The Wheatstone bridge is a commonly used circuit configuration to achieve highly accurate sensor measurements. The bridge is composed of four resistive elements creating two voltage dividers in parallel between an excitation voltage (V_EXC) and ground. In the most basic form, only one of the elements can vary in resistance. This change in resistance can create a difference in voltage between the two dividers, V_SIG+ and V_SIG-. A differential voltage measurement (V_DIFF) is taken between these two points. A large differential voltage corresponds to a large variation in resistance, and thus a large change in the sensor value being measured. Figure 2-2 depicts a classic Wheatstone bridge configuration and Equation 1 describes the relationship between V_DIFF, V_EXC, and the resistive bridge elements with respect to ground.

Figure 2-2 Basic Wheatstone Bridge Configuration

The input resistance of the selected sensor is approximately 1-kΩ and can consume more than 4-mA of current with a V_EXC of 4.096-V. Therefore, two, 500 Ω resistors were placed in series on either side of the bridge to limit the current to 2-mA while keeping the signal close to mid-supply to avoid common mode limitations in the subsequent INA stage. The current limiting resistors (R_LIMIT) are sized to produce an excitation voltage of 2.096-V across the Wheatstone bridge. Larger current limiting resistors can reduce the effective excitation voltage, thus decreasing bridge sensitivity. Figure 2-3, Equation 2, and Equation 3 describe the modified Wheatstone bridge and show the calculation for R_LIMIT.

Figure 2-3 Current Limited Wheatstone Bridge Configuration

This design uses a Wheatstone bridge load cell, however, any sensor that can be configured in Wheatstone bridge is applicable. The differential voltage developed at V_DIFF increases as the weight applied to the load cell increases. More detail on the selected load cell is included in the Appendix 1: Load Cell and Experimentation Setup.