In this work, an integrated digital thermometer was designed at transistor level in 0.18μm BiCMOS SiGe technology for a temperature range of -50 ˚C to 130 ˚C. The 1σ accuracy achieved at 100 ˚C is 1.21 ˚C. The digital thermometer consists of temperature sensor, Instrumentation amplifier (pre-amplifier), Sample-and-Hold, Comparator, Successive Approximation Register (SAR) and Digital-to-Analog Converter (DAC). The ADC designed is a successive approximation register (SAR) ADC having a sampling rate of 5 kS/s at 3V. The main thrust of this work was to have a minimum error. As the major error source is the sensor, three different sensor topologies were designed, tested and evaluated for minimum error and size. Monte Carlo simulations were performed to check for the process and mismatch variations. The data from Monte Carlo simulation was then used to measure the error.