Creating a Ratiometric Current Excitation in Sensors Using the MAX1464 Signal Conditioner - AN3364

The MAX1464 can be easily configured to generate constant current excitation for sensors that is ratiometric to the power supply voltage for resistive transducer applications. Applications utilizing sensing elements with high temperature coefficients, TCR, such as piezo resistive bridges, RTDs, etc. are typically implemented with constant current excitation. This application note suggests a simple resistive network that can be implemented to provide a ratiometric current source for sensor excitation. Applications using sensing elements with high temperature coefficients, TCR, such as piezo resistive bridges, RTDs, etc. are typically implemented with constant current excitation. The MAX1464 can be easily configured to generate constant current, ratiometric to the supply voltage, for resistive transducer applications. This application note suggests a simple resistive network to provide the ratiometric current source. To reduce external components, the uncommitted op-amps internal to the MAX1464 can be used. The MAX1464 is a highly integrated programmable sensor signal conditioner with two input/output channels and 16-bit resolution and provides for signal amplification, calibration, linearization, and temperature compensation. The MAX1464 supports a choice of analog or digital outputs including 4–20mA, ratiometric 0.5V to 4.5V, and PWM. Many of today's sensor applications use sensing elements, such has piezo-resistive sensors, that have very high temperature coefficient of resistance (TCR), and sensitivity (TCS). These two coefficients are typically of comparable values, but opposite signs. To minimize the overall temperature dependency and improve performance of the sensing elements, the sensing element is normally driven with constant current. The MAX1464 can be configured to generate a constant current for these applications. Figure 1 shows a simple resistive circuit that generates the constant current IB for excitation of the sensing element RB. The equations that apply to the circuit in Figure 1 are given below. Figure 2 shows the resulting ratiometric current for VDD ranging from 4.5V to 5.5V.