IE940758A1 - A ground compatible voltage-mode instrumentation amplifier - Google Patents

Abstract

A ground compatible voltage-mode instrumentation amplifier comprises an output difference stage (3) and an input buffer stage (2) which comprises two input operational amplifiers (OPIA) and (OPIB). To avoid the operational amplifiers (OPIA) and (OPIB) driving to ground due to loading of the difference stage (3) the internal circuitry of the operational amplifiers (OPIA) and (OPIB) are implemented with respective voltage level shift diodes (S) which are disposed in series with a constant current surce (Iss) and a MOSFET M1, the gate of which forms the positive input (1,3) of the respective operational amplifiers (OPIA) and (OPIB). The constant current sources (Iss), the voltage level shift diodes (S) and the MOSFETS M1 and M2 of the operational amplifiers (OPIA) and (OPIB) are matched so that the voltage level shift (Vs) of the input operational amplifiers (OPIA) and (OPIB) are identical, thereby providing an output voltage (Vc) provided by the function. <Fig. 1>

Description

The present invention relates to a ground compatible voltage-mode instrumentation amplifier of the type which would normally be implemented in solid state circuitry, and in particular, though not limited to an instrumentation amplifier implemented in MOS, CMOS, BiCMOS and BCDMOS. The invention also relates to an electronic circuit comprising the instrumentation amplifier and to a disc head position controller for a disc drive.

Such instrumentation amplifiers are extensively used in analog signal processing requiring ground compatible differential amplification with high input impedance low offset and gain errors and an output voltage referenced to an external reference voltage. Voltagemode instrumentation amplifiers in general comprise an input buffer stage followed by a voltage difference amplifier which implements the required transfer function. The buffer stage, in general, comprises a pair of input operational amplifiers which receive the respective input voltages. However, where such instrumentation amplifiers are required to provide differential amplification of signals whose common mode range includes ground, to avoid the.,.operational OPEN TO PUBLIC INSPECTION UNDLR SECTION 28 AND RULE 23 JNL No OF *94075$ amplifiers driving to ground and becoming saturated in the event of loading of the difference amplifier, it is necessary to level shift the input signals. This, in general, is achieved by providing an additional level shift stage for each input. The level shift stages of such ground compatible voltage-mode instrumentation amplifiers, in general, require matched current sources and matched transistors for providing a high impedance voltage level shift. Such matched constant current sources and matched transistors are required in addition to the matched constant current sources of the respective input operational amplifiers. Accordingly, not only are additional matched constant current sources and matched transistors required, which, in general, may be difficult to provide, but additional power is required to drive the voltage level shift stages, and furthermore, and most importantly, the additional components of the voltage level shift stage will increase offset voltage and also reduce circuit reliability.

There is therefore a need for a ground compatible voltage-mode instrumentation amplifier which overcomes these problems.

The present invention is directed towards providing such an instrumentation amplifier.

According to the invention, there is provided a ground compatible voltage-mode instrumentation amplifier having a main output and a main positive input and a main negative input and a reference voltage input, the instrumentation amplifier comprising: a difference amplifier having an output which forms the main output, and a pair of inputs, one of which inputs being associated with the reference voltage input, and a pair of input operational amplifiers, each having a first input, a second input and an output, the first inputs of the respective input operational amplifiers forming the respective main inputs, the outputs of the input operational amplifiers feeding the respective inputs of the difference amplifier and also being fed back to the second input of the corresponding input operational amplifier, each input operational amplifier comprising a differential transistor pair fed by a constant current source and being associated with the respective first and second inputs, the constant current sources and the differential transistor pairs of the respective input operational amplifiers being substantially matched, wherein a voltage shift element is disposed in each input operational amplifier in series with the constant current source and one transistor of the differential transistor pair associated with the first input for shifting the output voltage of the input operational amplifiers above zero volts, the voltage shift elements being substantially matched with each other.

In one aspect of the invention the voltage shift 5 element is a resistive element, and preferably, the voltage shift element is of relatively low voltage coefficient.

In one aspect of the invention the voltage shift element is a diode, and may be provided by a plurality of diodes connected in series. Preferably, each diode is a bi-polar diode.

Alternatively, the voltage shift element is a resistor.

Preferably, the first input of each input operational amplifier is a positive input and the second input of each input operational amplifier is a negative input.

In general, the differential transistor pair of each input operational amplifier comprises a pair of MOSFETS. Preferably, the gates of the respective MOSFETS of each input operational amplifier form the respective first and second inputs. Advantageously, the voltage shift element of each input operational amplifier is disposed in series with the constant current source and the source of the MOSFET, the gate of which MOSFET forms the first input.

In one aspect of the invention the gain of each input operational amplifier is unity. Alternatively, the gain of each input operational amplifier is greater than unity.

Generally, the difference amplifier is configured with associated resistors for providing an output voltage equal to the reference voltage summed with the product of the difference of the voltages applied the main inputs and the gain of the difference amplifier when the gain of the respective input operational amplifiers is unity.

Preferably, the difference amplifier comprises an output operational amplifier configured with a pair of matched first resistors, and a pair of matched second resistors associated with the main output and the reference voltage input to provide a gain equal to the quotient of the values of one of the second resistors divided by one of the first resistors.

In general, the instrumentation amplifier is implemented in solid state circuitry, and may be implemented in MOS circuitry, CMOS circuitry, BiCMOS 94075 circuitry and/or BCDMOS circuitry.

Additionally, the invention provides an electronic circuit comprising the ground compatible voltage-mode instrumentation amplifier according to the invention.

Preferably, the electronic circuit is implemented in solid state circuitry, and may be implemented in MOS circuitry, CMOS circuitry, BiCMOS circuitry and/or BCDMOS circuitry. The electronic circuit is suitable for signal processing, and preferably, the electronic circuit is a controlled-gain digital to analog converter circuit for header position control of a disc drive.

Further the invention provides a disc head position controller for disc drives which incorporate the electronic circuit according to the invention.

Preferably, the disc head position controller is for controlling high-end disc drives.

The invention will be more clearly understood from the following description of a preferred embodiment thereof, which is given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a ground compatible voltage-mode instrumentation amplifier according °40758 to the invention, and Fig. 2 is a circuit diagram of part of the circuitry of a buffer stage operational amplifier of the instrumentation amplifier of Fig. 1.

Referring to the drawings, there is illustrated a ground compatible voltage-mode instrumentation amplifier according to the invention indicated generally by the reference numeral 1. The instrumentation amplifier 1 is particularly suitable for use in solid state circuitry, and may be implemented in MOS, CMOS, BiCMOS and BCDMOS technologies and other similar type technologies. The instrumentation amplifier 1 comprises an input buffer stage 2 which receives input voltages Va and Vb on main inputs A and B. The buffer stage 2 feeds a difference amplifier 3 which provides an output function Vc on a main output C. A reference voltage input D to the difference amplifier 3 receives a reference voltage Vref. The buffer stage 2 comprises a pair of input operational amplifiers ΟΡΙΑ and OPIB, which in this case are P-channel type input for very high input impedance and ground compatibility, and are of unit gain. Each input operational amplifier ΟΡΙΑ and OPIB comprises a positive input and a negative input, namely, positive and negative inputs 1 and 2, respectively, of the input operational amplifier 0P1A and positive and negative inputs 3 and 4, respectively, of the input operational amplifier OP1B. The positive inputs 1 and 3 of the respective input operational amplifiers ΟΡΙΑ and OP1B form the main inputs A and B, respectively, to the instrumentation amplifier 1. Outputs 5 and 6 of the input operational amplifiers ΟΡΙΑ and OP1B, respectively, deliver respective output signals, as will be described below, which form the input signals to the difference amplifier 3. The outputs 5 and 6 of the respective input operational amplifiers ΟΡΙΑ and OP1B are fed back to their corresponding negative inputs 2 and 4, respectively.

The difference amplifier 3 comprises an operational amplifier OP2 with positive and negative input, namely, inputs 7 and 8, respectively, which receive inputs from the outputs 5 and 6 of the input operational amplifiers ΟΡΙΑ and OP1B, respectively. An output 9 of the operational amplifier 0P2 provides the output C of the instrumentation amplifier 1. The operational amplifier 0P2 of the difference amplifier 3 is configured with pairs of matched first resistors R1 and pairs of matched resistors R2 so that the gain K of the difference amplifier 3 is given by the formula K = R2 Rl. 94075( The two resistors R1 are identical, and the two resistors R2 are identical. The respective values of the resistors R2 and R1 will be selected depending on the gain which is required for the difference amplifier 3, as well as on the loading and the matching of the resistors which is processed dependent. In this particular embodiment of the invention, the value of the resistors R1 are each of 5 Ohms and the resistors R2 are each of 10 Ohms, thereby giving a gain of two.

Referring now to Fig. 2, the internal circuitry of each of the input operational amplifiers ΟΡΙΑ and OP1B of the buffer stage 2 will now be described. The internal circuitry of the input operational amplifier ΟΡΙΑ is illustrated in Fig. 2, although the internal circuitry of the input operational amplifier OP1B is identical to that the input operational amplifier ΟΡΙΑ. A constant current source Iss is provided in the operational amplifier ΟΡΙΑ and feeds a differential transistor pair provided by a pair of MOSFETS Ml and M2. The source of each MOSFET Ml and M2 is fed by the constant current source Iss and the drain from the MOSFETS Ml and M2 feeds conventional circuitry illustrated in block representation and indicated by the reference numeral 4 which provides an output on the output 5. Such circuitry 4 will be well known to those skilled in the art. The gate of the MOSFET Ml forms the positive input 1 while the gate of the M2 forms the negative input 2 of the input operational amplifier ΟΡΙΑ. A voltage shift element, in this case a bi-diode S is disposed in series with the constant current source Iss and the source of the MOSFET Ml for shifting the level of the input voltage by a value Vs. The constant current sources Iss and the respective MOSFETS Ml and M2 of the respective input operational amplifiers ΟΡΙΑ and OP1B of the buffer stage 2 are matched, and the voltage shift diodes S for providing the voltage shift Vs of the operational amplifiers 0P1A and OP1B are matched for providing substantially identical voltage level shifts Vs in each of the input operational amplifiers ΟΡΙΑ and OP1B. The MOSFETS Ml and M2 of the operational amplifier 0P1A are chosen to be equal and closely matched. The circuitry of block 4 in conjunction with the feed back connection from the output 5 to the negative input 2 of the operational amplifier ΟΡΙΑ ensures that the current II and 12 flowing from the drain of the MOSFETS Ml and M2 is equal, and furthermore, the respective currents II and 12 are equal to half the source current Iss. In this way, the voltage drop across the gate and the source of the MOSFET M2 is equal to the voltage drop across the gate and the source of the MOSFET Ml, since the source of the MOSFET Ml is connected to the voltage shift diode S. In other words, that is: 9 4 Ο 7 5 β Vgs M2 Vgs Ml As the source of the MOSFET M2 is referenced to a voltage which is Vs volts above the source of the MOSFET Ml, the voltage at the gate of the MOSFET M2 is also Vs volts above the voltage at the gate of the MOSFET Ml. That is: Vs M2 = Vg Ml + Vs Since the gain of the input operational amplifiers 0P1A and OP1B of the buffer stage 2 is unity, the output voltages appearing on the outputs 5 and 6 of the input operational amplifiers ΟΡΙΑ and OP1B are as follows: V5 = Va + Vs, and V6 = Vb + Vs Accordingly, the output voltage of the output 5 of the input operational amplifier ΟΡΙΑ will always be above ground for all values of input voltage Va equal to or greater than ground. Since the constant current source ISS, the MOSFETS Ml and M2 and the voltage shift diode S of the input operational amplifier ΟΡΙΑ are closely matched with the corresponding components of the input operational amplifier OP1B, similar comments apply to the input operational amplifier OP1B as to the input 0 7 5 operational amplifier ΟΡΙΑ. Accordingly, the output voltage on the output 6 of the input operational amplifier OP1B likewise will always be above ground for all values of the input voltage Vb equal to or greater than ground. Accordingly, saturation of the outputs 5 and 6 of the input operational amplifiers ΟΡΙΑ and OP1B due to loading of the difference amplifier 3 is avoided, and there is no danger of either of the input operational amplifiers ΟΡΙΑ and OP1B driving to ground.

The output Vc of the instrumentation amplifier 1 is provided by the following function: Vc = Vref + K (Va - Vb) The level shift voltage Vs is removed in the difference amplifier 3.

By deriving the level shift voltage Vs in the input operational amplifiers ΟΡΙΑ and OP1B full advantage is taken of the fact that reproducible level shift voltages Vs can be provided in the two input operational amplifiers ΟΡΙΑ and OP1B. The current sources Iss in the two operational amplifiers ΟΡΙΑ and OP1B are well defined current sources, and thus close matching of the two current sources can readily be achieved. By virtue of the fact that the input 94075( operational amplifiers ΟΡΙΑ and OP1B are provided with high impedance and low offset, the currents II which is equal to half the source current Iss will thus be closely matched in the two input operational amplifiers ΟΡΙΑ and OP1B. Thus, once the level shift element S whose voltage Vs will be a function of the current II flowing through it are closely matched in the two input operational amplifiers ΟΡΙΑ and OP1B a highly reproducible level shift voltage Vs will be provided, and accordingly, the level shift voltage Vs in each of the input operational amplifiers ΟΡΙΑ and OP1B will be substantially identical.

For both CMOS and BiCMOS technologies, the finite output resistance effects common to both the voltagemode (level shift) and current mode (current mirror stage) approaches of implementing a ground compatible instrumentation amplifier can be resolved by choosing the level shift element S to be a diode or low voltage co efficient resistor. In the case where the voltage level shift element is chosen to be a bi-polar diode as would be the case in BiCMOS type technologies, the level shift can be implemented with very close matching without introducing the effects of bias current loading. For low power applications, the instrumentation amplifier according to the invention 0*0758 offers the possibility to reduce current consumption.

By virtue of the fact that the voltage level shift Vs is derived across the voltage level shift elements, and the voltage level shift element is formed in the internal circuitry of the respective input operational amplifiers, and furthermore by virtue of the fact that the voltage level shift is derived from the internal constant current source of the input operational amplifiers, virtually no additional area in the integrated circuit is required for providing the voltage level shift. Additionally, other than the voltage level shift element, no additional components or constant current sources are required, thus, no additional power is required for operating the instrumentation amplifier, and furthermore, virtually no additional cost is required in the production of the instrumentation amplifier. Most importantly, since the constant current sources and internal circuitry of the respective input operational amplifiers can readily easily be matched, and the voltage level shift elements can also be readily easily matched, a particularly accurate instrumentation amplifier is provided which is ground compatible.

A particularly advantageous use of the instrumentation amplifier is as a header position control in disc drives, and in particular in high end disc drives for computers and the like.

Claims (31)

1. A ground compatible voltage-mode instrumentation amplifier having a main output and a main positive input and a main negative input and a reference voltage input, the instrumentation amplifier comprising: a difference amplifier having an output which forms the main output, and a pair of inputs, one of which inputs being associated with the reference voltage input, and a pair of input operational amplifiers, each having a first input, a second input and an output, the first inputs of the respective input operational amplifiers forming the respective main inputs, the outputs of the input operational amplifiers feeding the respective inputs of the difference amplifier and also being fed back to the second input of the corresponding input operational amplifier, each input operational amplifier comprising a differential transistor pair fed by a constant current source and being associated with the respective first and second inputs, the constant current sources and the differential transistor pairs of the respective input operational amplifiers being substantially matched, wherein a voltage shift element is disposed in each input operational amplifier in series with the constant current source and one transistor of the differential transistor pair associated with the first input for 9*0758 shifting the output voltage of the input operational amplifiers above zero volts, the voltage shift elements being substantially matched with each other.

2. An instrumentation amplifier as claimed in Claim 1 in which the voltage shift element is a resistive element.

3. An instrumentation amplifier as claimed in Claim 1 or 2 in which the voltage shift element is of relatively low voltage coefficient.

4. An instrumentation amplifier as claimed in any preceding claim in which the voltage shift element is a diode.

5. An instrumentation amplifier as claimed in any preceding claim in which the voltage shift element comprises a plurality of diodes connected in series.

6. An instrumentation amplifier as claimed in Claim 4 or 5 in which each diode is a bi-polar diode.

7. An instrumentation amplifier as claimed in any of Claims 2 to 4 in which the voltage shift element is a resistor.

8. An instrumentation amplifier as claimed in any preceding claim in which the first input of each input operational amplifier is a positive input and the second input of each input operational amplifier is a 5 negative input.

9. An instrumentation amplifier as claimed in any preceding claim in which the differential transistor pair of each input operational amplifier comprises a pair of MOSFETS.

10. 10 An instrumentation amplifier as claimed in Claim 9 in which the gates of the respective MOSFETS of each input operational amplifier form the respective first and second inputs .

11. An instrumentation amplifier as claimed in Claim 9 15 or 10 in which the voltage shift element of each input operational amplifier is disposed in series with the constant current source and the source of the MOSFET, the gate of which MOSFET forms the first input.

12. An instrumentation amplifier as claimed in any 20 preceding claim in which the gain of each input operational amplifier is unity.

13. An instrumentation amplifier as claimed in any of 440758 Claims 1 to 11 in which the gain of each input operational amplifier is greater than unity.

14. An instrumentation amplifier as claimed in any preceding claim in which the difference amplifier is 5 configured with associated resistors for providing an output voltage equal to the reference voltage summed with the product of the difference of the voltages applied the main inputs and the gain of the difference amplifier when the gain of the respective input 10 operational amplifiers is unity.

15. An instrumentation amplifier as claimed in Claim 14 in which the difference amplifier comprises an output operational amplifier configured with a pair of matched first resistors, and a pair of matched second 15 resistors associated with the main output and the reference voltage input to provide a gain equal to the quotient of the values of one of the second resistors divided by one of the first resistors.

16. An instrumentation amplifier as claimed in any 20 preceding claim in which the instrumentation amplifier is implemented in solid state circuitry.

17. An instrumentation amplifier as claimed in Claim 16 in which the instrumentation amplifier is * 4 ’M« 20 implemented in MOS circuitry.

18. An instrumentation amplifier as claimed in Claim 16 in which the instrumentation amplifier is implemented in CMOS circuitry.

19. An instrumentation amplifier as claimed in Claim 16 in which the instrumentation amplifier is implemented in BiCMOS circuitry.

20. An instrumentation amplifier as claimed in Claim 16 in which the instrumentation amplifier is implemented in BCDMOS circuitry.

21. A ground compatible voltage-mode instrumentation amplifier substantially as described herein with reference to and as illustrated in the accompanying drawings .

22. An electronic circuit comprising the ground compatible voltage-mode instrumentation amplifier as claimed in any preceding claim.

23. An electronic circuit as claimed in Claim 22 in which the electronic circuit is implemented in solid state circuitry. 8*0758

24. An electronic circuit as claimed in Claim 23 in which the instrumentation amplifier is implemented in MOS circuitry.

25. An electronic circuit as claimed in Claim 23 in 5 which the instrumentation amplifier is implemented in CMOS circuitry.

26. An electronic circuit as claimed in Claim 23 in which the instrumentation amplifier is implemented in BiCMOS circuitry. 10

27. An electronic circuit as claimed in Claim 23 in which the instrumentation amplifier is implemented in BCDMOS circuitry.

28. An electronic circuit as claimed in any of Claims 22 to 27 in which the circuit is suitable for signal 15 processing.

29. An electronic circuit as claimed in any of Claims 22 to 28 in which the electronic circuit is a controlled-gain digital to analog converter circuit for header position control of a disc drive. 20

30. A disc head position controller for disc drives incorporating the electronic circuit of any of Claims 94 0758 ' 22 to 29.

31. A disc head position controller as claimed in Claim 30 in which the head position controller is for high-end disc drives.