Hi Nikos, [quote user="Nikolaos Piliouras"]I am planning to use 1mA IDAC currents (taking into consideration the temperature range 0-50oC). For this reason in the upd2 schematic the REF resistor reduced from 3K3 to 820R. Do you believe that I have to stay with the initial design using 3K3 REF resistor?[/quote] Increasing the IDAC current to 1mA will not violate the IDAC compliance voltage @3.3V AVDD in your configuration. Below the Math: Vcomp = AVDD - 0.7V = 2.6V VIDAC1 = Vref + Vrtd(max) < 2.6V, with Rrtd(50°C) = 120 ohm, Vrtd(50°C) = 120 ohm x 1mA = 0.12V VIDAC1 = 820 ohm x 2 x 1mA + 0.12V = 1.76V < 2.6V, good! Now to the self-heating effect. Increasing the IDAC current will increase errors due to the self-heating of the RTD. The effects of self-heating on measurement accuracy are also based on the placement of the RTD in the application (open-air = insulates vs. moving fluid = dissipates). If this error -depending on your RTD type- is very little and negligible in your design and it can be tolerated that then that's okay! [quote user="Nikolaos Piliouras"]Also I was confused with Vincenzo advice on the initial design "if no RTD is connected you have 0V reference so you can't sense the pH sensor as well". Do you believe that there is an issue in the upd2 design with VREF in pH measurement if RTD's are not connected?[/quote] If no RTD is connected then there will be no voltage created over Rref, hence no external Vref will be available. However, you can still use the internal generated Vref (2.048V) for your pH measurements. Kind Regards, Mo.
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