Auto-calibration circuit to minimize input offset voltage in an integrated circuit analog input device

The invention relates to integrated circuit analog input devices and, more particularly, to an auto-calibration circuit for minimizing input offset voltage in the integrated circuit analog input device.

Integrated circuits are becoming far more sophisticated while continuing to drop in price. Combinations of both analog and digital functions fabricated on an integrated circuit die, or packaged in a multi-chip package (MCP), are becoming more prevalent and are further increasing the usefulness and reducing the cost of consumer and industrial products. The combination of a microcontroller, and analog and digital circuit functions on an integrated circuit die or in an MCP has also expanded the useful range of applications. Consumer and commercial products, such as, for example, but not limited to, appliances, telecommunications devices, automobiles, security systems, full-house instant hot water heaters, thermostats and the like, are being controlled by integrated circuit microcontrollers. Analog inputs for receiving sensor information and analog outputs for controlling functions are necessary for the application of these microcontrollers. Heretofore separate and discrete analog-to-digital and digital-to-analog interfaces were used to connect the digital microcontroller to the outside analog world.

Analog input devices such as an analog-to-digital converter (ADC) in conjunction with a separate operational amplifier (op-amp) were used to convert a time-varying analog signal into digital representations thereof for application to digital inputs and use thereof by the microcontroller. Voltage and current levels were also detected by discrete integrated circuit voltage comparators that changed a digital output state when a certain analog value was present on the input of the comparator.

The op-amp (and comparator) is generally a differential input (inverting and noninverting inputs) analog device, and the circuit of the op-amp has inherent direct current (DC) input offset voltage that causes the output of the op-amp to be nonzero with a zero input voltage between the differential inputs. Many applications require an op-amp with a very small input offset voltage. To achieve small input offset voltage, normally a calibration step in the production of the op-amp is required. This calibration step takes time during manufacturing/testing and is therefore generally expensive to perform. The calibration is typically performed at one operating point (e.g., temperature, common mode voltage, etc.) such that changes in operating environment are not compensated for. Technology has now advance to the point where the analog input and output devices can be fabricated on the same integrated circuit die on which the digital microcontroller and its support logic and memories are also fabricated. This creates an additional problem in that the equipment used to test the digital microcontroller functions are not capable of performing on-line calibration of analog functions efficiently.

What is needed is an integrated circuit having both analog and digital functions, where the analog functions may be easily calibrated during initial manufacturing/testing and, additionally, may be calibrated in a system application so as to remain in or meet the desired specifications and operating parameters over all operating temperature, voltage, current, speed, power, pressure, humidity, etc., that may be encountered during normal operation and can be mass-produced to reduce overall product costs.

The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing in a single integrated circuit package an auto-calibration circuit to minimize input offset voltage in an integrated circuit analog input device. The auto-calibration circuit may also be utilized to calibrate a plurality of analog input devices on an integrated circuit die or in an MCP. The auto-calibration circuit and analog input device(s) may be fabricated in combination with a microcontroller system on an integrated circuit die or in an MCP. The auto-calibration circuit of the present invention may quickly and easily compensate for input offset voltage of the analog input device. This compensation may be performed during initial manufacturing/testing and, in addition, may be performed at various times during the operation of the analog input device so as to more effectively maintain desired parameters and specifications over all of the operating conditions of temperature, voltage, current, speed, power, pressure, humidity, etc.

In an embodiment of the auto-calibration circuit of the present invention, the analog input device has input offset voltage compensation or a trimming circuit that counteracts or compensates for the device input offset voltage by applying, for example, but not limited to, an opposite polarity voltage between the differential inputs of the analog input device so as to minimize the resulting voltage error at the output of the analog device. Switching of resistances, current sources and/or voltage sources in the differential input circuit of the analog input device may be used to compensate for the input offset voltage error. In addition, a digital-to-analog converter (DAC) may be used to generate a compensation voltage to overcome the input offset voltage error. Other ways of compensating for the input offset voltage error now known or used in the future may be utilized in the present invention so long as they can be controlled by applying a digital value thereto.

A digital control circuit is used to apply a digital word to the input offset voltage compensation circuit for determining a digital value representative of the required input offset voltage compensation. A linear search or binary search of various digital values of the digital word may be used by the digital control circuit. In addition, the digital control circuit may also control switching the inverting and non-inverting inputs, the output, and the feedback gain determining resistors of the analog input device during a “calibration mode” thereof.

A voltage comparator compares the output of the analog input device and a voltage reference. When the output of the analog input device is equal to or greater than the voltage reference, the comparator output switches from a first logic level to a second logic level. The output of the comparator is connected to the digital control circuit and signals the digital control circuit by changing its output logic level.

The voltage reference may be programmably adjustable for selecting a desired reference voltage value to be applied to the analog input device and comparator inputs during the calibration cycle. This feature of the invention allows the ability to vary the voltage reference so as to facilitate calibration at the common mode voltage which is very close to that of the application of use. The appropriate voltage reference value may be written to a control register associated with the voltage reference circuit before initiating the auto calibration of the analog input device. Different voltage reference values may be used for different analog input devices during input offset voltage compensation calibration thereof.

The input offset voltage compensation circuit of the analog input device may have a storage register or memory that retains the digital value required for compensating the input offset voltage. This storage register may be volatile or nonvolatile depending upon the application desired.

In another embodiment of the invention, a plurality of analog input devices may have their input offset voltages compensated for by multiplexing the digital control circuit and comparator between each of the plurality of analog input devices. Thus circuits and die area are reduced, saving costs and improving reliability of the embodiment of the present invention.

A feature of the invention is auto-calibration of dc input offset voltage for an analog input device at any time during operation of the device.

Another feature is auto-calibration of dc input offset voltages for a plurality of analog input devices.

Another feature is auto-calibration of dc input offset voltage of an analog input device in combination with a microcontroller on an integrated circuit die.

Still another feature is a microcontroller-controlled auto-calibration of dc input offset voltage of an analog input device.

Yet another feature is a programmably adjustable voltage reference for selecting a desired reference voltage value to be applied to the analog input device and comparator inputs during the calibration cycle.

An advantage of the present invention is not having to calibrate an analog input device during manufacturing/testing of the device.

Another advantage is being able to dynamically calibrate the analog input device on demand and during operation thereof over all operating conditions.

Still another advantage is using only one auto-calibration circuit to calibrate a plurality of analog input devices on an integrated circuit die or in an MCP.

Another advantage is using the same auto-calibration circuit for calibrating both op-amp and comparator analog input devices.

Other and further features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram of a microcontroller system having integral analog and digital peripherals and input-output functions in a single integrated circuit;

FIG. 2 is a schematic block diagram of an analog input device having an auto-calibration circuit to minimize input offset voltage of the device, according to the present invention;

FIG. 3 is a schematic circuit diagram of another embodiment of the invention of FIG. 2;

FIG. 4 is a schematic circuit diagram of another embodiment of the invention having a plurality of analog input devices and one auto-calibration circuit.

The invention is an auto-calibration circuit used in combination with an analog input device to minimize input offset voltage of the analog input device. The invention may be used to maintain close tolerance specifications of the analog input device over a wide range of operating conditions. The auto-calibration circuit may also be used in combination with a microcontroller system fabricated on an integrated circuit die or in an MCP. The auto-calibration circuit of the present invention may also be used with a plurality of analog input devices fabricated on an integrated circuit die or in an MCP.

The auto-calibration circuit and analog input device(s) are preferably fabricated on the same integrated circuit die or in the MCP as a microcontroller system and therefore becomes a very low cost analog peripheral of the microcontroller. Furthermore, the embodiments of the invention will minimize the input offset voltage of the analog device(s) over conditions such as, for example, but not limited to, temperature, voltage, current, power, input common mode, packaging and variations of circuit component values over time, during operation of the analog input device. The analog input device may be, for example, but not limited to, an operational amplifier (op-amp) and/or a voltage comparator.

Referring now to the drawings, the details of preferred embodiments of the invention are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix.

Referring to FIG. 1, a schematic block diagram of a microcontroller system having integral analog and digital peripherals and input-output functions in a single integrated circuit is illustrated. The integrated circuit microcontroller system having integral analog and digital peripherals is generally represented by the numeral 100 and comprises a digital-to-analog converter (DAC) 102 , an analog-to-digital converter (ADC) 104 , digital input-output (I/O) 106 , a programmable comparator(s) 108 , a microcontroller 110 , random access memory (RAM) 112 , electrically programmable read-only memory (EPROM) 114 , a timer 118 , a serial interface 120 , a programmable switching mode controller (PSMC) 122 and a programmable operational amplifier(s) (op-amp) 124 . FIG. 1 is a schematic block diagram that is illustrative of a typical microcontroller system on an integrated circuit die or in an integrated circuit package having at least one integrated circuit die (MCP). It is contemplated and within the scope of the present invention that the microcontroller system 100 may have more or fewer functions than what is illustrated in FIG. 1 .

The microcontroller system 100 may be fabricated on one or more integrated circuit dice and enclosed in an integrated circuit package. The integrated circuit package may be, for example, but not limited to, plastic dual in-line package (PDIP), small outline (SO), shrink small outline package (SSOP), thin shrink small outline package (TSSOP), windowed ceramic dual in-line package (CERDIP), leadless chip carrier (LCC), plastic leaded chip carrier (PLCC), plastic quad flatpack package (PQFP), thin quad flatpack package (TQFP), pin grid array (PGA), ball grid array (BGA), T 0 - 220 , T 0 - 247 and T 0 - 263 .

Referring now to FIG. 2, a schematic block diagram of an analog input device having an auto-calibration circuit to minimize input offset voltage of the device, according to an embodiment of the invention, is illustrated. The analog input device and auto-calibration circuit are generally indicated by the numeral 200 and generally comprise, for example, but not limited to, an op-amp 202 having an offset trim circuit, a comparator 204 , digital control 206 , switches 230 , 232 and 234 , and gain-control feedback resistors 214 and 216 . The op-amp 202 has a noninverting input (+) and an inverting input (−), which are configured as differential inputs, and an output connected to an inverting input of the comparator 204 . The offset trim circuit of the op-amp 202 is generally comprised of a digital-to-analog converter (DAC) has a digital input adapted for receiving a digital value representative of the offset trim value required. The output of the offset trim circuit is an analog value (voltage and/or current) representative of the digital value. The DAC offset trim circuit adjusts a current and/or voltage through either the inverting or non-inverting signal input of the op-amp 202 to compensate for the offset voltage caused by device process mismatch, finite open loop gain, etc. The polarity of the offset determines which signal input is adjusted.

The digital control 206 has an output bus 208 , which controls the offset trim circuit and the switches 230 , 232 and 234 . The offset trim circuit of the op-amp 202 compensates for input offset voltage that would drive the output of the op-amp 202 to an undesired value when the inputs of the op-amp 202 are at substantially the same voltage potential. The digital control 206 is activated by a calibrate command on node 224 and is terminated when the digital control 206 has completed a search to determine a calibration digital word which best compensates for the op-amp 202 offset voltage.

At the start of calibration, analog switches 230 , 232 and 234 switch to position “a” from a normal operating position “b.” The inputs of the op-amp 202 are thereby connected to a reference voltage 222 , and a gain-control resistor feedback network, comprising resistors 214 and 216 , are connected to the op-amp 202 so as to establish a high-gain circuit for determining subsequent calibration input voltage offset compensation. When the switches 230 , 232 and 234 are in position “a” (calibrate), they effectively establish a high gain differential input amplifier circuit (op-amp 202 ) having both inputs connected to a voltage reference, Vref. Vref defines the input common mode voltage that the op-amp 202 will be calibrated at. The voltage reference, Vref, is also connected to an input of the comparator 204 so as to maintain a common reference. The output of the op-amp 202 is connected to the other input of the comparator 204 .

The output voltage (as referenced to ground) of the op-amp 202 is given by formula (1) below:

V out≈ V ref± V os(1 +R 2 / R 1 )  (1)

Where:

Vout=op-amp (amplifier) output voltage

Vref=reference input voltage

Vos=op-amp (amplifier) offset

R 1 , R 2 =resistors used to set feedback gain of op-amp when compensating for offset as shown by formula (1) above.

The output voltage of op-amp 202 is mainly determined by the ratio of R 2 and R 1 and the magnitude of the offset voltage of op-amp 202 . The output of op-amp 202 drives the inverting input, and Vref drives the noninverting input of comparator 204 . At the start of calibration, the output of op-amp 202 will cause the output of comparator 204 to a logic level state depending upon the magnitude and polarity of the op-amp 202 input offset voltage. The output voltage (as referenced to ground) of comparator 204 is given by formula (2) below:

V ocmp≈( V out− V ref± V oscmp)* A olcmp+ V cmo  (2)

Where:

Vocmp=comparator output voltage

Vout=op-amp output voltage

Vref=reference input voltage

Voscmp=comparator offset

Aolcmp=comparator open loop gain

Vcmo=comparator common mode output voltage

The op-amp 202 offset voltage is amplified using the resistor network R 1 , R 2 around the op-amp 202 so as to reduce errors that may be introduced by the comparator 204 offset and its finite open-loop gain. For example, given the following parameters:

Vocmp=3.5 volts (output voltage to drive digital cells to a correct state)

Vref=2.5 volts

Voscmp=1 mv

Aolcmp=80 dB

Vcmo=2.5 volts Using formula (1), Vout is determined to be 1.1 mv. This is the minimum output voltage required to drive the comparator 204 output to a magnitude sufficient to drive digital cells to the correct state. For a resistive gain of (1+R 2 /R 1 )=50, the minimum residual op-amp 202 offset error due to the comparator errors is determined to be 22 microvolts. These comparator errors define the minimum offset voltage to which the op-amp 202 may be calibrated.

The digital control 206 senses the output logic level from the comparator 204 and, based on the logic level state, sets the polarity of the op-amp 202 calibration network. The digital control 206 performs a search by adjusting the calibration word on the output bus 208 while sensing the logic level change at the output of the comparator 204 until the optimal calibration word for minimum offset voltage is found, within the quantization error of the offset trimming circuit of the op-amp 202 . The calibration trim step size is preferably larger than the error calculated from formula (2), due to the comparator 204 errors. Once calibration is complete, the digital control 206 controls the switches 230 , 232 and 234 so that they are back to position “b” for normal operation of the op-amp 202 . The aforementioned calibration process may be periodically repeated when an operating parameter(s) may have changed. The input offset error is thereby substantially compensated for under all operating conditions. This also allows the analog input device to maintain close tolerance to desired specifications for more critical or demanding applications.

Referring to FIG. 3, a schematic circuit diagram of another embodiment of the invention of FIG. 2 is illustrated. The op-amp 202 a, an offset compensation latch register 302 a, analog switches 230 a, 232 a and 234 a and gain-feedback resistors (not illustrated) comprise an analog input device adapted for input voltage offset compensation, generally represented by the numeral 310 . The digital control 206 and comparator 204 comprise an auto-calibration circuit, generally represented by the numeral 312 , for adjusting the offset trim of the op-amp 202 a for optimal performance as disclosed hereinabove.

The digital control 206 has a control output 324 that indicates and starts the calibration process by causing switches 230 a - 234 a to switch to position “a” (calibration mode) and enables the offset compensation latch register 302 a to begin accepting data at bus input 328 a. The digital control 206 sends digital values over bus 328 so as to minimize the input voltage offset of the op-amp 202 a. Linear search, binary search and the like may be used to generate the digital values from the digital control 206 . When the digital control 206 detects the comparator 204 output logic level change indicating that the output signal from the op-amp 202 a is at a minimum, the offset trim compensation circuit has the desired digital value from the bus 328 , and the offset compensation latch register 302 a stores this digital value. Once calibrated, the digital control 206 returns the analog input device 310 back to normal operation (switches 230 a - 234 a are returned to position “b”).

Referring to FIG. 4, a schematic circuit diagram of another embodiment of the invention having a plurality of analog input devices and one auto-calibration circuit is illustrated. A plurality of analog input devices 310 may be calibrated for minimum input offset with a single auto-calibration circuit 312 . One of the plurality of analog input devices 310 is selected by the auto-calibration circuit 312 for input offset calibration thereof. The remaining plurality of analog input devices 310 remain in their normal operating modes. In this way, each of the plurality of analog input devices 310 may be calibrated for minimum input offset, as described hereinabove.

It is contemplated and within the scope of the invention that the auto-calibration circuit 312 and the plurality of analog input devices 310 may be fabricated on an integrated circuit die (not illustrated) and, further, may be utilized with a microcontroller system (FIG. 1) fabricated on an integrated circuit die or in an MCP.

The invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While the invention has been depicted and described and is defined by reference to particular preferred embodiments of the invention, such references do not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts. The depicted and described preferred embodiments of the invention are exemplary only and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.