The circuit is a robust and flexible 2 wire loop-powered current transmitter that converts the differential voltage output from a pressure sensor to a 4 mA-to-20 mA current output. The design is optimized for a wide variety of bridge based voltage or current driven pressure sensors, utilizes only four active devices, and has a total unadjusted error of less than 1%. The loop supply voltage can range from 12 V to 36 V. The input of the circuit is protected for ESD and voltages beyond the supply rail, making it ideal for loop powered industrial applications.

• EVAL-CN0289-EB1Z circuit evaluation board
• +7V to +24V power supply or equivalent power supply
• Agilent 3458A or equivalent Current meter
• Differential Pressure sensor or Equivalent Dual channel DC Signal Source

• From +7V to +24V supply can be used to J2 for proper operation of the evaluation board

1. A differential output bridge type sensor should be attached to a 4 channel screw terminal block (J1) labeled with SUPPLYSNS on the PCB.
2. A differential sensor gauge like from Honeywell or from NXP can be used for evaluation.

The following configuration can be used to simulate the sensor characteristic for evaluation pusposes.

1. A dual power supply can be used to generate a common-mode voltage of 2.5 V from Vout1 and a 0 mV to 100 mV differential input voltage from Vout2 shown in the block diagram for test setup.
2. A 3 kΩ at R_bridge simulates the bridge type sensor impedance.

• The circuit can be switched to the current or voltage drive configuration by moving S1 to the position labeled on the PCB.

• In the voltage drive mode with VDRIVE = 10 V, the supply voltage VLOOP or system supply must be greater than 10.2 V

• In the current drive mode, the supply voltage VLOOP or system supply must be greater than 11.2 V in order to maintain sufficient headroom.

The output of the system can be measured in two ways, either through direct output current measurement or voltage output through the series resistor.

• Voltage output measurement is done across a series 250ohms resistor that is be placed in series with the negative terminal of the power supply and the system ground loop

• Current output measurement is done by connecting the positive terminal of the current meter at the system ground loop (negative terminal of J2) and the negative terminal of the meter connected to the negative terminal of the power supply.

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