Understanding Smart Sensors - Nomads.usp

84 Understanding Smart Sensors4.3.2 Integrated Active ElementsIntegrating the amplification and signal conditioning directly on the sensorcan be accomplished by the use of combined micromachining and microelectronics.The circuit of a fully signal conditioned pressure sensor is shown inFigure 4.11(a). The additional circuitry is integrated on the sensor using thesilicon area that is required to provide the mechanical support for the diaphragm[22]. The die size for the fully signal conditioned unit is 0.145 inch by0.130 inch. The diaphragm and piezoresistive element account for 20% of thedie, and the signal conditioning circuitry is 80%, including the area requiredfor wire bond pads. A quad operational amplifier frequently used for sensoroutput amplification is the LM324. The area of silicon in the LM324 is about0.050 inch by 0.050 inch. The sensor without signal conditioning is a 0.120-inch by 0.120-inch chip. In that case, the electronics has been combined withthe sensor with a very small increase in the total chip area because the mechanicalsupport for the diaphragm has been utilized. Common pressure ranges anda standard interface to 5V dc ADC allow the integrated approach to satisfy anumber of application requirements and justify the cost of the integrateddesign. The integrated sensor easily interfaces to an MCU with an onboardADC, as shown in Figure 4.11(b). The 50-pF capacitor and 51-kΩ resistor arerecommended as a decoupling filter.A more recent circuit design met the target specifications with one lessstage in the amplification circuitry than the earlier design [23]. As shown inFigure 4.12, it uses fewer resistors and is easier to trim. In addition a highersource/sink current can be obtained in the new design. Initial indications arethat improvement of at least 10 times has been obtained. The output stage hasbeen designed to drive CMOS A/D inputs from 0.2V to 4.7V (typical values)with a 5V supply.Along with the new circuit, a revised trim algorithm was developed.Although the basic principles are similar to the earlier version, there are severalimportant improvements. One of the most important is the balanced design,which allows for bidirectional trimming of all parameters, and a more independenttrim of each parameter. On the old circuit, parameters such as temperaturecoefficient of offset (Tco) and gain could be trimmed in only onedirection. If processing variations resulted in the initial value of those parametersbeing beyond the target values, the device could not be trimmed. Additionally,many parameters had to be trimmed with an iterative process in whichseveral parameters were roughly trimmed, and then they were all trimmedagain, to reach the final target. Any adjustment of one resistor affected morethan one parameter and limited the overall system accuracy. For the newdesign, all parameters can be trimmed in either direction, and each one is