A highly sensitive frontend IC for very robust capacitive vortex flowmeter sensors

This paper presents a capacitive sensor interface IC for vortex flow measurements. With this interface, we can measure flows down to half the minumum flow speed of the state of the art, for media temperatures up to 400 degrees C and pressures up to 200 bar in pipes from 15mm up to 300mm diameter, even in the presence of mechanical vibrations, for media ranging from air over oil to mercury. The mechanical sensor used is very robust in the presence of steam hammers. The sensor IC was realized in 0.35 mu m CMOS and operates over a temperature range of -50...125 degrees C. It adds input-referred noise far below the kT/C noise of the sensor, 1.17 aF/v root Hz at f(s) = 128 kHz. The IC consumes 1.5mA from a 3.3V supply and has an area of 10 mm(2). The main signal processing problems to solve were the compensation of time-varying offsets up to 8 pF while measuring 60 aF, and the digital detection of signal frequencies at 1.15 dB SNR, done by adaptive filtering. The sensor IC has a production yield of 96.5 %.