A compact instrumentation amplifier for MEMS thermal sensor interfacing

A compact CMOS instrumentation amplifier, based on a properly modified second order Gm-C low pass filter (LPF), is proposed as a possible readout channel for integrated thermal sensors. Low noise and low offset characteristics are obtained by applying chopper modulation to the input transconductor. The high input thermal noise density, typical of low frequency Gm-C filters, has been significantly reduced by adopting a two-stage topology for the first transconductor. Using this approach, an input noise density adequate for thermal sensor interfacing was obtained with no need of off-chip capacitors. The intrinsic filtering property of the amplifier effectively rejects the modulated offset ripple, allowing direct connection of the amplifier output to a low sampling rate AD converter. An original switching strategy involving swapping of the input and feedback ports is used to improve the gain precision. The effectiveness of the technique is proven by means of analytical arguments and electrical simulations performed on a prototype, designed with the STMicroelectronics BCD6s process.