ticomaaj Analog and MixedSignal Products Autozero amplifiers ease the design of highprecision circuits A wide variety of electronic applications deal with the conditioning of small input signals These systems require signal paths with very low offset ID: 22874
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Texas Instruments IncorporatedAmplifiers:Op Amps 2Q 2005www.ti.com/aajvoltage drift over time and temperature. With standard system-level auto-calibration. However, adding auto-The alternative is to use components with low offset andlow drift. The amplifiers with by far the lowest offset and withOffset tVINV 1 2+ Note: Labels for amplifiers such as AIN, GB, and AMare used to identify amplifiers in figures and to representamplifier gain in equations. Texas Instruments IncorporatedAmplifiers:Op Amps 20Analog Applications Journal www.ti.com/aaj2Q 2005back to dc. The demodulator consists of a switch S. An integrator then smoothsthe switch outputand presents the final dc output.stage and the dc gain of the integrator, easily reaches anopen-loop gain of 160 dB and reduces the gain error,voltage (also known as dc noise), the 1/f noise, and low-Offset and drift in the output integrator stage are nulleddc drifts in the ac stage are also irrelevant because The 1/f noise of the ac amplifier is modulated to higherfrequencies via the demodulator.tude modulation (AM), with the chopping frequency, fbeing the carrier, and the input voltage representing the OUT both sides of the odd harmonics of the chopper frequency.the demodulator, M, introduces sidebands on bothinput signal, however, Mroll-off of the subsequent low-pass filter limits the base-band to frequencies far below the chopper frequency.front end. Second, the carrier-based approach constituteslimited to a small fraction of the chopper frequency. Thechopper frequency, in turn, is restricted by ac amplifieringlow overall bandwidth. fN(f)fINVf)Vf)VIN(f)Vf)V014556f/f1M2 1 Texas Instruments IncorporatedAmplifiers:Op Amps 21Analog Applications Journal 2Q 2005www.ti.com/aajThe classic chopper-stabilized amplifiersolves the chopper amplifiers low-bandwidthstabilizing amplifier, a chopper type, biasesthe fast amplifiers positive terminal to forcethe summing point to zero.Fast signals directly drive the ac amplifier,chopper amplifier. The low-frequency cutofffrequency characteristics. With proper design,the chopper-stabilized approach yields band-characteristicof the chopper amplifier.Unfortunately, because the stabilizing amplifiercontrols the fast amplifiers positive terminal,the classic chopper-stabilized approach isrestricted to inverting operation only.fast amplifiers noise gain. Keeping outputchopper-stabilized design.Similar to the chopper-stabilized approach,offset correction. Figure 4 shows a block by Texas Instruments in the mid-80s.With the calibration path lying in parallelwith the signal path, both inputs of the mainThe main amplifier, A, and the nulling amplifier, Asignal inputs is given as Aloop gains of +Bphase. The oscillator, generating fphase by driving both switches into position 1. The inputsof the nulling amplifier are shorted together, while its out-on the C1 capacitor.position 2, this offset voltage remains on C1 and essentiallycorrects any error from the nulling amplifier. A COSN VAVBVCNOSNNC *SimilartothechopperamplifierinFigure1 Stabilizing Wideband VIN OscillatorS1VOSMS2VOUT1221C1C2BN+BMVC1VC2 MVOSN AN ++ Note: Labels for amplifiers such as A Texas Instruments IncorporatedAmplifiers:Op Amps 22Analog Applications Journal www.ti.com/aaj2Q 2005and subtracts it from the amplifiedamplifier. The potential, VMost obvious is the gain product of both the main andterm in Equation 3 explainswhy AZAs have extremely high open-loop gain. To under- VVABAVVOUTINNNNOSMOSN=++(). VVAABVAVOUTINMNNOSMMOSN=+++(). VAVVBVOUTMINOSMMC=++(). VAVCNINOSN VVAVVBVONCNINOSNNC==+(). AVVNINOSN(). input offset voltage of the complete amplifier, we shouldPutting Equation 4 into the form of Equation 5 gives usFrom here it is easy to see that k = A, to be in the region of 10,000 or higher, it quicklythe sum and difference of the input signal frequency, fand the auto-zero clock frequency, ffrequency, fof little importance. However, the difference frequency, /2. Older AZAbandwidth to less than half of the auto-zero frequency.clock frequencies in the range of only 400 to 500 Hz, thusTLC2654 was one of the first amplifiers that allowed high-OPA335. Modern process technology, with gate structures OSEffOSMOSN VABVOUTNNINOSMOSN VkVVOUTINOSEff(), VINVOS_EffVOUT + Texas Instruments IncorporatedAmplifiers:Op Amps 23Analog Applications Journal 2Q 2005www.ti.com/aajFigure 6 shows the inner structure of the OPA335. Thenate mode in parallel with the main amplifier, Athe amplification phase, correcting the main amplifiers(IMD) by keeping the amplifiers gain bandwidth constantamplifier consists of a multistage composite amplifier. Thisdown to 300 µA (versus the 1.5 mA of the TLC2654) whileLets return to the process of auto-zeroing. The nullingamplifier, whose switches are in position 1, is in the auto-tion 2), the output voltage of the nulling amplifier, Vadds to the output voltage of the main amplifier, V. With VGAVNBININOSN VGAVVAVNBININOSNZC=+(), VGAVAVVCBINOSNZCOSNBIN==(). OPA335 INGOVMVMVNAINAINGBGBAZAZVCVC1122 +++++++ Note: Labels for amplifiers such as AIN, GB, and AMare used to identify amplifiers in figures and to represent amplifier gain in equations. Texas Instruments IncorporatedAmplifiers:Op Amps 24Analog Applications Journal www.ti.com/aaj2Q 2005Inserting Equations 7 and 8 into Equation 9 yieldsHow low a filters 3-dB frequency can be to provide effec- OSMOSN VGGAVOUTOBININOSMOSN VGAVVGAVOUTOMOSMOSNBININOSN=+++(). VGVVOUTONM(). VAVVMMINOSM frequency, and the white noise region with constant spec-tral density. At corner frequency fnoise equals the magnitude of white noise density.For signal-to-noise ratio calculations we require the rmsing a first-order low-pass filter with a 3-dB cutoff at is the white noise spectral density, fVarious rms voltages have been calculated with the pre-order low-pass filter. The resulting plot is shown in Figure 8.It is important to notice that lowering the filters cutoff below10fresults in the positive slope in Figure 8. It can be seen thattive in establishing high dc accuracy.feedthrough. This term is broadly used to indicate visibility VvfrmsnwH157 VvfrmsnwC=+ln.,157 101001k10k100kFrequency(Hz)StandardOpAmpNoise(nV/Hz) 101001k10k100kFrequency(Hz)StandardOpAmprmsNoise(µV) Texas Instruments IncorporatedAmplifiers:Op Amps 25Analog Applications Journal 2Q 2005www.ti.com/aajoccurring during the sampling and holding of the amplifiersThe OPA335, however, has remarkably little noise.quency. low-frequency application that allows the OPA335 to besupply. The forward voltage of diode D1 has a negativejunction compensation via the resistor network R1 to R3.output amplifier. The single-supply amplifier providing anopen-loop gain of 130 dB allows 16-bit or better accuracyThus, AZAs ideally suit single-supply precision applicationswhere high accuracy, low drift, and low noise are imperative.clock frequency. Aliasing and intermodulation noise areacross its entire gain bandwidth. In addition, the amplifiersoutput provides rail-to-rail drive capability, allowing for ahigh signal-to-noise ratio at low supply voltages. 3.57k150k6.04k2.94k6.04k+5V0.1µF0.1µF4.096V+5VK-Type40.7µV/°C OPA335 11.8k21k2.74k680pF3.3nF330pF OPA335 + Texas Instruments IncorporatedAmplifiers:Op Amps 26Analog Applications Journal www.ti.com/aaj2Q 2005amplifier. Figure 11 shows the required circuit configura-tion in the form of a composite amplifier design.bias path of the wideband amplifier. The signal path stillruns from Vtwo functions. At low frequencies, it provides high gain to) ensures that the integrators closed-loop gain quicklyinput of the wideband amplifier.Note that the amplifiers input noise is amplified by thenoninverting closed-loop gain of the integrator. Thus, athigh frequencies, the OPA335 operates as a voltage follower(gain = 1), passing its input noise on to the widebandamplifier. To eliminate this noise, a low-pass filter (R2, C2)AZA. The same precaution is taken for the OPA353. Herethe low-pass filter (R1, C1) limits the output noise of thewideband amplifier.To compensate for the signal voltage drop across R1,the phase shift of the low-pass filter.True chopper- and chopper-stabilized amplifiers performlevels of output noise.method. Older-generation amplifier designs benefited from integrated circuit design. Aliasing and IMD, however,clock frequency. These devices required supply voltages ofTodays AZAs are by far the most sophisticated precisionentire gain bandwidth of the amplifier. Proprietary circuitquiescent current significantly, allowing for low-power 100k10nF22pF10k10nF1nF49.9kC=C1R+R1Bandwidth=1/2CR+R1Wideband OPA353OPA335 ++ Texas Instruments IncorporatedAmplifiers:Op Amps 27Analog Applications Journal 2Q 2005www.ti.com/aajthe OPA734/735 op amps with extended supply rangefrom 2.7 to 12 V;the OPA380 wideband transimpedance amplifier;the INA326 instrumentation amplifier for single-supplythe INA330 instrumentation amplifier for constant the PGA309, a fully integrated pressure sensor condi-Future design ideas aim to shape the noise floor offrom the baseband to higher, out-of-band frequencies. Thisamplifier.ti.comwww.ti.com/sc/device/with INA326, INA330, OPA335,OPA353, OPA380, OPA734, OPA735, PGA309, REF3040, IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reservestandard warranty. Testing and other quality control techniques areused to the extent TI deems necessary to support this warranty.applications using TI components. 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