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Design and characterization of CMOS circuitry

This result concerns the design and test of all readout circuits for detection, conditioning and amplification of the electrical signal generated by the cantilever-driver transducer on the mass sensor.

One of the main specifications of the design requiered to allow the detection of small currents coming from the nanocantilver-driver transducer, is the reduction of the capacitance associated to the connection interface betweeen transducer and circuitry. Such capacitance has been minimized by optimizing the dimensions of the metal line connnections and choosing conveniently the arquitecture of the input stage.

Two general different designs have been implemeted:
- Current amplifiers or transimpedance amplifiers are based on an operational amplifier with a resistive feedback, commonly called feedback ammeters.

The main advantage of this arquitecture is that the effect of the interface parasitic capacitance is negligible by virtually grounding it through the operational amplifier.

- Buffer amplifiers are based on the amplification of the ac voltage generated at the capacitnace divider formed by the cantilever-driver resonant capacitance and the interface parasitic capacitance.

These circuits are based on a buffer amplifier biased as source-follower (common drain configuration) and a voltage follower, introduced as an output stage for driving the output pad and input electrical test equipment capacitance. The polarization of this circuit is achieved through a capacitance coupled to the circuit input by a comb structure. This polarization capacitance is implemented in the same nanolithography process used to define the cantilever-driver transducer. Autopolarizations strategies based on a feedback path between output and input through a diode have been also implemented.

Both kinds of circuits allow to detect and amplify currents generated at the integrated cantilever-driver transducer of around 10-100 nA in a bandwidth of 1MHz.Additonal circuitry for phase detection has been also designed and tested in order to increase the functionality of the whole sensor system. Phase detection allows to externaly close the loop and implement a system for tracking the resonance frequency changes of the cantilever.

Informations connexes

Reported by

Department of Electronic Engineering
Edifici Q. Escola Tècnica Superior d’Enginyeria
08193 Bellaterra (Barcelona)