This application note proposes a field transmitter design using a precision voltage amplifier to condition the output of a Wheatstone bridge sensor. This document also shows how the voltage output of the amplifier can be translated into a 4-20-mA signal with a current loop transmitter. All sensor and amplifier circuitry are powered by the current loop transmitter. Additionally, this application note covers the selection of the amplifier and passive components to achieve the required gain and voltage-to-current conversion. Error sources, PCB layout considerations, and measured results are also discussed.
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Following are the design requirements:
The design goals and performance are summarized in Table 5-2. The measured transfer function and completed PCB design are shown below.
Weight | Goal | Calculated | Simulated | Measured | |
---|---|---|---|---|---|
Iout (Min) | 0lbs | 4mA | 4.0959mA | 4.0963mA | 4.0994mA |
Iout (Max) | 20lbs | 20mA | 20.1404mA | 20.1397mA | 20.1545mA |
Factory automation and control systems often require the status of several processes to be monitored and communicated to a control station to ensure proper operation. It is common practice for many sensors to be dispersed on a factory floor to convert process variables such as pressure, temperature and weight into electrical signals that can be transmitted to a central location. 4-20-mA current loop transmitters are ideal for this application because they allow remote processes to be monitored with only two wires: power for the sensor or surrounding circuitry and an output current corresponding to the sensor variable of interest. A Wheatstone bridge amplifier to 4-20-mA current loop transmitter circuit is proposed as a high accuracy factory automation signal chain solution.
The circuit can be divided into three sections:
Figure 2-1 shows a simplified version of the full circuit. The reference voltages and local ground (IRET) of TI's XTR116 are used to power all bridge and sensor circuitry. All current consumed by the XTR powered circuitry must return through the current loop (IRET); therefore, the monitoring circuitry must consume less than 4-mA to avoid interference with the 4-20-mA output current of interest. To account for variation in device, temperature, and supply it is good practice to limit the current consumption to 3.5-mA. The XTR116 is powered by the two-wire power supply (VLOOP).
The signal chain begins with a small differential voltage developed between the two outputs of the Wheatstone bridge corresponding to the variable of interest. These voltages are fed into the discrete 2-amp INA which can amplify their difference and the output voltage can be converted into a current through a resistor (RIN). The current loop transmitter then takes the current at the input (IIN) and returns the current multiplied by 100 at the output (IOUT). The following sections provide more detail on each portion of the circuit.
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 (VEXC) 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, VSIG+ and VSIG-. A differential voltage measurement (VDIFF) 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 VDIFF, VEXC, and the resistive bridge elements with respect to ground.
The input resistance of the selected sensor is approximately 1-kΩ and can consume more than 4-mA of current with a VEXC 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 (RLIMIT) 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 RLIMIT.
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 VDIFF 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.