Programmable gain instrumentation amplifier having improved dielectric absorption compensation and common mode rejection ratio

• US 20050088229A1
• Filed: 12/21/2004
• Published: 04/28/2005
• Est. Priority Date: 08/16/2004
• Status: Active Grant

• Current Assignee: National Instruments Corporation (Emerson Electric Company)
• Original Assignee: National Instruments Corporation (Emerson Electric Company)
• Inventors: Sjoboen, Lauren, Regier, Christopher G., Wangsanata, Antony, Daigle, Clayton H.
• Granted Patent: US 7,095,280 B2
• Time in Patent Office: Days
• US Class Current: 330/69
• CPC Class Codes: H03F 2200/228 A measuring circuit being coupled to the input of an amplifier
• CPC Class Codes: H03F 2200/261 Amplifier which being suitable for instrumentation applications
• CPC Class Codes: H03F 2200/447 the amplifier being protected to temperature influence
• CPC Class Codes: H03F 3/211 using a combination of several amplifiers H03F3/60 takes precedence
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit
• CPC Class Codes: H03F 3/45928 using IC blocks as the active amplifying circuit

Abstract

A PGIA for use in measurement devices (e.g., data acquisition device) having improved dielectric absorption (DA) compensation and common mode rejection ratio (CMRR). When a step function is applied to an input of the PGIA, a first and a second DA compensation circuit may generate DA compensation signals derived from the step function. The DA compensation signals may combine with an original response of the PGIA to cancel some of the dielectric absorptions effects and improve the overall step response of the PGIA. An input stage of the PGIA may include a CMRR enhancement circuit to increase symmetry at the inputs of the PGIA. The CMRR enhancement circuit may delay an input signal received at a negative input terminal a particular amount such that it is in phase with an input signal received at a positive input terminal of the PGIA, to improve the CMRR.

Claims

1. An instrumentation amplifier for use in a measurement device, the instrumentation amplifier comprising:

a negative input terminal;

an output terminal;

a first op-amp comprising a noninverting input terminal, an inverting input terminal, and an output terminal, wherein one of the input terminals of the first op-amp is coupled to the negative input terminal of the instrumentation amplifier;

a current-to-voltage (I-V) converter comprising a first input terminal, a second input terminal, and an output terminal, wherein the output terminal of the I-V converter is coupled to the output terminal of the instrumentation amplifier;

a first dielectric absorption (DA) compensation circuit coupled to the output terminal of the first op-amp, wherein the first DA compensation circuit is operable to generate a first DA compensation signal which is derived from an input step signal received at the negative input terminal of the instrumentation amplifier; and

a second DA compensation circuit coupled to one of the input terminals of the I-V converter, wherein the second DA compensation circuit is operable to generate a second DA compensation signal and provide the second DA compensation signal to the I-V converter;

wherein the first and second DA compensation signals are operable to reduce dielectric absorption and improve a step response of the instrumentation amplifier.

2. The instrumentation amplifier of claim 1, wherein the first DA compensation signal is a decaying current step and wherein a magnitude of the first DA compensation signal is dependent upon an amplitude of the input step signal and resistances of the first DA compensation circuit.

3. The instrumentation amplifier of claim 1, further comprising a first current source operable to provide a biasing current to a signal path of the instrumentation amplifier, wherein the first DA compensation circuit is coupled between the output terminal of the first op-amp and the first current source.

4. The instrumentation amplifier of claim 3, wherein the first DA compensation circuit is operable to inject the first DA compensation signal into the signal path of the instrumentation amplifier via the first current source to adjust the biasing current based on a polarity of the first DA compensation signal.

5. The instrumentation amplifier of claim 4, wherein if the first DA compensation signal is negative the biasing current may decrease momentarily by an amount proportionate to a magnitude of the first DA compensation signal, and if the first DA compensation signal is positive the biasing current may increase momentarily by an amount proportionate to the magnitude of the first DA compensation signal.

6. The instrumentation amplifier of claim 5, wherein an adjusted biasing current is operable to combine with an original response of the instrumentation amplifier to reduce dielectric absorption effects and improve a settling time of the instrumentation amplifier, to improve the step response of the instrumentation amplifier.

7. The instrumentation amplifier of claim 3, wherein the first DA compensation circuit comprises a first resistor, a second resistor, a third resistor, and a first capacitor, wherein the first resistor is coupled to the output terminal of the first op-amp and the first capacitor is coupled to the first current source, and wherein the second resistor is coupled in series with the first capacitor and the second resistor is also coupled to a junction of the first and third resistors.

8. The instrumentation amplifier of claim 1, further comprising a current mirror coupled between a left branch and a right branch of the instrumentation amplifier, wherein the second DA compensation circuit is coupled between one of the input terminals of the I-V converter and an output of the current mirror at the right branch of the instrumentation amplifier.

9. The instrumentation amplifier of claim 8, wherein the second DA compensation signal is a decaying voltage step and wherein a magnitude of the second DA compensation signal is dependent upon an amplitude of the input step signal, resistances of the second DA compensation circuit, resistances of the current mirror, and resistances of the I-V converter.

10. The instrumentation amplifier of claim 1, wherein the second DA compensation signal is operable to combine with a reference voltage signal provided to one of the inputs of the I-V converter to reduce a dielectric absorption and improve the step response of the instrumentation amplifier.

11. The instrumentation amplifier of claim 7, wherein the second DA compensation circuit comprises a fourth resistor, a fifth resistor, and a second capacitor, and wherein the second capacitor is coupled between one of the input terminals of the I-V converter and a junction of the fourth and fifth resistors.

12. The instrumentation amplifier of claim 1, further comprising a common mode rejection ratio (CMRR) enhancement circuit coupled in parallel to the first DA compensation circuit, wherein the CMRR enhancement circuit is operable to delay an input received at the negative input terminal of the instrumentation amplifier to improve a CMRR of the instrumentation amplifier.

13. The instrumentation amplifier of claim 12, further comprising a positive input terminal, wherein the CMRR enhancement circuit is operable to delay the input signal received at the negative input terminal a particular amount such that the input signal received at the negative input terminal will be in phase with an input signal received at the positive input terminal of the instrumentation amplifier, to improve the CMRR of the instrumentation amplifier.

14. The instrumentation amplifier of claim 12, further comprising a first current source, wherein the CMRR enhancement circuit comprises a resistor coupled in series to a capacitor, wherein the resistor is coupled to the output terminal of the first op-amp and the capacitor is coupled to the first current source.

15. The instrumentation amplifier of claim 12, wherein the CMRR enhancement circuit improves a symmetry between a positive input and a negative input of the instrumentation amplifier to improve the CMRR of the instrumentation amplifier.

16. The instrumentation amplifier of claim 1, configured as a programmable gain instrumentation amplifier (PGIA).

17. The instrumentation amplifier of claim 1, comprised in a data acquisition device.

18. An instrumentation amplifier for use in a measurement device, the instrumentation amplifier comprising:

a negative input terminal;

a positive input terminal;

a first op-amp comprising a noninverting input terminal, an inverting input terminal, and an output terminal, wherein one of the input terminals of the first op-amp is coupled to the negative input terminal of the instrumentation amplifier; and

a common mode rejection ratio (CMRR) enhancement circuit coupled to the output terminal of the first op-amp, wherein the CMRR enhancement circuit is operable to delay an input signal received at the negative input terminal a particular amount such that the input signal received at the negative input terminal will be in phase with an input signal received at the positive input terminal of the instrumentation amplifier, to improve the CMRR of the instrumentation amplifier.

19. The instrumentation amplifier of claim 18, further comprising a first current source, wherein the CMRR enhancement circuit is coupled between the output terminal of the first op-amp and the first current source, wherein the CMRR enhancement circuit comprises a resistor coupled in series to a capacitor.

20. The instrumentation amplifier of claim 19, further comprising one or more gain resistors coupled to the inverting input terminal of the first op-amp and to the first current source, wherein the particular amount of delay is dependant upon a resistance value of a selected gain resistor and a capacitance value of the capacitor of the CMRR enhancement circuit.

21. The instrumentation amplifier of claim 18, further comprising:

a first dielectric absorption (DA) compensation circuit coupled to the output terminal of the first op-amp, wherein the first DA compensation circuit is operable to generate a first DA compensation signal which is derived from an input step signal received at the negative input terminal of the instrumentation amplifier, wherein the first DA compensation signal generated by the first DA compensation circuit is operable to reduce dielectric absorption and improve a step response of the instrumentation amplifier.

22. The instrumentation amplifier of claim 21, further comprising:

an output terminal;

a current-to-voltage (I-V) converter comprising a first input terminal, a second input terminal, and an output terminal, wherein the output terminal of the I-V converter is coupled to the output terminal of the instrumentation amplifier; and

a second DA compensation circuit coupled to one of the input terminals of the I-V converter, wherein the second DA compensation circuit is operable to generate a second DA compensation signal and provide the second DA compensation signal to the I-V converter, wherein the second DA compensation signal generated by the second DA compensation circuit is operable to reduce dielectric absorption and improve the step response of the instrumentation amplifier.

23. A data acquisition device, comprising:

one or more analog-to-digital converters (ADCs) operable to convert received analog data into digital data; and

a programmable gain instrumentation amplifier (PGIA) coupled to the one or more ADCs, the PGIA comprising;

a negative input terminal;

an output terminal;

a first op-amp comprising a noninverting input terminal, an inverting input terminal, and an output terminal, wherein one of the input terminals of the first op-amp is coupled to the negative input terminal of the PGIA;

a current-to-voltage (I-V) converter comprising a first input terminal, a second input terminal, and an output terminal, wherein the output terminal of the I-V converter is coupled to the output terminal of the PGIA;

a first dielectric absorption (DA) compensation circuit coupled to the output terminal of the first op-amp, wherein the first DA compensation circuit is operable to generate a first DA compensation signal which is derived from an input step signal received at the negative input terminal of the PGIA;

a second DA compensation circuit coupled to one of the input terminals of the I-V converter, wherein the second DA compensation circuit is operable to generate a second DA compensation signal and provide the second DA compensation signal to the I-V converter, wherein the first and second DA compensation signals are operable to reduce dielectric absorption and improve a step response of the PGIA; and

a common mode rejection ratio (CMRR) enhancement circuit coupled in parallel to the first DA compensation circuit, wherein the CMRR enhancement circuit is operable to delay an input received at the negative input terminal of the PGIA to improve a CMRR of the PGIA.

24. The data acquisition device of claim 23, wherein the first DA compensation signal is a decaying current step and wherein a magnitude of the first DA compensation signal is dependent upon an amplitude of the input step signal and resistances of the first DA compensation circuit.

25. The data acquisition device of claim 23, further comprising a first current source operable to provide a biasing current to a signal path of the PGIA, wherein the first DA compensation circuit is coupled between the output terminal of the first op-amp and the first current source.

26. The data acquisition device of claim 25, wherein the first DA compensation circuit is operable to inject the first DA compensation signal into the signal path of the PGIA via the first current source to adjust the biasing current based on a polarity of the first DA compensation signal.

27. The data acquisition device of claim 26, wherein if the first DA compensation signal is negative the biasing current may decrease momentarily by an amount proportionate to a magnitude of the first DA compensation signal, and if the first DA compensation signal is positive the biasing current may increase momentarily by an amount proportionate to the magnitude of the first DA compensation signal.

28. The data acquisition device of claim 25, wherein the first DA compensation circuit comprises a first resistor, a second resistor, a third resistor, and a first capacitor, wherein the first resistor is coupled to the output terminal of the first op-amp and the first capacitor is coupled to the first current source, and wherein the second resistor is coupled in series with the first capacitor and the second resistor is also coupled to a junction of the first and third resistors.

29. The data acquisition device of claim 23, further comprising a current mirror coupled between a left branch and a right branch of the PGIA, wherein the second DA compensation circuit is coupled between one of the input terminals of the I-V converter and an output of the current mirror at the right branch of the PGIA.

30. The data acquisition device of claim 29, wherein the second DA compensation signal is a decaying voltage step and wherein a magnitude of the second DA compensation signal is dependent upon an amplitude of the input step signal, resistances of the second DA compensation circuit, resistances of the current mirror, and resistances of the I-V converter.

31. The data acquisition device of claim 23, wherein the second DA compensation signal is operable to combine with a reference voltage signal provided to one of the inputs of the I-V converter to reduce a dielectric absorption and improve the step response of the PGIA.

32. The data acquisition device of claim 28, wherein the second DA compensation circuit comprises a fourth resistor, a fifth resistor, and a second capacitor, and wherein the second capacitor is coupled between one of the input terminals of the I-V converter and a junction of the fourth and fifth resistors.

33. The data acquisition device of claim 23, further comprising a positive input terminal, wherein the CMRR enhancement circuit is operable to delay the input signal received at the negative input terminal a particular amount such that the input signal received at the negative input terminal will be in phase with an input signal received at the positive input terminal of the PGIA, to improve the CMRR of the PGIA.

34. The data acquisition device of claim 23, wherein the CMRR enhancement circuit comprises a resistor coupled in series to a capacitor, wherein the resistor is coupled to the output terminal of the first op-amp and the capacitor is coupled to a first current source.

35. The data acquisition device of claim 23, wherein the CMRR enhancement circuit improves a symmetry between a positive input and a negative input of the PGIA to improve the CMRR of the PGIA.

36. The data acquisition device of claim 23, comprised in a data acquisition system comprising a computer system for processing data obtained from one or more data acquisition processes.

37. The data acquisition device of claim 36, comprised in a data acquisition system further comprising a unit under test (UUT), wherein the UUT comprises a sensor operable to provide the data acquisition device with sensor data.

38. A PGIA for use in a data acquisition device, the PGIA comprising:

a negative input terminal;

an output terminal;

a first op-amp comprising a noninverting input terminal, an inverting input terminal, and an output terminal, wherein one of the input terminals of the first op-amp is coupled to the negative input terminal of the PGIA;

a current-to-voltage (I-V) converter comprising a first input terminal, a second input terminal, and an output terminal, wherein the output terminal of the I-V converter is coupled to the output terminal of the PGIA;

a first current source operable to provide a biasing current to a signal path of the PGIA;

a first DA compensation circuit coupled between the output terminal of the first op-amp and the first current source, wherein the first DA compensation circuit comprises a first resistor, a second resistor, a third resistor, and a first capacitor, and wherein the second resistor is coupled in series with the first capacitor and the second resistor is also coupled to a junction of the first and third resistors;

a second DA compensation circuit coupled to one of the input terminals of the I-V converter, wherein the second DA compensation circuit comprises a fourth resistor, a fifth resistor, and a second capacitor, and wherein the second capacitor is coupled between the one of the input terminals of the I-V converter and a junction of the fourth and fifth resistors; and

a CMRR enhancement circuit coupled in parallel to the first DA compensation circuit, wherein the CMRR enhancement circuit comprises a sixth resistor coupled in series to a third capacitor, wherein the sixth resistor is coupled to the output terminal of the first op-amp and the third capacitor is coupled to the first current source.

39. A data acquisition device, comprising:

one or more analog-to-digital converters (ADCs) operable to convert received analog data into digital data; and

a PGIA coupled to the one or more ADCs, the PGIA comprising;

a negative input terminal;

a positive input terminal;

an output terminal;

a first op-amp comprising a noninverting input terminal, an inverting input terminal, and an output terminal, wherein one of the input terminals of the first op-amp is coupled to the negative input terminal of the PGIA;

a current-to-voltage (I-V) converter comprising a first input terminal, a second input terminal, and an output terminal, wherein the output terminal of the I-V converter is coupled to the output terminal of the PGIA;

a first current source operable to provide a biasing current to a signal path of the PGIA;

a first DA compensation circuit coupled between the output terminal of the first op-amp and the first current source, wherein the first DA compensation circuit is operable to generate a first DA compensation signal which is derived from an input step signal received at the negative input terminal of the PGIA;

a second DA compensation circuit coupled to one of the input terminals of the I-V converter, wherein the second DA compensation circuit is operable to generate a second DA compensation signal and provide the second DA compensation signal to the I-V converter, wherein the first and second DA compensation signals are operable to reduce dielectric absorption and improve a step response of the PGIA; and

a CMRR enhancement circuit coupled in parallel to the first DA compensation circuit, wherein the CMRR enhancement circuit is operable to delay an input signal received at the negative input terminal a particular amount such that the input signal received at the negative input terminal will be in phase with an input signal received at the positive input terminal of the PGIA, to improve the CMRR of the PGIA.