Temperature Control Loop and Geometric Mean Circuitry

4 Temperature Control Loop and Geometric Mean Circuitry 

The schematic of the temperature control loop and the geometric mean circuitry of the single-ended mixed-signal architecture is shown in Fig. 5.7. 

The microhotplate temperature is measured using a polysilicon resistor as temperature sensor (Ry). The resistor is biased with a temperature-independent current source (Free). The voltage drop across the resistor is low-pass filtered and converted to the digital domain using a 10-bit successive-approximation ADC from the analog library of austriamicrosystems (Unterpremstatten, Austria). The inputs of the digi- 

The control voltage drives the voltage-to-current converter. The polysilicon reference resistor (Pref) used in the converter is located on the microhotplate in order to be at the same temperature as Pheat- This configuration eliminates the temperature dependence of the proportionality coefficient between I control and the power dissipated by Pheat (Pheat) as shown in Eq. (5.4)  

The digital PID controller is implemented as a recursive filter with a biquadratic transfer function. Controller wordlength, set-point input, controller output, and controller parameters are 10-bit fixed-point values in the range [0,1). The controller parameters also have an additional sign-bit. 

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