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WO1999060340A2 - Circuit de mesures utilisant une alimentation electrique double flottante - Google Patents

Circuit de mesures utilisant une alimentation electrique double flottante Download PDF

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Publication number
WO1999060340A2
WO1999060340A2 PCT/US1999/010996 US9910996W WO9960340A2 WO 1999060340 A2 WO1999060340 A2 WO 1999060340A2 US 9910996 W US9910996 W US 9910996W WO 9960340 A2 WO9960340 A2 WO 9960340A2
Authority
WO
WIPO (PCT)
Prior art keywords
power supply
voltage
analog signal
electrical
converter
Prior art date
Application number
PCT/US1999/010996
Other languages
English (en)
Inventor
Allen Gunion
Original Assignee
The Foxboro Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by The Foxboro Company filed Critical The Foxboro Company
Publication of WO1999060340A2 publication Critical patent/WO1999060340A2/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D3/00—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups
    • G01D3/028—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups mitigating undesired influences, e.g. temperature, pressure

Definitions

  • This invention relates to circuits for measuring signals from sensors. More particularly, the invention is directed to the use of dual floating power supplies in circuits for measuring signals from remotely located sensors, such as loop powered transmitters.
  • Measurement circuits monitor sensor output signals representative of a variety of parameters, such as pressure, temperature, voltage, current, ground fault, flow rate and the like. Some analog sensors provide single-ended output voltage or current signals, while other sensors provide differential output signals. It has long been a challenge to provide measurement circuits that accommodate various types of sensor output signals.
  • Loop powered transmitters include an input stage and an output stage.
  • the input stage is essentially a sensor circuit.
  • the output stage is essentially a transmitter and typically is powered from a 24-Vdc power supply.
  • a typical transmitter provides a loop current output signal between 4 n A and 20 mA, which is responsive to the magnitude of the parameter being sensed by the sensor circuit.
  • the loop current can be coupled to a sampling resistor, across which the voltage can be sensed. By Ohm's law, the voltage across the sampling resistor is proportionate to the sensor output current.
  • Prior circuits typically employ 50 ohm to 60 ohm sampling resistors, thus providing 0.2 Vdc to 1.2 Vdc output signals.
  • An analog-to-digital (a/d) converter circuit converts the sensed voltage to a digital representation of the magnitude of the parameter being measured.
  • Loop powered transmitters are typically located a few thousand feet or more from the a/d converter circuitry and the 24-Vdc power supply.
  • Prior measurement circuits generally include a/d converter circuitry or some type of buffering amplifier powered from a bipolar power supply, along with a 24-Vdc loop power supply. The bipolar and loop power supplies are typically common grounded.
  • common grounding provides reduced common mode noise rejection and renders it difficult to utilize the prior art circuits to measure differential sensor signals.
  • an object of the invention is to provide a measurement circuit having simpler power supply requirements.
  • Another object of the invention is to provide a measurement circuit having reduced noise coupling and increased common mode noise rejection.
  • a further object of the invention is to provide a measurement circuit that can accommodate both single-ended and differential sensor signals.
  • the invention attains the foregoing objects by providing a measurement circuit that includes an electrical element having a differential input; first and second power supplies; and a voltage reference.
  • the electrical element includes a differential amplifier.
  • the electrical element includes an a/d converter.
  • the differential analog input has a relatively high- voltage side and a relatively low- voltage side and is adapted for receiving an output signal from an external sensor.
  • the first power supply provides power and signal ground to the electrical element.
  • the second power supply provides current to the external sensor.
  • the voltage reference is powered from the first power supply and provides a common mode bias to the differential input.
  • the first power supply is a single-ended supply and the a/d converter is adapted for being powered from the single-ended supply.
  • the voltage reference can also provide a selected reference voltage to the a/d converter.
  • the external sensor is a remotely located loop powered transmitter.
  • the first and second power supplies are not common grounded (i.e. the first and second power supplies are floating with respect to each other). By floating the power supplies, the return from the loop powered sensor can be decoupled from the signal ground of the a/d converter, thereby diminishing the coupling of noise from the loop powered transmitter. Additionally, decoupling the grounds enables a circuit according to the invention to measure both single-ended and differential sensor output signals and to provide improved common mode noise rejection.
  • a measurement circuit includes only one single-ended power supply for providing power and signal grant to the a/d converter.
  • the invention provides an improved circuit for measuring an analog signal from a sensor, such as a loop powered transmitter.
  • a measurement circuit reduces the number of power supplies, from the prior requirements of a single-ended and a bipolar power supply to two single-ended power supplies. Additionally, the invention reduces the introduction of electrical noise, by floating the two power supplies with respect to each other and by decoupling the loop transmitter return from the signal ground.
  • FIGURE 1 is a schematic diagram of a typical prior art circuit for measuring an electrical signal from a remotely located sensor
  • FIGURE 2 is a schematic diagram of a circuit according to the invention for measuring an electrical signal from a remotely located analog sensor.
  • An electrical measurement circuit employs dual floating power supplies, i.e., the circuit employs two electrical power supplies that do not have a common electrical ground connection.
  • the measuring circuit of the invention requires only two single-ended power supplies and yet provides at least the same performance as prior circuits that require the more complex and more costly combination of both a single-ended supply and a bipolar supply.
  • a further feature of the measuring circuit of the invention is that it is less susceptible to the introduction of electrical noise, and has higher rejection of common-mode noise, compared to prior circuits having a bipolar power supply.
  • FIGURE 1 is a schematic diagram of a typical prior art circuit 100 for measuring a signal from a remotely located sensor 102.
  • the illustrated sensor 102 is a loop powered transmitter having a sensor portion 102a and a transmitter portion 102b.
  • the measurement circuit 100 includes a 24-Vdc power supply 104, a bipolar ⁇ 5-Vdc power supply 106 and an a/d converter 108.
  • the power supply 104 supplies voltage to the loop powered transmitter 102 by way of the connection 110 and the interface terminal 112.
  • the sensor portion 102a of the loop powered transmitter 102 monitors a selected parameter such as temperature, pressure, flow rate, ground fault current or the like.
  • the transmitter portion 102b in response, provides an output signal, typically a dc current between 4 mA and 20 mA, which is responsive to the magnitude of the sensed parameter.
  • the transmitter portion 102b electrically couples the output current signal to measurement circuit 100 by way of an interface terminal 118.
  • the output current signal is then passed through the sampling resistor 120 by way of lines 122 and 124.
  • the voltage developed across the sampling resistor 120 due to the transmitter output current is coupled to the differential inputs 108a and 108b of the a/d converter 108 by way of the lines 126 and 128.
  • the bipolar supply 106 provides a +5-Vdc supply voltage to the terminal 106c of the a/d converter 108, by way of the line 130.
  • the supply 106 also provides a -5-Vdc supply voltage to the terminal 108d of a/d converter 108, by way of the line 132.
  • the supply 106 further provides signal ground to the terminal 108e of the a/d converter 108, by way of the line 134.
  • the supply 106 also powers the voltage reference 136.
  • the voltage reference 136 provides a 2.5-Vdc reference voltage across the reference terminals 108f and 108g of the a/d converter 108.
  • the capacitor 138 filters the output voltage from the reference 136.
  • the a/d converter 108 employs the reference voltage 136 across the terminals 108f and 108g in a known manner to perform an a/d conversion of the voltage presented across the terminals 108a and 108b.
  • the crystal 140 along with the capacitors 142 and 144, provide a clock signal across terminals 108h and 108i of the a/d converter 108.
  • the a/d converter 108 employs this clock signal in a known manner to aid in the a/d conversion of the voltage signal presented across the input terminals 108a and 108b.
  • Operation of the a/d converter 108 can be controlled by an external processor 150 in a known fashion by way of the optically isolated control signals 148a-148d. Additionally, by way of the optically isolated serial ports 146a and 146b, the a/d converter 108 provides the external processor 150 with a digital representation of the analog signal applied across the input terminals 108a and 108b.
  • the measurement circuit 100 of FIGURE 1 illustrates the above mentioned drawbacks of prior approaches to measuring signals from remotely located sensors, such as loop powered transmitters.
  • the a/d converter 108 employs the bipolar voltage supply 106. Additionally, according to the configuration of the circuit 100, the return from the loop powered transmitter 102b connects to the signal ground from the power supply 104 at point 152. With the loop powered transmitter 102 a few thousand feet or more from the measurement circuit 100, connecting signal ground to the loop return increases the likelihood of coupling noise into circuit 100. Tying the loop return to signal ground also reduces common mode noise rejection typically associated with a differential input, such as that provided by inputs 108a and 108b.
  • FIGURE 2 depicts a measurement circuit 200 according to an illustrative embodiment of the invention.
  • circuit 200 measures an output signal from a loop powered transmitter 201
  • the circuit 200 can be employed to measure signals from other types of sensors, such as thermocouples.
  • the illustrated measurement circuit 200 includes an a/d converter 202, a 24-Vdc power supply 204, a 5-Vdc power supply 206 and a 2.5-Vdc voltage reference 208.
  • the a/d converter 202 can be a AD7705BRU, which uses a single-ended 5-Vdc supply voltage and is available from Analog Devices, Inc.
  • the power supply 204 provides current to the remotely located loop powered transmitter 201 by way of a terminal 210 and a line 212.
  • the power supply 204 also provides a return signal to a terminal 214 by way of a line 216.
  • the loop powered transmitter includes a sensor portion 201a and a transmitter portion 201b
  • the transmitter portion 201a provides a current signal, typically between 4 mA and 20 mA, which is representative of the magnitude of a parameter which the sensor portion senses.
  • the output current signal from the transmitter 201b couples into the measurement circuit 200 by way of an interface terminal 218. Lines 222 and 224 pass the signal to a sampling resistor 220.
  • the resultant voltage developed across sampling resistor 220 is coupled to the differential inputs 202a and 202b of the a/d converter 202 by way of lines 226 and 228. In this way, the input terminal 202a is maintained at a relatively higher voltage with respect to the input terminal 202b.
  • the power supply 206 provides a dc voltage, illustratively 5-Vdc, to a supply terminal 202c of the a/d converter 202 and to a reference input terminal 208a of the voltage reference 208, by way of line 230.
  • the power supply 206 also provides signal ground to a ground terminal 202d of the a/d converter 202 and to a ground terminal 208b of the voltage reference 208.
  • the voltage reference 208 couples a dc reference voltage, illustratively 2.5-Vdc, across reference input terminals 202e and 202f of the a/d converter 202, from a reference output terminal 208c.
  • the voltage reference 208 also couples the dc reference voltage through a coupling resistor 234 to one differential input 202b of the a/d converter 202.
  • coupling resistor 234 typically on the order of 10 kohms, the voltage reference 208 effectively provides a common mode bias, of 2.5-Vdc in the illustrated example, to the differential inputs 202a and 202b.
  • a capacitor 236, in the FIGURE 2 measurement circuit filters the output voltage from reference 208.
  • a crystal 238, along with capacitors 240 and 242, provides a clock signal to the a/d converter 202; and optically isolated signals 248a-248d enable the a/d converter 202 to be controlled from a digital controller 244 in a known fashion.
  • the optically isolated serial ports 246a and 246b provide a digital representation of the analog voltage across the terminals 202a and 202b to the processor 244.
  • the voltage developed across the sensing resistor 220 couples across the differential input terminals 202a and 202b of the a/d converter 202.
  • the voltage across the differential inputs 202a and 202b instead has a materially higher level, typically ranging between approximately 2.74 Vdc -3.7 Vdc, which enables the use of single-ended dc power supply 206.
  • the a/d converter 202 under the control of the optically isolated signals 248a-248d, converts the analog voltage across the differential input terminals 202a and 202b to a digital representation.
  • the foregoing measurement circuit 200 provides several important advantages over prior art circuits such as the FIGURE 1 circuit 100.
  • the common mode bias applied to the differential inputs 202a and 202b eliminates the need for a bipolar power supply. Another advantage is that uncoupling the signal ground from the loop return reduces the opportunity for noise to couple from the loop powered transmitter to the signal ground.
  • a further advantage of floating the supply 204 with respect to the supply 206 is that the measurement circuit 200 provides a true differential input across the sampling resistor 220, thus providing improved common mode noise rejection.
  • the measurement circuit 200 of the invention requires no further electrical supplies to attain the foregoing advantage. Instead, it utilizes the same single-ended power supply to power voltage reference 208 as it uses to power the a/d converter 202.
  • the reference 208 provides both a reference voltage to the a/d converter 202 and also provides a common mode bias voltage to the differential inputs 202a and 202b.
  • the common mode bias of the invention draws minimal current from the reference supply 208.
  • the power supply 204 is shown as part of circuit 200, in alternate embodiments it can be removed from circuit 200 and located proximate to the remote sensor 201. To that end, the circuit 200 provides a terminal 214 for connecting the return from the remotely located power supply 204 back to the circuit 200.
  • the circuit 200 depicts the loop return current being sampled across the resistor 220, with the voltage developed across the resistor 220 being measured by the a/d converter 202.
  • the combination of the resistor 220 and the a/d converter 202 is only illustrative and can be replaced with any electrical/electronic processing circuit adapted for processing a differential input voltage, wherein the processing circuit is powered by a single single-ended power supply, and the differential input is common mode biased.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Arrangements For Transmission Of Measured Signals (AREA)
  • Measurement Of Current Or Voltage (AREA)

Abstract

La présente invention concerne un circuit électrique, destiné à mesurer une tension analogique d'un capteur extérieur, comprenant un numériseur, des première et seconde alimentation électrique et une tension de référence. Le numériseur comprend une entrée différentielle avec un côté de tension relativement élevée et un côté de tension relativement faible. L'entrée différentielle est adaptée à recevoir un signal différentiel provenant d'un capteur extérieur. La première alimentation électrique à sortie simple fournit une puissance et un signal de terre au numériseur. La seconde alimentation électrique flotte par rapport à la première alimentation et fournit un courant à un capteur extérieur, tel qu'un émetteur à boucle alimentée. La tension de référence est donné par la première alimentation électrique à sortie simple et fournit une polarisation de mode commun à l'entrée différentielle du numériseur.
PCT/US1999/010996 1998-05-20 1999-05-19 Circuit de mesures utilisant une alimentation electrique double flottante WO1999060340A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US8613098P 1998-05-20 1998-05-20
US60/086,130 1998-05-20

Publications (1)

Publication Number Publication Date
WO1999060340A2 true WO1999060340A2 (fr) 1999-11-25

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PCT/US1999/010996 WO1999060340A2 (fr) 1998-05-20 1999-05-19 Circuit de mesures utilisant une alimentation electrique double flottante
PCT/US1999/011154 WO1999060541A2 (fr) 1998-05-20 1999-05-20 Circuit de mesure utilisant une double alimentation flottante

Family Applications After (1)

Application Number Title Priority Date Filing Date
PCT/US1999/011154 WO1999060541A2 (fr) 1998-05-20 1999-05-20 Circuit de mesure utilisant une double alimentation flottante

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100424728C (zh) * 2004-03-23 2008-10-08 横河电机株式会社 传送器系统
US7496458B2 (en) * 2001-01-22 2009-02-24 I F M Electronic Gmbh Electrical transducer

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10908029B2 (en) * 2018-01-02 2021-02-02 Nxp B.V. Voltage and temperature monitoring in power supplies

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3602830A (en) * 1969-10-21 1971-08-31 Honeywell Inc A constant current control circuit
US5677476A (en) * 1996-02-06 1997-10-14 Endress + Hauser Conducta Gesellschaft Fuer Mess- Und Regeltechnik Mbh & Co. Sensor and transmitter with multiple outputs

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7496458B2 (en) * 2001-01-22 2009-02-24 I F M Electronic Gmbh Electrical transducer
CN100424728C (zh) * 2004-03-23 2008-10-08 横河电机株式会社 传送器系统

Also Published As

Publication number Publication date
WO1999060541A3 (fr) 2000-06-02
WO1999060541A2 (fr) 1999-11-25

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