Temperature transmitter with on-line calibration using johnson noise

• US 5,746,511 A
• Filed: 01/03/1996
• Issued: 05/05/1998
• Est. Priority Date: 01/03/1996
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Lenz, Gary, Eryurek, Evren
• Primary Examiner(s): Chilcot, Richard
• Assistant Examiner(s): Oen, William L.
• Application Number:
  US08/582,514
• Time in Patent Office: 853 Days
• Field of Search: 73/118.1, 73/116, 73/168, 73/659, 364/550, 364/551, 364/552, 364/553, 364/551.01, 364/140, 364/571.07, 395/11, 395/23, 395/27, 395/22, 374/120, 374/2, 374/175
• US Class Current: 374/2
• CPC Class Codes: G01K 15/00 Testing or calibrating of thermometers
• CPC Class Codes: G01K 7/30 using thermal noise of resistances or conductors

Abstract

A temperature transmitter and a process control system includes a sensor input for coupling to a temperature sensor having a temperature dependent resistance. The resistance measuring circuitry couples to the sensor input and provides a resistance output related to resistance of the temperature dependent resistance. Analog to digital conversion circuitry couples to the sensor input and provides a digital output related to AC signals across the sensor input. Digital signal processing circuitry isolates a Johnson noise component of the AC signals and provides a digitized Johnson noise output. Temperature measurement circuitry provides a calibrated temperature output based upon the resistance output and the digitized Johnson noise output. Output circuitry transmits the calibrated temperature output over the process control loop.

Claims

1. A transmitter in a process control system, comprising:

a sensor input for coupling to a temperature sensor having a temperature dependant resistance;

resistance measuring circuitry coupled to the sensor input having resistance output related to resistance of the temperature dependent resistance;

an amplifier coupled to the sensor input providing an amplified output having an AC noise component and a Johnson noise component;

analog to digital conversion circuitry coupled to the amplified output providing a digital output;

digital signal processing circuitry coupled to the digital output for isolating the Johnson noise component and providing a digitized Johnson noise output;

temperature measurement circuitry coupled to the resistance measurement circuitry and the digital signal processing circuitry providing a calibrated temperature output based upon the resistance output and the digitized Johnson noise output; and

output circuitry for coupling to a process control loop and transmitting the calibrated temperature output over the loop.

4. The transmitter of claim 1 wherein the digital signal processing circuitry performs a Fast Fourier Transform on the digital output.

5. The transmitter of claim 1 wherein the digital signal processing circuitry performs a wavelet transform on the digital output.

6. The transmitter of claim 1 including a power source coupled to the process control loop for generating power for the transmitter.

7. The transmitter of claim 1 wherein the amplifier comprises a wideband AC-coupled amplifier.

8. The transmitter of claim 1 including a memory coupled to the temperature measurement circuitry storing a calibration value related to a comparison of the resistance output and the digitized Johnson noise output.

9. The transmitter of claim 1 wherein the output circuitry transmits a signal related to the digitized Johnson noise output.

10. The transmitter of claim 9 wherein the signal is a digital signal.

11. The transmitter of claim 1 wherein the output circuitry transmits a signal by controlling current through the control loop.

12. The transmitter of claim 1 wherein the output circuitry digitally transmits a signal on the control loop.

13. The transmitter of claim 1 including a reference for calibrating the Johnson noise output.

14. The transmitter of claim 12 including a switch for selectively coupling the reference to the amplifier.

15. The transmitter of claim 1 including a capacitor connected in series between the amplifier and the sensor input.

16. The transmitter of claim 1 wherein the resistance measuring circuitry comprises:

a current source connected in series with the sensor input;

a differential amplifier for connection across the sensor input and providing a difference output; and

conversion circuitry coupled to the differential amplifier providing a digitized difference output related to resistance of the sensor.

17. The transmitter of claim 16 including a multiplexer for coupling the sensor input to the differential amplifier.

2. The transmitter of claim 2 including:

a second wideband AC-coupled amplifier coupled to the sensor input providing an amplified output having an AC noise component and a Johnson noise component; and

second analog to digital conversion circuitry coupled to the amplified output of the second amplifier and providing a digital output to the digital signal processing circuitry.

3. The transmitter of claim 2 wherein the digital signal processing circuitry calculates Cross Power Spectral Density based upon the digitized outputs.