AN1525 The ABCs of Signal-Conditioning Amplifier Design for Sensor Applications

Although fully signal-conditioned, calibrated, and temperature compensated monolithic sensor ICs are commercially available today, there are many applications where the flexibility of designing custom signal-conditioning is of great benefit. Perhaps the need for a versatile low-level sensor output is best illustrated by considering two particular cases that frequently occur: (1) the user is in a prototyping phase of development and needs the ability to make changes rapidly to the overall transfer function of the combined sensor/amplifier subsystem, (2) the specific desired transfer function does not exist in a fully signal-conditioned, precisiontrimmed sensor product (e.g., a signal-conditioned device is precision trimmed over a different pressure range than that of the application of interest). In such cases, it is obvious that there will always be a need for low-level, nonsignalconditioned sensors. Given this need, there is also a need for sensor interface amplifier circuits that can signal condition the “raw” sensor output to a usable level. These circuits should also be user friendly, simple, and cost effective. Today's unamplified solid-state sensors typically have an output voltage of tens of millivolts (Freescale’s basic 10 kPa pressure sensor, MPX10, has a typical full-scale output of 58 mV, when powered with a 5.0 V supply). Therefore, a gain stage is needed to obtain a signal large enough for additional processing. This additional processing may include digitization by a microcontroller's analog to digital (A/D) converter, input to a comparator, etc. Although the signalconditioning circuits described here are applicable to lowlevel, differential-voltage output sensors in general, the focus of this paper will be on interfacing pressure sensors to amplifier circuits. This paper presents a basic two operational-amplifier signal-conditioning circuit that provides the desired characteristics of an instrumentation amplifier interface: