Understanding Smart Sensors - Nomads.usp

74 Understanding Smart Sensorsand the desired measurement accuracy are important factors in deciding thetype of signal conditioning circuitry that is required.The instrumentation amplifier’s dedicated differential-input gain blockdifferentiates it from an ordinary op amp. Although the signal level for the sensormay be only a few millivolts, it can be superimposed on a common-modesignal of several volts. A high CMRR keeps the common-mode voltage fluctuationsfrom causing errors in the output [3]. A simple circuit using a standardinstrumentation amplifier with a gain of 10,100, CMRR of 100 dB, and bandwidthof 33 kHz is shown in Figure 4.1 [4]. The pressure sensor has passivecalibration for zero offset, full-scale span, and temperature compensation, butno additional amplification. The bridge current in the circuit is less than 3.6mA with a 9V supply, and the output can be directly interfaced to an ADC ofan MCU or a DSP.Another circuit that demonstrates the amplification and calibration for apressure sensor is shown in Figure 4.2 [5]. This circuit has a regulated voltagefor the bridge and amplifier supplied from voltage regulator U1. The supplyvoltage can range from 6.8 to 30V [6]. The low-cost interface amplifier providesa 0.5–4.5V output and can achieve an accuracy of ±5% by using 1%9VLT1078+−9V100kLT10041.2V+−LT1101A=100100k0.33 mF+−LT1078Output0to5V=0 to 30PSI100k*10k*2k − GainNotes: *1% film resistorSensor = Motorola MPX2200APZin = 1800Figure 4.1 Pressure sensor circuit using an instrumentation amplifier.