Figure 2
From: Miniaturized thermal acoustic gas sensor based on a CMOS microhotplate and MEMS microphone
Sensor interfacing. (a) Block diagram of the experimental setup. The microheater is electrically modulated using a custom current drive circuit. Signals from the active and reference MEMS microphones are amplified and their differential signal digitised using a National Instruments data acquisition unit, with a software based lock-in amplifier used to help remove noise. (b) Modulated acoustic signal showing the pressure changes which occur during electro-thermal modulation of the sensor at a frequency of 43 Hz. (c) Acoustic frequency response of the sensor, showing the rise in the acoustic signal with modulation frequency and gradual tail off in the response due to thermal inertia of the membrane which limits the thermal modulation depth. Inset: relative change in pressure with humidity. (d) Response of the sensor to pulses of ~ 20–60 % CO\(_2\) gas concentrations showing the drop in the thermal acoustic signal with increasing CO\(_2\) concentration.
