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This version (31 Jul 2017 02:06) was approved by erbe.

CN0289 Flexible, 4 mA-to-20 mA, Loop-Powered Pressure Sensor Transmitter with Voltage or Current Drive

OVERVIEW

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.


DEMO REQUIREMENTS

  • 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

HARDWARE SETUP

Power Supply

  • From +7V to +24V supply can be used to J2 for proper operation of the evaluation board
In order for the circuit to operate properly, the supply voltage, must be greater than 7 V in order to provide sufficient headroom.

Input Signal

Using Actual Sensor

  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.
  • Vsense is where the analog voltage output of the sensor is being taken for measurement. Pin polarity should be take into account.
  • V/Iforce is the sensor excitation or is being supplied either from current or voltage source.
Pin polarity for Vsense and for V/Iforce should be take into consideration since it varies from manufacturer.

Using Simulated Input Sensor

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.

For lower than 5 kΩ bridge resistance, the drive voltage can be decreased down to 5 V using a buffer configuration by removing resistor R6.

Sensor Drive

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

Voltage Drive Configuration

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

Current Drive Configuration

  • 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.
Current drive configuration can be used for four-wire ohm circuit configuration where in the main advantage of this configuration is that the voltage is measured directly across the load cell, neglecting the load effect of the lead wires for long distance measurements.
Both for voltage and current drive mode, the minimum loop supply voltage is dependent on the configuration of the drive circuit for the bridge.

Output Measurements

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 Measurement

  • 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 Measurement

  • 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.
The system has an onboard resistor load (R7) of 250ohms that can be de-soldered to replace with a desired resistor load value

SCHEMATICS, PCB LAYOUT, BILL OF MATERIALS

End of Document

resources/eval/user-guides/circuits-from-the-lab/cn0289.txt · Last modified: 28 Jul 2017 08:45 by erbe