Hi Pavel, The ADS1259 can measure differential voltages up to +/- Vref. Therefore, your reference voltage will need to be at least 3.6V, but I see no issues with using a larger reference, such as 5V. I would recommend using a low-noise reference, such as the REF6241 or REF6250 . These references have buffered outputs and would be better suited for driving multiple ADC reference inputs. A benefit of using the REF6241 is that you could use the same 5V supply for both the ADC and reference ICs. I would not recommend gaining up the reference voltage, if given the choice. Adding an amplifier to the circuit adds additional noise and error (such as offset, gain, and drift errors). Also, since this is a 24-bit ADC with thermal noise that is larger than the ADC's LSB size, there is no benefit or additional resolution that can be achieved by using a smaller reference voltage (1 LSB with a 5V reference is below the noise floor, so making the LSB size even smaller will only increase the number of unstable bits) ...That being said, the input to the ADS1259 is unbuffered, so if your signal is not buffered you will need to add an amplifier to the input of the ADS1259 , and in this case it may make sense to gain up your input signal to the same amplitude as your reference voltage.
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