Residual offset correction method and circuit for chopper stabilized amplifiers

#The present invention relates to operational amplifiers. More particularly, the present invention relates to a technique and circuit for providing residual offset correction in a chopper stabilized amplifier circuit.

#The demand for improved operational amplifiers, and in particular instrumentation amplifier circuits for high-precision data acquisition and instrumentation applications, such as multi-channel data acquisition systems, current shunt monitors, and industrial or physiological sensors, continues to increase. Instrumentation amplifier circuits are generally designed to amplify the difference between two voltage inputs with a defined gain, wherein a single-ended output is provided which is referenced to a known reference point, for example, ground.

#Such operational amplifier circuits generally include an input stage circuit and an output stage circuit comprised of various amplifier devices and other current sources. To facilitate the reduction of error caused by mismatch of transistor devices within the various amplifier and/or current sources, operational amplifier circuits are being configured with chopper stabilization. In this technique, a pair of commutating switch blocks can be configured at the input and output of an input stage circuit in a manner to zero the voltage offset of the input stage. During operation, the switching block is configured to reverse the polarity of an input offset voltage during alternating phases, which operates to stabilize the voltage offset of the input stage. In addition, chopper stabilization can also reduce input referred low frequency noise.

#A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and:

#FIG. 1 illustrates a schematic diagram of a prior art chopper stabilized operational amplifier circuit;

#FIG. 2 illustrates a schematic diagram of an exemplary switching circuit in accordance with an exemplary embodiment of the present invention;

#The present invention may be described herein in terms of various functional components and various processing steps. It should be appreciated that such functional components may be realized by any number of hardware or structural components configured to perform the specified functions. For example, the present invention may employ various integrated components, such as buffers, current mirrors, and logic devices comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes and the like, whose values may be suitably configured for various intended purposes. In addition, the present invention may be practiced in any integrated circuit application. Such general applications that may be appreciated by those skilled in the art in light of the present disclosure are not described in detail herein. However for purposes of illustration only, exemplary embodiments of the present invention will be described herein in connection with chopper stabilized circuits for use with an operational amplifier. Further, it should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection through other components and devices located thereinbetween.

#As discussed above, currently available chopper stabilization configurations for use in operational amplifiers include a residual DC offset term that is not corrected by such current chopper stabilization techniques when operating at higher switching frequencies. This residual offset term results from a change in voltage at the output of the input stage that occurs due to offset voltage of the following stage, e.g., an output stage, reacting with parasitic capacitances at the output of the input stage. However, in accordance with various aspects of the present invention, a circuit and technique are provided that facilitate residual offset correction during chopper stabilization of an operational amplifier circuit.

#In accordance with an exemplary embodiment, a chopper stabilized operational amplifier having an input stage and a following stage, such as an output stage, can be configured with an additional gain stage configured between the input and following stages. For chopper stabilization, a first switching block is configured at the input of the input stage. However, instead of a second switching block being configured at the output of the input stage, the second switching block is coupled to the output of the additional gain stage. As a result, the offset voltage of the additional gain stage appears across the output of the input stage in the same polarity during chopper stabilization, regardless of the chopping phase of the second switching block, and thus remains constant at the end of each of the chopping phases. Accordingly, any residual offset voltage, for example that due to changes in voltage across parasitic capacitances on the output of the input stage, can be eliminated.