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Deepa Devendran
(From: Madras Institute of Technology; To: Maxim, Bangalore)
Thesis
Title: Design of a Thermocouple Signal Conditioner
This thesis describes the design of a low noise, low offset amplifier using chopper stabilization. Low-offset amplifiers are widely used in measurement systems. Typical applications include strain gauges, thermocouples, photodiodes etc. In CMOS process, offset and 1/f noise are primary bottlenecks that severely limit the accuracy and dynamic range of such amplifiers. Chopping is used to modulate the offset and 1/f noise to higher frequencies after which it can be easily filtered out, providing continuous offset reduction.
An illustrative CMOS prototype of a chopper stabilized amplifier in a 0.18 µm process is presented which achieves an input referred noise of 1 µV in 1 Hz to 1 kHz bandwidth. It consumes 0.65 mW of power and occupies 0.215 mm² of area.
The offset after chopping is not zero since the mismatch between the cascode devices in the first stage opamp is not chopped. Also the residual offset drifts with time and temperature. This is corrected using nested chopping which employs two levels of chopping wherein lower and more stable residual offsets can be attained.
Finally a full system design required to measure signal from a K type thermocouple is illustrated. This involves the design of a chopper amplifier, a cold junction compensation buffer, an oscillator and finally an output differential to single ended converter. The integrated circuit designed in 0.35 µm CMOS process has an output sensitivity of 2 mV/°C. The chopper amplifier, CJC buffer and output differential to single ended converter consumes 1.98 mW, 2 mW and 1.935 mW of power respectively. The thermocouple signal conditioning block consumes around 6 mW occupying 1 mm² of area.