Abstract: This paper introduces the design of a liquid level sensor conditioning circuit and the different output forms required by the transmitter in different applications. Keywords: transmitter, liquid level sensor, temperature drift. Sensor technology has become increasingly mature, and liquid level transmitters are widely used in industrial process control. Although sensor manufacturing technology is developing rapidly, the output signals of sensors (sensitive devices) still have some problems, such as temperature drift. Various sensors have zero-point and sensitivity temperature drift, which must be reduced to meet the accuracy requirements of detection and control. Various sensors also have nonlinearity problems, so the output signals of the sensors need to be linearized. The output signals of most sensors are relatively small, especially the output signals of metal film sensors, which are even smaller. It is necessary to amplify and standardize the output signals. Designing a transmitter signal conditioning circuit can solve the above problems. In the development and application of transmitters, it is often encountered that the output of the required transmitter is different from the output of the existing transmitter, or the output of the user's existing transmitter cannot meet the new requirements, which requires changing the original output of the transmitter. In order to meet the needs of various customers, transmitters with multiple outputs are needed. For example, the standard output of a Type II transducer is usually 0-10mA or 0-10V, while the currently used Type III transducers are 4-20mA or 1-5V. How to convert between these outputs is a problem we must solve. 1. Transmitter Signal Conditioning Circuit Design 1.1 Temperature Drift Handling Sensor temperature drift can be divided into zero-point temperature drift and sensitivity temperature drift. Zero-point temperature drift is the drift in the sensor's output caused by temperature changes when the sensor is not under pressure. In sensor applications, constant current power supply is often used. The zero-point and temperature drift compensation method can be the series-parallel resistor method. The circuit shown in Figure 1 can effectively solve the zero-point temperature drift problem. For sensors with constant current power supply bridge circuits, the sensitivity temperature compensation circuit is usually shown in Figure 2. In the R network, Rt is a thermistor with a temperature coefficient in the same direction as the sensitivity temperature drift, and Rs, Rp, and Rz are resistors with negligible temperature coefficients used to adjust the temperature coefficient of Rt. After the above zero-point and sensitivity temperature compensation, the sensor's output signal can be considered independent of temperature changes within a certain temperature range. 1.2 Amplification and Nonlinearity Processing The nonlinearity of any force sensor has magnitude and sign. During signal processing and transmission, linearization is required to ensure the final signal is linearly related to the liquid level. The linearization circuit is designed based on the magnitude and sign of the nonlinearity. Linearization can be performed at different stages of signal processing, sometimes in the analog signal stage and sometimes in the digital signal stage. In the circuit of Figure 3, connecting pin 12 to pin 6 and adjusting resistor R8 adjusts the positive nonlinearity; connecting pin 12 to pin 1 and adjusting resistor R8 adjusts the negative nonlinearity. For general applications requiring accuracy (±0.5%FS0), simple positive and negative feedback correction is sufficient within an appropriate range. However, when a small-range sensor is used in a large range, the nonlinearity increases, and sometimes simple positive and negative feedback correction for linearization becomes difficult. In such cases, digital linearization methods are preferred, or multi-point correction methods can be used. For sensors with very small output signals, even only a few mV, when making 4-20mA level transmitters, a high-performance instrumentation amplifier, such as the INA118, can be used. This amplifier comprehensively considers temperature compensation, linearization, amplification, and output, allowing for the design of a level transmitter circuit that meets the requirements. Alternatively, a transmitter circuit block, such as the xTR106 (a product of BB Corporation in the United States), can be used. The specific circuit is shown in Figure 3. Transmitters assembled using this circuit have shown excellent performance in all aspects after long-term operation. R1 is used to adjust the zero point, W2 to adjust the range, R3 to adjust the sensitivity and temperature drift, and R8 to adjust the linearity. 2. Transmitter Output Conversion 2.1 4-20mA to 0-5V Conversion The OP295 is a dual operational amplifier circuit with low noise, high accuracy, and can input and output both positive and negative signals. This amplifier operates from a single power supply of 3-36V, with a low offset voltage of 300μV, a high open-loop gain of 1000V/mV, a maximum power supply current of 150μA per amplifier, an output current of ±18mA, and an operating temperature range of -40 to 125℃. It is a very good amplifier. Figure 4 shows the schematic diagram of a 4-20mA to 0-5V converter using an OP295 amplifier. R1 is the sampling resistor, W1 and W2 are potentiometers for zero and full-scale adjustment respectively, the diode group is used for common-mode adjustment, and the 8V voltage can be obtained using an LM317. This circuit is easy to assemble and debug, and has high precision. 2.2 4-20mA to 0-10mA Conversion: The RCV420 is a product of BB Corporation. It can convert 4-20mA to 0-5V. Its rated power supply voltage is ±15V, the quiescent current is 3mA, and the operating temperature range is -55 to 125℃. Although the power supply given in the manual is a dual power supply, it can also be applied to single power supply 0-24V applications, which is very convenient as no external components are needed (see Figure 5). The circuit for converting 4-20mA to 0-10mA is shown in Figure 6. Although this circuit is relatively complex, its performance is stable and reliable. Let the voltage drop across R1 (sampling voltage) be Vi. Through derivation, the current flowing through RL can be obtained as I = (Vi-1)/R11. Obviously, if the transmitter outputs 4mA at zero position, the voltage drop across the 250Ω sampling resistor is 1V, so I = 0. If the transmitter output is 20mA, Vi = 5V, then I = (5-1)/R11. By appropriately selecting R11, I = 10mA. 2.3 Dual 4-20mA Outputs Sometimes, for ease of application, a transmitter needs to use two or more 4-20mA outputs. Figure 7 shows a successful solution in practical applications. The actual circuit consists of an OP295 and a 9015 PNP transistor. Adjusting R1 to make the output 4-20mA results in a dual 4-20mA output. This circuit provides a stable and reliable output. 3. Conclusion The signal conditioning circuit for the level sensor and the conversion circuits for various transmitters described in this article are a summary of the author's practical work. The level transmitter designed using this method has proven to have excellent performance and reliable operation after years of practical application.