Offset cancel circuit and offset cancel method using the same

Abstract: An offset cancel circuit according to the present invention includes two terminals, an output terminal and two intermediate nodes. When the offset cancel circuit is in a first state, the offset cancel circuit supplies a potential difference responsive to a difference in potential between the two terminals between the two intermediate nodes and supplies a potential responsive to both the potential difference supplied between the two intermediate nodes and a first potential to the output terminal. On the other hand, when the offset cancel circuit is in a second state, the offset cancel circuit supplies a potential difference responsive to the difference in potential between the two terminals between the two intermediate nodes and supplies a potential responsive to both the potential difference supplied between the two intermediate nodes and a second potential to the output terminal.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to an offset cancel circuit for canceling an offset voltage of a pressure sensor or the like and to an offset cancel system using it. Description of the Related Art: An offset cancel circuit comprises a preamplifier unit and a level shifting unit. The preamplifier unit is electrically connected to a device for outputting a small potential difference, such as a pressure sensor or the like. The preamplifier unit serves as a circuit for receiving such a small potential difference therein, amplifying it and outputting the amplified potential difference therefrom. The level shifting unit serves as a circuit for outputting a potential responsive to a reference potential settable to a desired level and the amplified potential difference to an AD converter. Thus, the level shifting unit sets the potential outputted to the AD converter so as to fall within the range of input standards of the AD converter. Operations of the offset cancel circuit and a system using the offset cancel circuit will now be described. A potential difference outputted from the pressure sensor is first regarded as X1. Next, the potential difference X1 is supplied to an input terminal of the preamplifier unit. At this time, an output potential Y1 supplied to an output terminal of the level shifting unit satisfies the relations in Y1 = a X 1 + bA + c (where a and b: coefficients and A: reference potential). The preamplifier unit and the level shifting unit are respectively composed of a plurality of operational amplifiers. Further, each of the operational amplifiers has an offset voltage. Symbol c indicates a term that will result from the offset voltage. The AD converter receives the Y1 therein and converts the analog value Y1 into a digital value Y3, which is in turn outputted to an arithmetic device. Next, a potential difference "0" is supplied to the input terminal of the preamplifier unit. At this time, an output potential Y2 supplied to the output terminal of the level shifting unit meets the relations in Y2 = a·0 + bA + c = bA + c . The AD converter receives the Y2 therein and converts the analog value Y2 into a digital value Y4, followed by output to the arithmetic device. The arithmetic device actually performs an arithmetic computation on Y3 - Y4 based on the Y3 and Y4 obtained previously. For simplicity of illustration, however, a description will be made below on the assumption that the arithmetic device performs an arithmetic computation of Y3 - Y4 = Y1 - Y2 (because Y3 - Y4 is associated with Y1 - Y2). The arithmetic device calculates Y1 - Y2 so as to obtain a potential difference Y1 - Y2 = aX1 . As a result, the potential difference irrelevant to the offset voltage of each operational amplifier can be obtained. However, the pressure sensor or the like for supplying a small potential difference to the preamplifier unit also includes an offset voltage. Thus, since an output error "e" called offset output is produced, an output potential difference X1 of the pressure sensor becomes d + e instead of being to ideally become "d" when the pressure sensor is under a given pressure level. As a result, Y1 = a·(d + e) + bA + c and Y2 = a·0 + bA + c. From both equations, Y1 - Y2 becomes a(d + e) (= a·d + a·e). Accordingly, the term a·e based on the offset output of the pressure sensor appears at an output node of the level shifting unit. The offset voltage varies according to products. Correspondingly, the output error e also varies according to the products. It was therefore necessary to adjust the output error e. JP-A-1,129,832 discloses a circuit to eliminate an offset by employing a switch through which the input voltage of an operational amplifier can be set to zero. By measuring the offset voltage a correction can be made by subtracting the offset voltage from the actual output voltage. EP-A-0,367,927 discloses a sensor arrangement particularly suitable for pressure sensors. Differential output signals from a pressure sensor are switched between two amplifiers and fed to a microcomputer where they are stored as two pairs of signals in digital form. The two pairs of signals are combined to cancel amplifier offset voltages. SUMMARY OF THE INVENTION With the foregoing in view, it is therefore an object of the present invention to adjust an offset output derived from an offset voltage varied according to products such as a pressure sensor, etc. connected to a preamplifier unit. An offset cancel circuit and an offset cancel method according to the present invention are defined in claims 1 and 2, respectively. BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: Fig. 1 is a view showing a circuit configuration of an offset cancel system including an offset cancel circuit according to a first embodiment of the present invention; and Fig. 2 is a view illustrating a circuit configuration of an offset cancel system including an offset cancel circuit according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. A first embodiment will be described below with reference to Fig. 1. Symbols N1 through N16 indicate nodes respectively. Symbols A1 through A4 indicate Operational Amplifiers (hereinafter called "Op amps") respectively. Symbols S1 through S4 designate analog switches respectively. Symbols R1 through R8 indicate resistors respectively. Symbol R9 designate a variable resistor. One of output terminals of a pressure sensor is electrically connected to an input terminal IN1, whereas the other thereof is electrically connected to an input terminal IN2. The node N1 is electrically connected to the input terminal IN1, a positive input (hereinafter called "non-inverting input") of the Op amp A1 and one analog input/output of the analog switch S1. The node N2 is electrically connected to control inputs of the analog switches S1, S2, S3 and S4. The node N3 is electrically connected to one analog input/output of the analog switch S2 and a non-inverting input of the Op amp A2. The node N4 is electrically connected to a negative input (hereinafter called "inverting input") of the Op amp A1, one end of the resistor R1 and one end of the resistor R2. The node N5 is electrically connected to an inverting input of the Op amp A2, one end of the resistor R3 and the other end of the resistor R2. The node N6 is electrically connected to the input terminal IN2 and the other analog input/output of the analog switch S2. The node N7 is electrically connected to the output of the Op amp A1, the other end of the resistor R1 and one end of the resistor R4. The node N8 is electrically connected to the output of the Op amp A2, the other end of the resistor R3 and one end of the resistor R6. The node N9 is electrically connected to the other end of the resistor R4, one end of the resistor R5 and an inverting input of the Op amp A4. The node N10 is electrically connected to the other end of the resistor R6, one end of the resistor R7 and a non-inverting input of the Op amp A4. The node N11 is electrically connected to the output of the Op amp A4, the other end of the resistor R5 and the input of an Analog Digital converter (hereinafter called "AD converter"). The node N12 is electrically connected to the other end of the resistor R7, the output of the Op amp A3 and an inverting input of the Op amp A3. The node N13 is electrically connected to a non-inverting input of the Op amp A3, one analog input/output of the analog switch S3 and one analog input/output of the analog switch S4. The node N16 is electrically connected to the other analog input/output of the analog switch S3, one end of the resistor R8 and one end of the resistor R9. The node N14 is electrically connected to the other end of the resistor R9. The node N14 is supplied with a source potential VDD. The node N15 is electrically connected to the other end of the resistor R8 and the other analog input/output of the analog switch S4. The node N15 is supplied with a ground potential GND. A preamplifier unit is composed of the analog switches S1 and S2, the Op amps A1 and A2 and the resistors R1 through R3. A level shifting unit is composed of the analog switches S3 and S4, the Op amps A3 and A4, the resistors R4 through R8 and the variable resistor R9. The ratio among resistance values of the resistors R1, R2, R3 and R4 will be defined as 1 : 1 : 1 : 1. The variable resistor R9 serves so as to adjust a potential level at the node N16 and adjust a potential level at the node N13 when the analog switch S3 is turned on. Operations of the offset cancel circuit according to the first embodiment and the offset cancel system using it will now be described. The offset cancel circuit according to the first embodiment has two states of an input potential difference measurement mode and an offset measurement mode. The operation of the offset cancel circuit in the input potential difference measurement mode will first be described. <Input potential difference measurement mode> When the offset cancel circuit is placed in the input potential difference measurement mode, the node N2 is set to an L level. Thus, the switches S1 and S4 are turned OFF and the switches S2 and S3 are turned ON. At this time, a predetermined pressure is applied to the pressure sensor so that a potential difference ΔV is outputted. The small potential difference ΔV outputted from the pressure sensor is inputted between the nodes N1 and N3. If potentials at the nodes N1 and N3 are defined as VN1 and VN3 respectively, then the difference in potential therebetween ΔV is given by the following expression: ΔV = VN3 - VN1 The preamplifier unit amplifies the potential difference ΔV by (R1 + R2 + R3)/R2 times. The level shifting unit supplies a potential VN11 that meets the following relation, to the node N11 based on the amplified potential difference (R1 + R2 + R3)/R2·ΔV and a potential VN13 at the node N13. VN11 = (R1 + R2 + R3)/R2·ΔV + VN13 Here, the expression 2 is of a theoretical expression at the time that it is assumed that no offset voltage appears between the non-inverting and inverting input terminals of each of the individual Op amps A1, A2, A3 and A4. In practice, however, offset voltages VOFF1, VOFF2, VOFF3 and VOFF4 respectively exist between the non-inverting and inverting input terminals of the Op amps A1, A2, A3 and A4. If the offset voltages are taken into account, then the expression (2) is rewritten as follows: VN11 = (R1 + R2 + R3)/R2·(ΔV + VOFF1 - VOFF2) + VN13 + VOFF3 + 2·VOFF4 An error Vop (corresponding to a shift or displacement of the expression 3 from the expression 2) developed at the node N11 by the offset voltages of the individual Op amps is given by the following expression: Vop = (R1 + R2 + R3)/R2·(VOFF1 - VOFF2) + VOFF3 + 2·VOFF4 The error Vop differs according to each product. Now, the VN13 can be set to a desired value. Thus, when the input potential difference inputted to each products is kept identical, the VN11 can be set to the same value between products by adjusting the VN13 in advance at each product. However, the offset voltages vary according to temperatures, the source potential, etc. Thus, even when the same pressure is applied to the pressure sensor, the output potential VN11 varies according to the state of usage of a single product even in the case of the single product. Further, the pressure sensor also has an offset voltage. Thus, an error occurs even in the potential difference ΔV produced from the pressure sensor and the potential difference ΔV is shifted from an ideal value. Namely, assuming that when the pressure applied to the pressure sensor is of a given value, an ideal potential difference supplied between the input terminals IN1 and IN2 is defined as d and an output potential difference error caused by the offset voltage of the pressure sensor is defined as e, the following expression is given: ΔV = d + e An output potential error Vp produced at the node N11 by the offset voltage of the pressure sensor is given by the following expression: Vp = (R1 + R2 + R3)/R2·e In order to cancel product-to-product shifts or displacements of the output potential errors (corresponding to the shift from the ideal value) produced at the node N11 by the offset voltage of each Op amp and the offset voltage of the pressure sensor, the offset measurement mode is set to the offset cancel circuit. The operations of the offset cancel circuit and the offset cancel system at the time of its offset measurement mode will be described below. <Offset measurement mode> When the offset cancel circuit is placed in the offset measurement mode, the node N2 is brought to an H level. As a result, the analog switch S1 is turned ON, the analog switch S2 is turned OFF, the analog switch S3 is turned OFF and the analog switch S4 is turned ON. Thus, potentials equal to each other are inputted to the nodes N1 and N3 respectively. Namely, ΔV = 0 in the expression 1. The ground potential GND is supplied to the node N13. At this time, an output potential VN11' supplied to the node N11 is given by the following expression: VN11' = (R1 + R2 + R3)/R2·(VOFF1 - VOFF2) + VOFF3 + 2·VOFF4 The AD converter is supplied with the VN11 and performs AD conversion on the VN11. The AD converter outputs a value based on the VN11 to an arithmetic device. Further, the AD converter is inputted with the VN11' and performs AD conversion on the VN11'. The AD converter outputs a value based on the VN11' to the arithmetic device. The arithmetic device computes the difference between the two values outputted from the AD converter. For simplicity of illustration, the following description will be made on the assumption that an input ahead of its AD conversion and an output obtained after its AD conversion are equal to each other. The difference between the potential VN11 (expression 3) and the potential VN11' (expression 4) both supplied to the node N11 is given by the following expression: VN11 - VN11' = (R1 + R2 + R3)/R2·ΔV + VN13 Thus, the error developed due to the offset voltage of each Op amp is canceled. When ΔV = d + e is now substituted into the expression 5, the above difference is rewritten as follows: VN11 - VN11' = (R1 + R2 R3)/R2·(d + e) + VN13 An output potential error Vp produced at the node N11 by the offset voltage of the pressure sensor is given by the following expression: Vp = (R1 + R2 + R3)/R2·e The offset voltage of the pressure sensor varies under the influence of temperatures and a source potential. However, the influence of the output potential difference error e developed due to the offset voltage of the pressure sensor on the temperatures and the source potential is known in advance. Thus, even when conditions given to the temperature and the source potential vary, a change in output potential VN11 due to their variations can be calculated by the arithmetic device. Therefore, such a change can be canceled. Thus, when the same pressure is applied to the pressure sensor, the difference VN11 - VN11' can be equally set between products by setting the VN13 to a desired value at each product. Namely, a shift in the error e at each product produced due to the offset voltage of the pressure sensor can be canceled. In the first embodiment as described above, the potential at the node N13 is switched between the input potential difference measurement mode and the offset voltage measurement mode. Thus, the term of VN13 appears in the expression 5 indicative of the difference between the value (expression 3) measured under the input potential difference measurement mode and the value (expression 4) measured under the offset voltage measurement mode. Therefore, the difference VN11 - VN11' can be set to a given value by adjusting and shifting the VN13 to the desired value at each product. As a result, the influence of the offset voltages of the Op amps and the shift developed at each product due to the offset voltage of the pressure sensor can be eliminated. Fig. 2 is a circuit diagram showing an offset cancel system according to a second embodiment of the present invention. The same elements of structure as those shown in Fig. 1 or the elements of structures similar to those shown in Fig. 1 are identified by like reference numerals and their description will therefore be omitted. The analog switch S3 shown in Fig. 1 is omitted from the offset cancel system shown in Fig. 2. A node N13 is electrically connected to a non-inverting input of an Op amp A3 and one ends of resistors R8 and R9. A node N14 is electrically connected to the other end of the resistor R9 and a source potential VDD. A node N15 is electrically connected to the other end of the resistor R8, one analog input/output of an analog switch S4 and a ground potential GND. A node N12 is electrically connected to the other analog input/output of the analog switch S4, the output of the Op amp A3, an inverting input of the Op amp A3 and one terminal of a resistor R7. A control input of the analog switch S4 is electrically connected to a node N2. An output enable input OE of the Op amp A3 is electrically connected to the node N2. The operation of the second embodiment will now be described. <Input potential difference measurement mode> The operation of the second embodiment placed in the input potential difference measurement mode is identical to that of the first embodiment. Thus, an output potential difference supplied to the node N11 is given by the expression 3. <Offset measurement mode> When the second embodiment is placed in the offset measurement mode, the node N2 is brought to an H level. Thus, the analog switch S4 is turned ON and the output of the Op amp A3 is disabled. The node N12 is supplied with a potential VN15 applied to the node N15. Accordingly, the expression 4 is rewritten as follows: VN11' = (R1 + R2 + R3)/R2·(VOFF1 - VOFF2) + VN15 + 2·VOFF4 If the difference between the expression 3 and the expression 6 is taken, then the following expression is obtained: VN11 - VN11' = (R1 + R2 + R3)/R2·ΔV + VOFF3 + VN16 - VN15 Thus, the term of an error produced by an offset voltage of the Op amp A3 remains. According to the second embodiment as described above, the term of the error produced by the offset voltage of the Op amp A3 is left on the above expression. Since, however, the Op amp A3 is disabled when the second embodiment is placed in the offset measurement mode, an advantage can be brought about that the current to be consumed by the Op amp A3 can be set to zero. While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description, without departing from the wording of the claims.

Claims: An offset cancel circuit comprising: two input terminals (N1, N3) and an output terminal (N11); an amplifier circuit (S1, S2, A1, A2, R1, R2, R3) coupled to the two input terminals (N1, N3) and being adapted to apply at its output a first potential difference obtained by amplifying a second potential difference between the two input terminals when the offset cancel circuit is in a first state, and being adapted to apply at its output a third potential difference obtained by amplifying a fourth potential difference between the two input terminals when the offset cancel circuit is in a second state; said second potential difference being a non-zero value and said fourth potential difference being a zero-value; a level shifting circuit (S3, S4, A3, A4, R4, R5, R7, R8, R9) setting a potential applied to the output terminal so as to fall within a predetermined range, said level shifting circuit being coupled to the output of said amplifier circuit and said output terminal, said offset cancel circuit being further characterized by: said level shifting circuit being adapted to apply a first potential to a level shifting node in said first state and being adapted to apply a second potential to said level shifting node in said second state; said level shifting circuit being adapted to apply a potential responsive to both said first potential difference and said first potential to said output terminal when the offset cancel circuit is in said first state (VN11), and further being adapted to apply a potential responsive to both said third potential difference and said second potential to said output terminal when said offset cancel circuit is in said second state (VN11'); arithmetic means being adapted to determine the difference of the potentials at the output terminal in said first and said second state (VN11 -VN11'); and adjusting means (R9) for setting said difference of the potentials to the given value. An offset cancel method, said method comprising: applying a first potential difference at an output of an amplifier by amplifying a second potential difference between two input terminals of said amplifier during a first state; applying a third potential difference at the output of said amplifier by amplifying a fourth potential difference between said two input terminals of said amplifier during a second state; said second potential difference being a non-zero value and said fourth potential difference being a zero-value; performing a level shifting operation setting an output potential so as to fall within a predetermined range, said method being further characterized by said level shifting operation comprising applying a first potential to a level shifting node during said first state and applying a second potential to said level shifting node during said second state; said level shifting operation comprising applying a potential responsive to both said first potential difference and said first potential to an output terminal during said first state, and applying a potential responsive to both said third potential difference and said second potential to said output terminal during said second state; determining the difference of the potentials at said output terminal in said first and said second state; and setting the difference of said potentials to a given value by an adjusting means.

Inventors: TAKASHIMA SUMIHIRO, TAKASHIMA, SUMIHIRO, Takashima, Sumihiro, Oki Electric Ind. Co., Ltd
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1996
Main classification: G01D3
IPC codes: G01D3, G01L1, H03F3