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OP227GY Datenblatt(PDF) 11 Page - Analog Devices |
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OP227GY Datenblatt(HTML) 11 Page - Analog Devices |
11 / 16 page REV. A OP227 –11– Dynamic range is limited by A1 as well as A2. The output of A1 is: V R R VV N O dCM 1 12 =+ Ê ËÁ ˆ ¯˜ + – If the instrumentation amplifier was designed for a gain of 10 and maximum Vd of ±1 V, then RN/RO would need to be four and VO would be a maximum of ±10 V. Amplifier A1 would have a maximum output of ±5 V plus 2 VCM, thus a limit of ±10 V on the output of A1 would imply a limit of ±2.5 V on VCM. A nominal value of 10 k W for RN is suitable for most applications. A range of 20 W to 2.5 kW for RO will then provide a gain range of 10 to 1000. The current through RO is Vd/RO, so the amplifiers must supply ±10 mV/20 W (or ±0.5 mA) when the gain is at the maximum value of 1000 and Vd is at ±10 mV. Rejecting common-mode inputs is important in accurately amplifying low level differential signals. Two factors determine the CMR in this instrumentation amplifier configuration (assuming infinite gain): ∑ CMR of the op amps ∑ Matching of the resistor network ratios (R3/R4 = R2/R1) In this instrumentation amplifier configuration error due to CMR effect is directly proportional to the CMR match of the op amps. For the OP227, this DCMR is a minimum of 97 dB for the “G” and 110 dB for the “E” grades. A DCMR value of 100 dB and a common-mode input range of ±2.5 V indicates a peak input- referred error of only ±25 mV. Resistor matching is the other factor affecting CMR. Defining Ad as the differential gain of the instrumentation amplifier and assuming that R1, R2, R3, and R4 are approximately equal (RN will be the nominal value), then CMR for this instrumentation amplifier configuration will be approxi- mately Ad divided by 4 R/RN. CMR at differential gain of 100 would be 88 dB with resistor matching of 0.01%. Trimming R1 to make the ratio R3/R4 equal to R2/R1 will raise the CMR until limited by linearity and resistor stability considerations. The high open-loop gain of the OP227 is very important to achieving high accuracy in the two op amp instrumentation amplifier configuration. Gain error can be approximated by: Gain Error A A A AA d O d OO 1 1 2 1 2 11 + < , where Ad is the instrumentation amplifier differential gain and AO2 is the open loop gain of op amp A2. This analysis assumes equal values of R1, R2, R3, and R4. For example, consider an OP227 with AO2 of 700 V/mV. Id the differential gain Ad were set to 700, then the gain error would be 1/1.001, which is approximately 0.1%. Another effect of finite op amp gain is undesired feedthrough of common-mode input. Defining AO1 as the open-loop gain of op amp A1, then the common-mode error (CME) at the output due to this effect would be approximately: CME A A A A V d d O O CM 2 1 1 2 1 + , For Ad/A01 < 1, this simplifies to (2Ad/A01) 3 VCM. If the op amp gain is 700 V/mV, VCM is 2.5 V, and Ad is set to 700, then the error at the output due to this effect will be approximately 5 mV. A compete instrumentation amplifier designed for a gain of 100 is shown in Figure 3. It has provision for trimming of input offset voltage, CMR, and gain. Performance is excellent due to the high gain, high CMR, and low noise of the individual ampli- fiers combined with the tight matching characteristics of the OP227 dual. 3 4 10 11 21 14 13 12 7 6 10k OFFSET V+ V– V+ VO = 100Vd V– 5 ADJUST CMR 50 9.95k 2.5k 191 10k 0.1% VCM – 1/2Vd GAIN VCM – 1/2Vd 10k , 0.1% 10k , 0.1% OP227 Figure 3. Two Op Amp Instrumentation Amplifier Using OP227 Dual A three op amp instrumentation amplifier configuration using the OP227 and OP27 is recommended for applications requir- ing high accuracy over a wide gain range. This circuit provides excellent CMR over a wide frequency range. As with the two op amp instrumentation amplifier circuits, the tight matching of the two op amps within the OP227 package provides a real boost in performance. Also, the low noise, low offset, and high gain of the individual op amps minimize errors. A simplified schematic is shown in Figure 4. The input stage (A1 and A2) serves to amplify the differential input Vd without amplifying the common-mode voltage VCM. The output stage then rejects the common-mode input. With ideal op amps and no resistor matching errors, the outputs of each amplifier will be: V R R V V V R R V V VV V R R V VA V O d CM O d CM O O d Od d 1 2 21 1 21 2 1 21 2 1 21 =+ Ê ËÁ ˆ ¯˜ + =+ Ê ËÁ ˆ ¯˜ + == + Ê ËÁ ˆ ¯˜ = – – – |
Ähnliche Teilenummer - OP227GY |
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Ähnliche Beschreibung - OP227GY |
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