digital thermometer using pt100

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Joined Aug 4, 2012
i want to build a digital thermometer using RTD PT 100 and analog to digital converter ICL7107. the datasheet of ICL 7107 gives the following formula for the count.


i have set Vref to 1V. now PT 100 has a resistance of 100Ω at 0°C and about 138°C at 100°C. i have used a current source of 1mA, that passes through PT 100 which gives a voltage of 0.1mV and 0.138mV at 0 and 100°C respectively. i have amplified this signal by an amplifier having a gain of 27 which results in the low and high voltages of 2.7 and 3.726V respectively. i want to see zero count on the seven segment display for 2.7 volt Vin (the major problem, help me if you have some idea regarding this problem) and 100 count for 3.726 volt Vin. according to the formula if i change the value of Vref then still it does not give zero count.
the circuit diagram which i am using for PT100, current source and amplifier is also attached.


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