Im new here and also electronics ;). I need to measure +/- 3.3V signal. I usually use microcontollers with level shifter but now I need some more precise ADC. I picked LTC1403. How can I measure negative voltages with differential input? Do I need level shifter and IN connect to ground? Or to minus of my of my power supply? Its my first time with external ADC.
Before signal I have some filters.
If you can use the LTC1403-1, an easy solution is to use a fully differential amplifier like the ADA4945 between your filter circuit and the ADC input. Use the same +/-3V supplies for the ADA4945. Use figure 104 of the ADA4945 data sheet and connect VOCM to 1.5V and use 4.75k for RG and 1.96k for Rf. To minimize noise you may want to add an RC filter between the ADA4945 outputs and LTC1403-1 inputs. 51ohms and 47pF is a good starting point.
I have really noise envrionemnt, limited space for board so adding additonal component isn;t quite solution for me. Maybe another ADC? I looking something for 12 bits or more and more than 1Msps.
In terms of an ADC that can be directly driven from the output of the LT1112, the best fit is the LTC2328-16. This is a 16-bit, 1Msps ADC that requires a +5V supply. This might be overkill for your application but it is the best solution I can offer.
Thanks for your answer. I reconsidered additonal driver do ADC. What do you think about LTC1992 before ADC LTC1403?
The inputs of the LTC1403 will have a small transient on them when the ADC goes into acquire mode. This transient must settle before the start of the conversion. This is the acquisition time of the ADC. For the LTC1403 running at its full speed the acquisition time is only 39ns. If you look at the small signal step response of the LTC1992 you will see that it takes over a 1us to settle. If you look at the settling time of the ADA4945 in low power mode, it settles to a 16-bit level in 150ns. This is to an 8V step. Settling to 12bits from the small transient on the ADC inputs should be possible in 39ns. The supply current of the ADA4945 in low power mode is not that much more than the LTC1992.