High Precision Readout Circuits for TCD gas sensors using AC modulation


This project is about  the design, fabrication and test of CMOS integrated circuits for the read-out of MEMS thermal conductivity detectors (TCD’s) using modulation and coherent detection. The system aims for stable and low-cost detection of hydrogen for safety, and/or  CO2 for indoor air quality measurements.


Project description

The photos in figure 1 show two types of micro TCD’s. Circuits need to be developed for:

  • A commercially available platinum based thin film sensor
  • Our own MEMS TCD using thermo-electric sensors, fabricated at DIMES.
Figure 1a) MEMS TCD  fabricated at DIMES
          1b) Commercial thin film device from Posifa MicroSystems Inc.
Figure 2: Possible basic circuit topology

Figure 2 shows a possible solution for the AC modulation method. DC drift is eliminated by synchronous chopping of  the thermal actuator and the detector signal after amplification. Operation conditions of the sensor should be either at constant power or at constant temperature.


Main challenges:

  • Design of a low noise instrumentation amplifier for the thermo-electric sensor.
  • Design of choppers for synchronous detection for accurate demodulation of the thermal actuation signal.
  • Modelling and running simulations on the complete read-out system in the Cadence design environment.
  • Building a control loop for constant power or constant temperature operation.

We offer an innovative project for a motivated MSc student interested in circuit design and sensor interfacing.



Ger de Graaf