I finished this variant of level shifting and I got successful result. I had doubts about noise levels due fact that the voltage reference use ground which is the -2.5V output from the linear regulator. But the measured noise when the ADC inputs are shortcut to GND shows noise level less than noise specified in the voltage reference datasheet.
I used low cost +5V ADR395 voltage reference from Analog Devices for the breadboarding because I had only +2.5V version of the LTC6655. The noise level of this voltage reference is 8uV p-p according the datasheet, but when I short inputs of the ADC, I got less then 1uVp-p noise.
The ADC LTC2440 can measure now +/- 2.5V on the input. The picture below shows my setup with power supply module made with cheap LDOs. This supply gives +/- 2.5V, +/- 5V and GND to rest of the breadboards.
The breadboard at bottom contain the ADR395 and the LTC2440 IC.
The breadboard in the middle contain the level translator for outputs and inputs from the ADC to the uController. The outputs from the ADC are level translated with bipolar transistor MMBT3904 as this is shown in the LTC2442 datasheet page 32. The inputs for ADC are level translated with 74HC4053 which is triple single-pole double-throw analog switch.
In the bottom right corner is shown the Cutecom application which trace measured results from Pro Micro Arduino board.
Measuring negative voltage less then -0.3V (I tried with AA battery) which is the maximum allowed from the LTC2440 datasheet does not cause malfunction of the ADC.
The schematics is shown below and can be found in opendcm git repository in the following link. This schematics was made with the LTC6655 voltage reference, because I was too lazy to make new component for the ADR395.
Next step is to implement the automatic voltage range on the breadboard and to connect it to the ADC. I already received all parts and hope that next week will have enough time to finish it. This is the final step before start working on the first PCB prototype.
