=3V with the same ADS1246 chip, the +/-2.5V supply will not work.
So I use unbalanced bipolar supply like VDD=4.3V and VSS=-0.7V, in which VDD is from 5V minus a diode VF, and VSS from GND minus a diode VF. Decoupling capacitors are large enough.
I want to realize the discriptions on bipolar supplies:
1. Datasheet gives the AVSS value not lower than -2.8V to DGND, so ADS1246 will run with -0.7 and 4.3V supplies, but will ADS1246 run correctly and accurately with unbalanced supplies, -0.7 and 4.3V, as well as with +/-2.5V balanced supplies, or +5V Unipolar supplies.
2. Diode VF is not very stable to temperature, if one of the two moves, the AVDD-AVSS becomes larger or smaller, it is likely to affect the effective num of bit, is it? Datasheet does not provide the PSRR for AVDD and AVSS, if my -0.7/+4.3V supply changed a bit (in long term DC, not AC ripple), how much it will affect the conversion performance?
3. If two diodes VF are traced at the same temperature, I can put two diodes very close on the board, AVDD-AVSS will keep stable even if the ambient temperature changes, only the absolute value of AVDD or AVSS changeds, does it degrade the specifications noted in the datasheet?
4. If +/-2.5V or my -0.7/+4.3V supplies are used, with VREF=2.5V, does ADS1246 meet the performance in table 1 and table 2 in page 12 in the datasheet and other specifications in electrical characteristic?
5. I suppose that AVDD and AVSS only drive input MUX and PGA, so -2.8V limit of AVSS to DGND is from the analog switches in MUX and PGA. With -0.7/+4.3V supply, MUX and PGA of ADS1246 will work very similar as it works under a +5V unipolar supply, since they can even work well with +/-2.5V supply, is it right?
Thanks a lot.
