1、 Circuit functions and advantages

The circuit shown in Figure 1 is a flexible current transmitter that converts the differential voltage output of the pressure sensor into a 4 mA to 20 mA current output.

The circuit is optimized for various bridge voltage or current driven pressure sensors. Only five active devices are used, and the total non adjustable error is less than 1%. The power supply range is 7 V to 36 V, depending on the component and sensor driver configuration.

The input of the circuit has ESD protection function and can provide voltage protection higher than the power supply rail. It is an ideal choice for industrial applications.

  Figure 1. Pressure sensor signal conditioning circuit with 4 mA to 20 mA output (shown as sensor voltage drive mode), schematic diagram: not all connections and decoupling are shown)

Figure 1. Pressure sensor signal conditioning circuit with 4 mA to 20 mA output (shown as sensor

Voltage driven mode), schematic diagram: not showing all connections and decoupling)

2、 Circuit description

The design provides a complete 4 mA to 20 mA transmitter pressure sensor measurement solution. There are three important circuit levels: sensor excitation drive, sensor output amplifier and voltage current converter.

The total current required by the circuit (excluding bridge drive current and output current) is 5.23 MA (maximum), as shown in Table 1.

Table 1. Maximum circuit current at 25 ° C

Excitation: voltage driven configuration

Voltage drive or current drive is required, depending on the selected pressure sensor. The circuit uses a quarter of ada4091-4 (u2a) and selects different configurations through switch S1, supporting one of two options. Figure 2 shows the voltage drive configuration, with S1 position closest to the identification mark (see the complete circuit layout and schematic diagram in cn0295 design support package: http://www.analog.com/CN0295-DesignSupport The voltage drive usually adopts the gain (1 + R5 / R6) of this stage to be configured as 6 V bridge drive voltage. Other drive voltages can be obtained by appropriately changing the resistance ratio:

  

Please note that the power supply voltage Vcc should be at least 0.2 V higher than the bridge drive voltage so that u2a has sufficient margin. ADA4091-4:

  

  

The circuit selects the operational amplifier ada4091-4 because it has low power consumption (250 per amplifier) μ A) Low offset voltage (250) μ 5) And rail to rail input and output characteristics.

Because of the accuracy (Class A: 0.1%, class B: 0.06%) and low quiescent current (0.8 MA) characteristics of adr02, adr02 is selected as the 5 V reference voltage source.

  Figure 2. Sensor voltage drive configuration (schematic: not all connections and decoupling shown)

Figure 2. Sensor voltage drive configuration (schematic: not all connections and decoupling shown)

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