Initially I though to use LTC1167 and level shifting schematics for the breadboarding prototype, but unfortunately the following reasons stop me from this decision:
- The complex schematics of the level shifting and the input offset of LTC1167 increase significant the total price.
- When the op-amp input is touched by the finger during measurement, the output voltage level is decreasing with several hundred mV. I do not know what is the reason, but I was unpleasantly surprised from this fact. This means that I can not use this op-amp as front-end for sure.
- The noise (DC to 10Hz) of LTC1167 is 1.5 uV peak to peak, which is less then my target of 6.5 digits of resolution for the 2.5V peak to peak range (-1.25V to +1.25).
So, I changed my mind in the following direction:
- Instead to use level shifting, I will try with setting negative voltage on the ADC's ground pin (-2.5V) and level shifting the digital output pins as this is shown in the LTC2442 ADC datasheet. This will give me full 5V peak to peak range (-2.5V to +2.5V) and maximum 1.56uV noise for reaching 6.5 digits of resolution.
- Instead LTC1167 op-amp, I will tried the LTC2057 which does not change output voltage level when fingers touch the op-amp input pins and have the following properties:
- 0.2 uV p-p typical value for DC-10Hz noise.
- 5uV maximum input offset voltage (0.5uV typical) for +/-30V supply.
- 0.025 μV/°C input offset voltage drift against temperature.
- Low input current bias - 200pA max (30pA typical) for 25°C.
Most of the parts are already ordered and next blog post will contain the result of breadboarding.
