Isolation circuitry for transmitter electronics in process control system

• US 5,610,552 A
• Filed: 07/28/1995
• Issued: 03/11/1997
• Est. Priority Date: 07/28/1995
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Kyro, Bruce E., Schlesinger, Morton L.
• Primary Examiner(s): Callahan, Timothy P.
• Assistant Examiner(s): ZWEIZIG, JEFFERY SHAWN
• Application Number:
  US08/508,551
• Time in Patent Office: 592 Days
• Field of Search: 327/4, 327/560, 327/561, 327/563, 327/588, 327/509
• US Class Current: 327/560
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

A transmitter in a process control system includes isolation circuitry which isolates transmitter electronics from a sensor bridge circuit. The isolation circuitry includes high impedance isolators in series with high impedance buffers to electronically isolate the transmitter electronics from the bridge. A power supply is isolated using a series of capacitors connected to a periodic signal.

Claims

1. A transmitter for sensing and transmitting a process variable, comprising:

electronic circuitry comprising;

process variable input circuitry for receiving an input related to the process variable; and

output circuitry transmitting the process variable;

a reference level generator generating a reference level;

a sensor having a sensor output related to the sensed process variable;

isolation circuitry coupling the sensor to the electronic circuitry comprising;

a first high impedance buffer connected to the reference level providing a buffered reference; and

a first high impedance isolator coupling the buffered reference to an input of the sensor.

2. The transmitter of claim 1 wherein the isolation circuitry further includes:

a second high impedance buffer coupling the buffered reference to an input of the sensor;

a second high impedance isolator coupled to the sensor output; and

a second high impedance buffer coupling the second high impedance isolator to the process variable input circuitry.

3. The transmitter of claim 2 wherein the second high impedance buffer comprises a differential amplifier and the second high impedance isolator comprises a first resistor connected between an input to the differential amplifier and the output of the sensor and a resistor connected between a second input to the differential amplifier and the output from the sensor.

4. The transmitter of claim 1 including an amplifier connected between the first high impedance isolator and the sensor input.

5. The transmitter of claim 1 wherein the first high impedance buffer comprises a first operational amplifier and a second operational amplifier and the first high impedance isolator comprises a first resistor connected in series with an output of the first operational amplifier and a second resistor connected in series with the output of the second operational amplifier.

6. The transmitter of claim 1 wherein the process variable input circuitry of the electronic circuitry includes an analog-to-digital converter for receiving the input relating to the process variable, the analog-to-digital converter including a reference level input generated from the reference level whereby changes in the buffered reference are reflected in the reference level input to the analog-to-digital converter.

7. The transmitter of claim 1 wherein the electronic circuitry includes a clock and the isolation circuitry includes power isolation circuitry which generates an isolated power supply using a clock signal from the clock.

8. The transmitter of claim 7 wherein the power isolation circuitry includes a plurality of capacitors connected in series with the clock signal providing the isolated power supply.

9. The transmitter of claim 8 including a rectifier and filter circuit connected to the plurality of capacitors to rectify and filter the isolated power supply.

10. The transmitter of claim 1 wherein the isolating circuitry includes an open sensor detector providing an open sensor output to a third high impedance isolator responsive to an open sensor condition of the sensor.

11. The transmitter of claim 1 including a high impedance guard isolator connected to guard foil proximate the output from the sensor.

12. The transmitter of claim 1 wherein the sensor is a bridge circuit.

13. A transmitter for sensing and transmitting a process variable, comprising:

circuitry for receiving an input related to the process variable and transmitting the process variable;

an AC signal source for generating an AC signal;

a process variable sensor providing an output to the transmitter circuitry in response to the process variable;

sensor circuitry coupled to the process variable sensor used in operation of the sensor;

sensor power supply circuitry comprising;

at least three capacitors connected in series and coupled to the AC signal source, the capacitors allowing pressure of the AC signal and blocking passage of the DC signal;

a rectifier connected to the AC signal source through the three capacitors for rectifying the AC signal;

a holding capacitator connected in parallel with the rectifier providing an isolated power source for the process variable sensor; and

a filter coupled to the holding capacitor providing a substantially DC power supply output to the sensor circuitry.

14. The transmitter of claim 13 including:

a high impedance isolator coupled to the process variable sensor output; and

a high impedance buffer coupling the high impedance isolator to transmitter circuitry.

15. The transmitter of claim 13 including:

a reference level generator providing a reference level;

a high impedance buffer coupled to the reference level;

a high impedance element coupling the high impedance buffer to the process variable sensor thereby providing a buffered reference level.

16. The transmitter of claim 15 including:

an analog-to-digital converter for converting the process variable output to a digital value, the analog to digital converter responsive to the reference level whereby changes in the buffered reference level are reflected in operation of the analog-to-digital converter.

17. The transmitter of claim 13 wherein the AC signal source is a clock.

18. The transmitter of claim 13 including a high impedance element connected to an open sensor signal from the process variable sensor.

19. The transmitter of claim 13 including a high impedance guard isolator connected to guard foil proximate connections to the output from the process variable sensor.

20. The transmitter of claim 13 wherein the process variable sensor comprises a bridge circuit.