Source: http://www.google.com/patents/US7719351?dq=6680675
Timestamp: 2016-06-24 22:31:53
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Patent US7719351 - Autozeroing current feedback instrumentation amplifier - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsAn embodiment is directed to an instrumentation amplifier. The instrumentation amplifier includes an output stage for generating an output voltage, a low-frequency path coupled with the output stage, and a high-frequency path coupled with the output stage. The high-frequency path dominates the low-frequency...http://www.google.com/patents/US7719351?utm_source=gb-gplus-sharePatent US7719351 - Autozeroing current feedback instrumentation amplifierAdvanced Patent SearchPublication numberUS7719351 B2Publication typeGrantApplication numberUS 11/804,492Publication dateMay 18, 2010Filing dateMay 17, 2007Priority dateMay 17, 2007Fee statusPaidAlso published asDE102008023384A1, US20080284507Publication number11804492, 804492, US 7719351 B2, US 7719351B2, US-B2-7719351, US7719351 B2, US7719351B2InventorsMichiel Pertijs, George ReitsmaOriginal AssigneeNational Semiconductor CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (5), Non-Patent Citations (8), Referenced by (7), Classifications (14), Legal Events (2) External Links: USPTO, USPTO Assignment, EspacenetAutozeroing current feedback instrumentation amplifier
US 7719351 B2Abstract
an auto-zeroing circuit coupled with said first input stage, said first feedback stage, and said output stage, said auto-zeroing circuit for generating a nulling current, wherein said nulling current compensates for errors in said first intermediate current and said first feedback current resulting from input offsets in said first input stage and said first feedback stage; and
a pre-charge circuit coupled with said first input stage, wherein said pre-charge circuit is operable to charge an input of said first input stage to a pre-charge voltage, wherein said pre-charge voltage is based on said differential input, and wherein said pre-charge circuit is operable to charge said input of said first input stage to said pre-charge voltage without loading said differential input.
a current buffer stage coupled with said first input stage, said first feedback stage, said auto-zeroing circuit, and said output stage, said current buffer stage for buffering at least one of said first intermediate current, said first feedback current, and said nulling current; and
wherein said current buffer stage increases an input impedance observed at an input of said output stage.
3. The instrumentation amplifier as recited in claim 2 wherein said first input stage, said first feedback stage, and said auto-zeroing circuit comprises a high frequency path coupled with said output stage; and
further comprising a high-frequency path coupled with said output stage, wherein said high-frequency path dominates said low-frequency path at frequencies above a particular frequency, and wherein further said low-frequency path dominates said high-frequency path at frequencies below said particular frequency.
4. The instrumentation amplifier as recited in claim 3 wherein said high-frequency path comprises:
a second input stage coupled with said output stage, said second input stage for sensing said differential input and generating a second intermediate current based thereon; and
a second feedback stage coupled with said output stage, said second feedback stage for generating a second feedback current based on said output voltage.
5. The instrumentation amplifier as recited in claim 1 wherein said auto-zeroing circuit is operable to detect offset currents corresponding to input offsets of said input stage and said feedback stage generate a nulling current based on said detected offset currents.
a tranconductance amplifier coupled with said integrator, said tranconductance amplifier for generating said nulling current.
7. The instrumentation amplifier as recited in claim 1 wherein said auto-zeroing circuit comprises:
a plurality of switches coupled with said first input stage and said first feedback stage, wherein said plurality of switches are operable to short inputs of said first input stage to an input common mode voltage, and wherein further said plurality of switches are operable to short inputs of said first feedback stage to a feedback common mode voltage.
8. The instrumentation amplifier as recited in claim 7 wherein said plurality of switches are operable to temporality disconnect said first input stage and said first feedback stage from said output stage while said nulling current is calibrated.
9. The instrumentation amplifier as recited in claim 1 wherein said pre-charge circuit comprises a buffer.
an auto-zeroing circuit coupled with said input stage and said feedback stage, said auto-zeroing circuit for switching said instrumentation amplifier between an amplification configuration and an auto-zeroing configuration, wherein said auto-zeroing circuit is operable to detect offset currents corresponding to input offsets of said input stage and said feedback stage in said auto-zeroing configuration, and wherein further said auto-zeroing circuit is operable to generate a nulling current based on said detected offset currents in said amplification configuration;
a pre-charge circuit coupled with said input stage, wherein said pre-charge circuit is operable to charge an input of said input stage to a pre-charge voltage, wherein said pre-charge voltage is based on said differential input, and wherein said pre-charge circuit is operable to charge said input of said first input stage to said pre-charge voltage without loading said differential input; and
11. The instrumentation amplifier as recited in claim 10 wherein, in said auto-zeroing configuration, said auto-zeroing circuit is operable to de-couple said output stage from said input stage and said feedback stage, to couple said input stage and said feedback stage with an auto-zero loop, to couple said input stage with a common mode input voltage, and to couple said feedback stage with a common mode feedback voltage.
12. The instrumentation amplifier as recited in claim 10 further comprising:
a current buffer stage coupled with said input stage, said feedback stage, said auto-zeroing circuit, and said output stage, said current buffer stage for buffering at least one of said intermediate current, said feedback current, and said nulling current.
generating an intermediate current based on an input voltage of a current feedback instrumentation amplifier;
generating a feedback current based on an output voltage of a current feedback instrumentation amplifier, wherein said intermediate current and said feedback current comprise offset components corresponding to input offsets of an input stage and a feedback stage respectively;
generating a nulling current based on said offset components, wherein said nulling current compensates for said offset components; and
pre-charging said input stage with a buffered version of said input voltage prior to applying said input voltage to said input stage.
applying a common mode input voltage to said input stage;
applying a common mode feedback voltage to said feedback stage; and
measuring said offset components while said common mode input voltage and said common mode feedback voltage are applied to said input stage and said feedback stage.
15. The method as recited in claim 13 wherein generating said nulling current comprises:
switching said instrumentation amplifier from an amplification configuration to an auto-zero configuration;
measuring said offset components; and
generating said nulling current based on the measuring of said offset components.
16. The method as recited in claim 15 wherein said switching comprises:
de-coupling said output stage from said input stage and said feedback stage;
coupling said input stage to a common mode input voltage; and
coupling said feedback stage to a common mode feedback voltage.
buffering said intermediate current, said feedback current and said nulling current.
utilizing a high-frequency path to generate said output voltage at frequencies above a particular frequency.
The present Application for Patent is related to co-pending U.S. patent application Ser. No. 11/804,490, now U.S. Pat. No. 7,573,327, entitled “AUTOZEROING CURRENT FEEDBACK INSTRUMENTATION AMPLIFIER”, by Michiel Pertijs et al., filed May 17, 2007, assigned to the assignee hereof, and expressly incorporated by reference herein.
Generally speaking, embodiments provide technology for reducing input offsets in current feedback instrumentation amplifiers. The technology involves using auto-zeroing circuitry to null an offset of an input stage. In one embodiment, this is achieved by periodically switching-in the auto-zeroing circuitry. As a result, embodiments are able to achieve very low input-referred offset, low input current, and low-level spurious switching signals at the output. Additionally, spurious signals may be further reduced by adding a high-frequency feedforward path.
Exemplary Circuits, in Accordance with Various Embodiments
FIG. 3 illustrates a block diagram of a current feedback amplifier 300, in accordance with various embodiments of the present invention. Amplifier 300 includes an input stage 320, an output stage 310, a feedback stage 330, and a feedback network 350. The feedback network is operable to generate a feedback voltage Vfb from the output voltage Vout and the reference voltage Vref and thus defines the gain of the amplifier. The amplifier 300 also advantageously includes an auto-zero circuit 340 coupled to the input stage 320, the output stage 310, and the feedback stage 330. The auto-zero circuit 340 is operable to switch the amplifier 300 between an amplification configuration corresponding to an amplification phase and an auto-zeroing configuration corresponding to an auto-zeroing phase. During the amplification phase, the amplifier 300 is operable to perform normal amplification operations. During the auto-zeroing phase, the auto-zero circuit 340 is operable to null offset currents generated by the input stage 320 and the feedback stage 330.
At high frequencies, the feedforward path comprising amplifiers 617-618 is dominant. The feedforward path, together with the output amplifier 611, forms a regular Miller-compensated two-stage amplifier with approximately 20 dB/dec roll-off. This type of frequency compensation is known as “multi-path nested-Miller compensation” and has been used in conventional op-amps, but without application to auto-zeroed instrumentation amplifiers.
The above-referenced mixing problems may alternatively be solved by using a dual-input-stage “ping-pong” architecture. FIG. 7 illustrates a schematic of a current feedback instrumentation amplifier 700, including parallel input stages, in accordance with various embodiments of the present invention. Amplifier 700 includes first and second input stages 720 and 725, an output stage 710, first and second feedback stages 730 and 735, and a feedback network 750. The amplifier 700 also advantageously includes a first auto-zero circuit 740 coupled to the input stage 720, the output stage 710, and the feedback stage 730. The amplifier 700 further includes a second auto-zero circuit 745 coupled to the input stage 725, the output stage 710, and the feedback stage 735.
During operation, the auto-zeroing circuits 740 and 745 of amplifier 700 periodically switch amplifier 700 between the first configuration and the second configuration, ensuring that the input stages 720 and 725 and feedback stages 730 and 735 are periodically recalibrated. Thus, this “ping-pong” operation ensures that there is continuously an offset-free stage in the signal path.
Switches 831-836 and 841-846, tranconductance amplifiers 814 and 815, and capacitors 853 and 854 function together as a first auto-zero circuit, such as auto-zero circuit 740 of amplifier 700. Similarly, switches 871-876 and 881-886, tranconductance amplifiers 824 and 825, and capacitors 858 and 859 function together as a second auto-zero circuit, such as auto-zero circuit 745 of amplifier 700. It should be appreciated that switches 831-836, 841-846, 871-876, and 881-886 may be any of a number of devices capable of performing a switching function. In one embodiment, the switches 831-836, 841-846, 871-876, and 881-886 serve to switch the amplifier 800 between first and second configurations corresponding to first and second phases of operation. For example, the first configuration may correspond to switches 831-836 and 871-876 being closed and switches 841-846 and 881-886 being open. Conversely, a second configuration may correspond to switches 841-846 and 881-886 being closed and switches 831-836 and 871-876 being open.
During the second phase, while the first input stage and the first feedback stage are performing amplification functions, the second input stage and the second feedback stage are auto-zeroed. In other words, the inputs of the tranconductance amplifiers 822 and 823 are shorted to the input common mode voltage Vcmin and the feedback common mode voltage Vcmfb, respectively. Any input offsets of amplifiers 822 and 823 cause an offset current that flows into the integrator formed by tranconductance amplifier 824 and capacitors 858 and 859. The output of this integrator then drives the tranconductance amplifier 825 to generate a nulling current, which effectively nulls the offset current.
During operation, the auto-zeroing circuits of amplifier 800 periodically switch amplifier 800 between the first configuration and the second configuration, ensuring that the input stages and feedback stages are periodically recalibrated. Thus, this “ping-pong” operation ensures that there is continuously an offset-free stage in the signal path.
During operation, the auto-zeroing circuits 940 and 945 of amplifier 900 periodically switch amplifier 900 between the first configuration and the second configuration, ensuring that the input stages 920 and 925 and feedback stages 930 and 935 are periodically recalibrated. Thus, this “ping-pong” operation ensures that there is continuously an offset-free stage in the signal path.
During operation, the auto-zeroing circuits of amplifier 1000 periodically switch amplifier 1000 between the first configuration and the second configuration, ensuring that the input stages and feedback stages are periodically recalibrated. Thus, this “ping-pong” operation ensures that there is continuously an offset-free stage in the signal path.
It should be appreciated that the tranconductance amplifiers 412, 612, 812, 822, 1012, and 1022 of FIGS. 4, 6, 8, and 10 have associated amounts of input capacitance. As such, the amplifiers 412, 612, 812, 822, 1012, and 1022 may act as switched-capacitor loads to the signal source Vin because they are periodically discharged during auto-zeroing phases and need to be recharged during amplification phases. In various embodiments, this effect may be reduced by using a pre-charging technique. FIG. 12 illustrates an input stage 1200 of an amplifier (such as amplifier 400) that includes pre-charging circuitry, in accordance with various embodiments of the present invention. It should be appreciated that similar configurations may be used in amplifiers 300, 500, 600, 700, 800, 900, and 1000 as well. In FIG. 6, the input capacitance of amplifier 412 is depicted by capacitor 1255. Input stage 1200 includes additional switches 1271 and 1272, which allow for a “pre-charging” configuration of the input stage 1200, in addition to the amplification configuration and the auto-zeroing configuration. During the pre-charging phase, switches 431-432 and 441-442 are opened and switches 1271-1272 are closed. As a result, the inputs of amplifier 412 are coupled with a buffered version of the input signal Vin via buffers 1281 and 1282, so that the current needed to charge the input capacitance 1255 is provided by the buffer amplifiers 1281-1282, rather than by the signal source. In one embodiment, the buffers are 1281-1282 are unity gain buffers. Thus, in a subsequent amplification phase, the signal source only needs to provide current to correct for any small offset errors of the buffer amplifiers 1281-1282, rather than the full input voltage Vin. It should be appreciated that while an input stage 1200 is depicted in FIG. 12, the input capacitances of other tranconductance amplifiers may be pre-charged in a similar fashion. For example, feedback amplifiers 413, 613, 813, 823, 1013, and 1023 may be pre-charged to Vfb to reduce loading from their respective feedback networks.
Exemplary Operations in Accordance with Various Embodiments
The following discussion sets forth in detail the operation of present technology for reducing effects of offsets in current feedback instrumentation amplifiers. With reference to FIGS. 13-17, flowcharts 1300, 1350A, 1410A, 1600, and 1625A each illustrate example operations used by various embodiments of the present technology for reducing effects of offsets in current feedback instrumentation amplifiers. Flowcharts 1300, 1350A, 1410A, 1600, and 1625A include processes that, in various embodiments, are carried out by circuitry in an integrated circuit. Although specific operations are disclosed in flowcharts 1300, 1350A, 1410A, 1600, and 1625A, such operations are examples. That is, embodiments are well suited to performing various other operations or variations of the operations recited in flowcharts 1300, 1350A, 1410A, 1600, and 1625A. It is appreciated that the operations in flowcharts 1300, 1350A, 1410A, 1600, and 1625A may be performed in an order different than presented, and that not all of the operations in flowcharts 1300, 1350A, 1410A, 1600, and 1625A may be performed.
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