CHOPPER STABLIZED AMPLIFIER WITH FILTERING
PRIORITY ENTITLEMENT
[001] This application is entitled to priority based on Provisional Patent Application Serial Number 61/262,484 filed on November 18, 2009. This application and the Provisional Patent Application have at least one common inventor.
TECHNICAL FIELD
[002] The invention relates to amplifier circuits and systems used in electronics and integrated circuitry. More particularly, the invention relates to chopper stabilization and noise reduction in operational amplifier circuits and associated systems.
BACKGROUND OF THE INVENTION
[003] In integrated circuit operational amplifiers it is desirable to have a low offset voltage, low offset drift, low noise, and a stable output signal. To some extent, these sought-after characteristics can be contradictory. An auto-zeroing approach is sometimes used, providing low ripple noise at the expense of higher in-band noise. Auto-zeroing techniques also tend to have substantially higher quiescent current requirements relative to chopper stabilized circuits. A chopper stabilization approach can generally provide lower in-band noise and lower offset drift, but with a relatively higher ripple noise level.
[004] Due to the foregoing and possibly additional problems, improved apparatus and systems and related methods for chopper stabilized amplifiers having low offset and low noise, particularly low ripple noise, would be a useful contribution to the arts.
SUMMARY OF THE INVENTION
[005] In carrying out the principles of the present invention, in accordance with preferred embodiments, the invention provides chopper stabilized amplifier circuitry and techniques having integrated filters for noise reduction. The embodiments described herein are intended to be exemplary and not exclusive. Variations in the practice of the
invention are possible and preferred embodiments are illustrated and described for the purposes of clarifying the invention. All possible variations within the scope of the invention cannot, and need not, be shown.
[006] According to one aspect of the invention, in a preferred embodiment, a chopper stabilized amplifier circuit includes a gain stage having both an input and an output equipped with chopping circuitry such that an input signal may be chopped prior to reaching the gain stage and an intermediate signal output by the gain stage may also be chopped prior to reaching the output of the chopper stabilized amplifier circuit. Integrator circuitry and a low pass filter are interposed between the gain stage output chopping circuitry and the chopper stabilized amplifier circuit output. This arrangement provides a chopper stabilized amplifier circuit adapted for producing an output signal having relatively minor ripple noise.
[007] According to another aspect of the invention, in a preferred embodiment thereof, a chopper stabilized amplifier circuit as described above includes a transconductance operational amplifier within the gain stage circuitry.
[008] According to another aspect of the invention, in a preferred embodiment thereof, a chopper stabilized amplifier circuit as described above includes low pass filter circuitry in a switched capacitor filter configuration.
[009] According to another aspect of the invention, in a preferred method thereof, a technique for amplifying a signal includes steps for receiving and chopping an input signal, and thereafter amplifying the chopped input signal to produce an intermediate signal. The intermediate signal is also chopped, integrated, and filtered to reduce ripple noise, resulting in an output signal.
[010] According to yet another aspect of the invention, in a preferred implementation of the embodiment described immediately above, a filtering step also includes a step of
sampling the intermediate signal synchronously with the chopping frequency.
[Oil] According to still another aspect of the invention, the above-described method includes a filtering step of sampling the intermediate signal at twice the chopping frequency.
[012] According to another aspect of the invention, the above-described method may include a variation in which the filtering step further includes sampling the intermediate signal at a frequency lower than the chopping frequency.
[013] According to another aspect of the invention, in preferred method embodiments, steps include receiving and chopping an input signal, and thereafter amplifying the chopped input signal to produce an intermediate signal. The intermediate signal is integrated and filtered to reduce ripple noise by operating a transconductance-capacitor low pass filter.
[014] The invention has advantages including but not limited to providing one or more of the following features, low noise, low offset voltage, and high efficiency in stable amplifier circuitry. These and other advantages, features, and benefits of the invention can be understood by one of ordinary skill in the arts upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[015] The present invention may be more clearly understood from consideration of the description and drawings in which:
[016] Figure 1 is a schematic diagram of a preferred embodiment of a chopper stabilized amplifier circuit with integrated filter;
[017] Figure 2 is a schematic diagram of an exemplary preferred embodiment of a filter component of the circuit introduced in Figure 1 ; and
[018] Figure 3 is a schematic diagram of an alternative preferred embodiment of a chopper stabilized amplifier circuit according to the invention.
[019] References in the detailed description correspond to like references in the various drawings unless otherwise noted. Descriptive and directional terms used in the written description such as front, back, top, bottom, upper, side, et cetera, refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating principles and features as well as advantages of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[020] The making and using of various exemplary embodiments of the invention are discussed herein, but it should also be appreciated that the apparatus and techniques for its use exemplify inventive concepts which can be embodied in a wide variety of specific contexts. It should be understood that the embodiments of the invention may be practiced in various applications and implementations without altering the principles of the invention. For purposes of clarity, detailed descriptions of functions, components, and systems familiar to those skilled in the applicable arts are not included. In general, the invention provides circuits and methods for implementation of chopper stabilized amplifiers with noise reduction filtering. The invention is described in the context of representative example embodiments. Although variations and alternatives for the details of the embodiments are possible, each has one or more advantages over the prior art.
[021] Referring primarily to Figure 1, in an example of a preferred embodiment, a chopper stabilized amplifier having filtering circuitry is shown 100. The circuitry 100
typically includes inputs 102 for receiving input signals from an associated system and an output 104 for outputting an amplified signal. Preferably, the input signals are within the range of about 0.01 Hz to about 10 Hz, such as those common in low-power sensor applications, although other frequency ranges may be suitable in other applications. The amplifier circuit 100 preferably has two internal signal paths. A first signal path 106 includes operational amplifiers, preferably transconductance operational amplifiers: gml; gm2; and gm3, as shown. A second signal path 108 preferably includes operational amplifier gm4, and also gm3. A gain stage 107 is included in the first signal path 106, here implemented with operational amplifier gml, the gain stage 107 has its inputs 110 and outputs 112 chopped. The effect of this chopper stabilized amplifier gml on the chopped input signal is to provide gain while minimizing noise and offset. However, it has been found that this method of cancelling offset creates ripple noise at the chopping frequency of the intermediate signal at the output 112 of the gain stage amplifier gml . As a solution to this problem, integrator circuitry 113, herein implemented with capacitive filter CI , is connected to the output 112 of the gain stage 107. The integrator circuitry 113 is connected to an active low pass filter 120 which integrates and filters the sampled error further reducing output ripple noise. The integrator circuitry 113 is preferably sampled at the chopping frequency, greatly reducing output ripple noise in the intermediate signal at the gain stage gml output 112. The low pass filter circuitry 120 in presently preferred embodiments is implemented in a switched capacitor filter configuration adapted for sampling in synchronization with the chopping frequency. The filtered, integrated, chopped, gain stage output signal is subsequently conducted to the output 104 of the chopper stabilized amplifier circuit 100.
[022] Now referring primarily to Figure 2, an example of a preferred embodiment of an active low pass filter 120 is shown in a simplified schematic diagram. The low pass filter 120 preferably samples the intermediate signal output of the gain stage 107 (Figure 1) at twice the chopping frequency, significantly reducing output ripple noise. Preferably, the low pass filter 120 may be adjusted for gain and cut-off frequency. The example shown and described herein may in some instances be implemented in combination with other
topologies, for example the topology shown in Figure 1, without departure from the principles of the invention. The low pass filter circuitry 120 may optionally be configured with a cutoff frequency lower than the chopping frequency, achieving increased noise reduction at the expense of tolerating more ripple.
[023] Another example of a preferred embodiment of a chopper stabilized amplifier circuit with integrated filtering is shown in the simplified schematic of Figure 3. The circuit 300 uses a low pass transconductance-capacitive (gm-C) filter 302 to reduce output ripple noise. The low pass gm-C filter 302 is created by coupling an operational amplifier gm6 and an integrating capacitive filter C2 as shown in the example, or equivalent circuits. Operational amplifier gm6 has its inputs and outputs chopped. The gm-C filter low pass cut-off frequency is selected to be much lower than the chopping frequency in order to greatly reduce output ripple noise. The capacitor C2 integrates the output of gm6 over time. Because the signal need not reach its final value during each chopping phase, C2 creates the further benefits of allowing for higher chopping frequencies, facilitating the use of ultra-low currents in the circuit 300. The use of ultra- low currents is advantageous in applications for which the level of energy consumption is a concern.
[024] Again referring primarily to Figure 1, the chopper stabilized amplifier circuit 100 may include one or more stabilizing capacitor C3 operably coupled with the low pass filter circuitry 120. Providing one or more stabilizing capacitors, e.g., C3, on the output side of the low pass filter 120, in which the filter output capacitance is significantly lower than the filter input capacitance, facilitates fast signal settling and stability in the operation of the gain stage 107.
[025] The circuits and methods of the invention provide one or more advantages including but not limited to one or more of; greatly reducing output ripple noise otherwise found in chopper stabilized amplifiers, and reduced costs. While the invention has been described with reference to certain illustrative embodiments, those described
herein are not intended to be construed in a limiting sense. For example, variations or combinations of features or materials in the embodiments shown and described may be used in particular cases without departure from the invention. Although the presently preferred embodiments are described herein in terms of particular examples, modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the arts upon reference to the drawings, description, and claims.