4mA to 20mA pressure sensor transmitter with integrated voltage or current driver

　　1. Circuit functions and advantages

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

Optimized for a variety of bridge voltage or current driven pressure sensors, this circuit uses only 5 active components and has a total unadjusted error of less than 1%. The power supply range is 7 V to 36 V, depending on the component and sensor driver configuration.

The inputs of this circuit are ESD protected and provide voltage protection above the supply rails, making it ideal for industrial applications.

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

　　2. Circuit description

This design provides a complete 4 mA to 20 mA transmitter pressure sensor measurement solution. There are three important circuit stages: sensor excitation driver, sensor output amplifier, and voltage-to-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.

　　Excitation: Voltage Drive Configuration

Either voltage drive or current drive is required, depending on the pressure sensor selected. This circuit uses a quarter of an ADA4091-4 (U2A) and supports one of two options with different configurations selected via switch S1. Figure 2 shows the voltage driver configuration, with S1 positioned closest to the identification mark (see the complete circuit layout and schematic in the CN0295 Design Support Package: http://www.analog.com/CN0295-DesignSupport). Voltage drivers typically use the gain of this stage. (1 + R5/R6) configured for a 6 V bridge drive voltage. Other drive voltages can be obtained by appropriately changing the resistor ratio:

Note that the supply voltage VCC should be at least 0.2 V higher than the bridge drive voltage to give U2A enough headroom. ADA4091-4:

The ADA4091-4 op amp was chosen for this circuit because of its low power consumption (250 μA per amplifier), low offset voltage (250 μV), and rail-to-rail input and output characteristics.

ADR02 was selected as the 5 V reference voltage source because of its accuracy (Grade A: 0.1%, Grade B: 0.06%) and low quiescent current (0.8 mA).

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