Analog Applications Journal Texas Instruments Incorporated Amplifiers Op Amps Q www - PDF document

Texas Instruments Incorporated 4Q 2005www.ti.com/aajstage, however, can limit the input common-mode range significantly. Thus, common-following processing circuitry. This coulddesign hints that extend the inputcommon- VVV VVVININDININ and . tor, R VVVGG2122 where GRRFG112=+. GVV1121=Š=+ and , IRVVDFCM=ŠŠ=++ and . ININ Senior Systems Engineer, Industrial Systems V/2 V/2 V=V+V/2IN+CMDV=V+GV/2)2CM1D Figure 1. Classic three-op-amp INA and its voltage nodes Texas Instruments Incorporated 26Analog Applications Journal www.ti.com/aaj4Q 2005/2 are constant, the onlyis to reduce the input gain from Gto G/2), thus providing an expansionand Gto G CMCM=+Š VcV1211==+=onstant ±±+ = GGG output is now passed on 1:1 to the input. Applying gain toresistor of A3 in Figure 2. A common solution uses astand-alone difference amplifier, which provides access topin. The input stage isthen realized by a dual low-noise amplifier, with externalTo raise the gain of a unity-gain amplifier, external resis-tors can be switched in series to R. However, the internalgain, however, the external resistors can reach prohibitivewith moderate resistor values. Voltage follower A4 pro-CMRR of the difference amplifier. CMCM R=RR=RG'=R'/R221G'=1+R/R243 Texas Instruments Incorporated 27Analog Applications Journal 4Q 2005www.ti.com/aajbody. The currents create different potentialsat different parts of the body, which are sensedA typical electric potential is a 0.5- to 1.5-mV AC signal with a bandwidth of 0.05 toDC offset potential of ±500 mV and a largecommon-mode voltage of up to 1.5 V. TheTo determine the input signal of the INA ina patient’s right arm has a DC offset of 450 mVand an AC signal of 0.5 mVon the left arm has a 50-mVoffset and 1.5-mVsentative signal that is of use to a following signal process-ing system, a total gain of 1000 or more is required.The solution to this problem is performed in three steps:(1)Limit the input gain, G1, to avoid saturation of A1 (2)implement low-pass filtering in the output stage to (3)apply high gain in the output stage, boosting the AC To determine G, the INA is assumed to operate from atypical ±5-V supply. For simplification, A1 to A3 have rail-to-rail inputs and outputs, and the common-mode potentialis at a maximum of V= 1.5 V.= 10; thus, the differential input signal of A3 consists of a 4-V DC component and a 10-mV AC component. Togain, G, is increased by the buffered voltage divider, R SATCMDDC221755 V1.5 V VVVVVVVDDDCDACDCRDCLACPPRACPPL__()_()_400 mV1 To determine GWith the low-pass filter in the feedback loop of A3, thepass characteristics. One would now assume that the filter’sHowever, establishing the transfer function reveals that f VjffjffjffO0200=+ =+ . jRCINTINT=+= jff=+=Š2222 and , INTINT === DAC=== R=R filter and gain stage Texas Instruments Incorporated 28Analog Applications Journal www.ti.com/aaj4Q 2005To return to the specified fStandard INAs with unity-gain output stages, such asGain-booster A4 and integrator A5 can be designed withthe dual low-noise amplifier OPA2132 with an input-The adjustment of GThe RC values defining the integrator time constantnow reflect the real lower-bandwidth limit, fThe final gain stage A4 allows independent adjustment G=1+2R/RG=–G/G=–R/R3TOTFILTFILTG=1½OPA2132½OPA213240k40k40k25k25k40k A5 Figure 4. INA128 with OPA2132 providing low-pass filter and external gain stage OPA2335OPA2335OPA2335390k390k390k390k200k40k20k40k3.2M750pF1µF10µF0.0015µF0.1µF0.1µF1µFOPA2335OPA335 V=2.5V Figure 5. High-precision analog front end of a portable ECG application Texas Instruments Incorporated 29Analog Applications Journal 4Q 2005www.ti.com/aajINA326 and the dual precision amplifier OPA2335, operateThe input gain of the INA326 is set to 5 via G). The 750-pF capacitor parallel to Rtor A4 is set to 0.05 Hz, while the output stage around A3/5 k= 200. The precisionthe 2.5-V bias voltage to the amplifiers and the 16-bit, 100-kSPS, SAR-ADC ADS8321.To further reject 50/60-Hz noise, the input common-www.ti.com, keyword “instrumentation amplifier.”amplifier.ti.comwww.ti.com/sc/device/INA326, OPA335, OPA2132, OPA2227, OPA2335, or 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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