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Achieving SAR ADC Maximum Dynamics

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iheb:
Hello i am currently having an application using C8051F06 Microcontroller and i am designing a buffer driver for its input i have been working with the Circuit from the reference design but , i can't quite manage to drive the ADC deferential mode without having to modify the gain inside the Microcontroller, that also i am having the issue that i was never able to reach the specifications in the data sheet and attain the full dynamics without distortions on the signal the input voltage across the ADC's is between 0 and +/-Vref and v ref is 2.5v, and I'm only able to reach 1.5V at one side and 1.7V at the other side of the ADC when working in differential mode. I also notice an imbalance in the internal resistance when measuring it directly for each ADC.

If its possible anyone can help me why do this imbalance is from ?
What is the best way to drive the ADC in differential mode without having to alter the code for gain and offset errors?

thank you

moffy:
It is hard to respond in a constructive manner to your issue as it is probably code dependent and also development environment. I assume you are using the 'Simplicity Studio' development package by SiLabs? I have no experience with it though I have used the 8051 core before, but long ago, and I programmed in assembly, long time ago. I have found some code: AN118sw.zip which uses the 16 biy ADC, if you don't have it it might provide a good starting point for your code.

iheb:
Thank you for your suggestion, i have read that application note about improving ADC resolution. it says a bit about the dynamic range but , i'm not quite sure how to prove that the ADC driver that i did would occupy the full dynamic range with minimum software calibration. i can provide the design if anyone can help 

moffy:
Have you tried to compile the code in AN118sw.zip and run it? It should give you a known good starting point to work from. The configuration of the peripheral , in this case the ADC, can be difficult, so using an example that does it for you can be a big help. I prefer to modify a working example, to writing from scratch. You just have to have 1 bit in an obscure and apparently unrelated register not set and things go awry.

Benta:
I suspect you're driving the input voltage beyond what the device will accept.
But without a schematic, your question flies in the wind.

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