Digital thermometer using a dual slope A/D converter without a high precision voltage source

• US 3,978,471 A
• Filed: 03/26/1975
• Issued: 08/31/1976
• Est. Priority Date: 03/26/1975
• Status: Expired due to Term

• Current Assignee: Ametek Incorporated
• Original Assignee: Weston Instruments, Inc. (Schlumberger NV)
• Inventors: Kelly, Austin T.
• Primary Examiner(s): Sloyan, Thomas J.
• Application Number:
  US05/562,194
• Time in Patent Office: 524 Days
• Field of Search: 340/347 NT, 73/362 AR, 73/362 R, 73/1 F, 323/40, 323/75 H, 324/104, 324/99 D
• US Class Current: 341/118
• CPC Class Codes: G01K 1/028 arrangements for numerical indication
• CPC Class Codes: G01K 7/20 in a specially-adapted circuit, e.g. bridge circuit
• CPC Class Codes: H03M 1/52 Input signal integrated with linear return to datum

Abstract

A measuring device such as a digital thermometer uses dual slope analog-to-digital converter techniques but elminates the usual need for a high precision voltage source. An input capacitor is connected through novel switching to the temperature responsive arm and the temperature insensitive arm of a resistance bridge, to the low precision voltage source and to the output of the integrator and detector amplifiers. The switching sequence is such that long term changes in the voltage source and in the amplifier offset and drift are cancelled out during measurement cycles, thereby providing highly accurate measurements without the usual need for an expensive high precision voltage source.

Claims

1. A measurement device comprising:

an integrator amplifier and a detector amplifier each having an inverting and a non-inverting input and an output, an integrating capacitor connected between the output and the inverting input of the integrator amplifier, an integrating resistor connecting the inverting input of the integrator amplifier to ground, and means for connecting the inverting input of the detector amplifier to the output of the integrator amplifier and the noninverting input of the detector amplifier to the inverting input of the integrator amplifier, and an input capacitor having a right side connected to the noninverting input of the integrator amplifier;

a voltage source and a resistance bridge connected across the source, said bridge having a temperature responsive arm providing an output voltage by dividing the source voltage in accordance with the temperature of a portion of the arm and a temperature insensitive arm providing an output voltage by dividing the source voltage in accordance with a fixed ratio;

a first switch connecting the temperature insensitive bridge arm to the left side of the input capacitor to apply thereto the output voltage of the insensitive bridge arm when closed, a second switch connecting the temperature responsive arm to the left side of the input capacitor to apply thereto the output voltage of said temperature responsive arm when closed, a third switch connecting the source to the left side of the input capacitor to apply thereto the source voltage when closed, and a fourth switch connecting the right side of the input capacitor to the output of the detector amplifier to apply thereto the drift and offset of said amplifiers when closed;

means for maintaining the second and fourth switches closed and the first and third switches open for an initial time interval of sufficient duration to place on the input capacitor an initial charge reflecting the difference between the output voltage of the temperature sensitive bridge arm and the offset and drift of the amplifiers;

means for maintaining the first switch closed and the remaining switches open for a subsequent time interval of fixed duration which is short as compared to the initial time interval, to thereby apply to the integrator amplifier a measurement signal reflecting the difference between the output voltages of the two bridge arms; and

means for maintaining the third switch closed and all other switches open for a subsequent variable time interval which ends when the two inputs of the detector amplifier are at the same level, to thereby apply to the integrator amplifier during the variable time interval a reference signal reflecting the difference between the voltage of the source and the output voltage of the temperature insensitive bridge arm, the duration of the variable time being short as compared to the initial time interval and being a function of the difference between the resistances of the two bridge arms, thus being a measure of the temperature affecting the temperature responsive arm.

2. A measurement device as in claim 1 including means for measuring the duration of the variable time interval and for providing an indication of the measured duration.

3. A measurement device as in claim 1 including means for carrying out successive cycles each comprising an initial, a fixed and a variable time interval.

4. A measurement device comprising:

integrator means having an input and an output and responding to a voltage signal applied to the input to provide at the output a corresponding voltage ramp, detector means connected to the output of the integrator means for detecting when the ramp provided thereby crosses a defined threshold, and an input capacitor connected to the input of the integrator means;

a power source providing an output signal and an impedance bridge connected to the power source to receive the output signal therefrom, said bridge having a condition responsive arm providing an output signal by dividing the power source output signal in accordance with an ambient condition and a condition insensitive arm providing an output signal by dividing the power source output signal in accordance with a fixed ratio;

means for storing in the input capacitor an initial signal representing the output signal of the condition responsive bridge arm;

means for applying for a fixed time interval the output signal of the condition insensitive bridge arm to the input of the integrator means through said input capacitor to thereby apply to the input of the integrator means for said fixed time interval a measurement signal representing the combination of the output signal of the condition insensitive bridge arm and the initial signal stored in the input capacitor; and

means for subsequently applying for a variable time interval the output signal of the power source to the input of the integrating means through the input capacitor to thereby apply to the input of the integrator means for said variable time interval a reference signal representing the combination of the power source output signal and the initial signal stored in the input capacitor, said variable time interval ending when the detecting means detects the crossing of a defined threshold by the integrator means output, and the duration of said variable time interval representing the difference between the output signals of the two bridge arms.

5. A measurement device as in claim 4 including means for providing a signal representing the offset and drift of the integrator and detector means and wherein the means for storing said initial signal in the input capacitor comprises means for storing an initial signal representing the combination of the output signal of the condition responsive arm and the signal representing the offset and drift of the integrator and detector means.

6. A measurement device as in claim 4 including means for measuring the duration of the variable time interval and for indicating the measured duration.

7. A measurement device as in claim 5 wherein the polarities of the measurement signal and of the reference signal are opposite each other.

8. A measurement device as in claim 5 wherein the condition responsive bridge arm is responsive to ambient temperature, the bridge is a resistance bridge and the power source is a voltage source.

9. A measurement device as in claim 8 wherein the initial signal is a charge representing the difference between the output signal of the temperature responsive bridge arm and the signal representing the offset and drift of the integrator and detector means, the measurement signal represents the difference between the output signal of the temperature insensitive bridge arm and the initial charge on the input capacitor, and the reference signal represents the voltage of the output signal of the voltage source.

10. A measurement method comprising:

providing an integrator which provides at its output a voltage ramp corresponding to an applied input voltage, a detector connected to the output of the integrator for detecting when the ramp crosses a defined threshold, an input capacitor connected to the integrator input, a power source providing an output signal, and an impedance bridge connected to the power source to receive the output signal therefrom, said bridge having a condition responsive arm providing an output signal by dividing the power source output signal in accordance with an ambient condition and a condition insensitive arm providing an output signal by dividing the power source output signal in accordancee with a fixed ratio;

storing in the input capacitor an initial signal representing the combination of the output signal of the condition responsive bridge arm and a signal representing the offset and drift of the integrator and of the detector;

applying for a fixed time interval the output signal of the condition insensitive bridge arm to the input of the integrator through the input capacitor to thereby apply to the integrator input for said fixed time interval a measurement signal representing the combination of the output signal of the condition insensitive bridge arm and the initial signal stored in the input capacitor; and

applying for a variable time interval the power source output signal to the integrator input through the input capacitor to thereby apply to the integrator input for said variable time interval a reference signal representing the combination of the power source output signal and the initial signal stored in the input capacitor, said variable time interval ending when the detector detects the crossing of said defined threshold by the integrator output and the duration of the variable time interval representing the difference between the output signals of the two bridge arms.

11. A method as in claim 10 wherein the condition responsive arm is responsive to ambient temperature, the bridge is a resistance bridge, the power source is a voltage source, the initial signal represents the difference between the output signal of the temperature responsive arm and the offset and drift of the integrator and detector, the measurement signal represents the difference between the output signal of the temperature insensitive bridge arm and the initial signal and the reference signal represents the difference between the voltage source output and the initial signal.

12. A method as in claim 11 including measuring the duration of the variable time interval and providing an indication of the measured duration.

13. A measurement device comprising:

integrator means having an input and an output and responding to a voltage signal applied to the input to provide at the output a corresponding voltage ramp, detector means connected to the output of the integrator means for detecting when the ramp provided thereby crosses a defined threshold, and an input capacitor connected to the input of the integrator means;

a power source providing an output signal and an impedance bridge connected to the power source to receive the output signal therefrom, said bridge having a condition sensitive arm providing an output signal by dividing the power source output signal in accordance with a sensed condition and a condition insensitive arm providing an output signal by dividing the power source output signal in accordance with the fixed ratio;

means for storing in the input capacitor an initial signal representing the output signal of one of the bridge arms;

means for applying, for a fixed time interval, one of the output signal of the other bridge arm and the output signal of the power source to the input of the integrator means through said input capacitor to thereby apply to the input of the integrator means for said fixed time interval a first signal representing the combination of one of the output signal of said other bridge arm and the output signal of the power source with the initial signal stored in the input capacitor; and

means for subsequently applying, for a variable time interval, the other of the output signal of the other bridge arm and the output signal of the power source to the input of the integrating means through the input capacitor to thereby apply to the input of the integrator means for said variable time interval a second signal representing the combination of one of the output signal of the other bridge arm and the output signal of the power source with the initial signal stored in the input capacitor, said variable time interval ending when the detecting means detects the crossing of a defined threshold by the integrator means output, and the duration of said variable time interval representing the difference between the output signals of the two bridge arms.

14. A measurement method comprising:

providing an integrator which provides at its output a voltage ramp corresponding to an applied input voltage, a detector connected to the output of the integrator for detecting when the ramp crosses a defined threshold, an input capacitor connected to the integrator input, a power source providing an output signal, and an impedance bridge connected to the power source to receive the output signal therefrom, said bridge having a condition sensitive arm providing an output signal by dividing the power source output signal in accordance with a sensed condition and a condition insensitive arm providing an output signal by dividing the power source output signal in accordance with a fixed ratio;

storing in the input capacitor an initial signal representing the combination of the output signal of one of the bridge arms and a signal representing the offset and drift of the integrator and of the detector;

applying, for a fixed time interval, one of the output signal of the other bridge arm and the power source output signal to the input of the integrator through the input capacitor to thereby apply to the integrator input for said fixed time interval a first signal representing the combination of one of the output signal of the other bridge arm and the power source output signal with the initial signal stored in the input capacitor; and

applying, for a variable time interval, the other of the output signal of the other bridge arm and the power source output signal to the integrator input through the input capacitor to thereby apply to the integrator input for said variable time interval a second signal representing the combination of said other of the output signal of the other bridge arm and the power source output signal with the initial signal stored in the input capacitor, said variable time interval ending when the detector detects the crossing of said defined threshold by the integrator output and the duration of the variable time interval representing the difference between the output signals of the two bridge arms.