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OP220FZ データシート(PDF) 9 Page - Analog Devices |
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OP220FZ データシート(HTML) 9 Page - Analog Devices |
9 / 12 page REV. A OP220 –9– In this instrumentation amplifier configuration, error due to CMRR effect is directly proportional to the differential CMRR of the op amps. For the OP220A/E, this combined CMRR is a minimum of 98 dB. A combined CMRR value of 100 dB and 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 CMRR. 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 CMRR will be approximately AD divided by 4 DR/RN. CMRR at differential gain of 100 would be 88 dB with resistor matching of 0.1%. Trimming R1 to make the ratio R3/R4 equal to R2/R1 will directly raise the CMRR until it is limited by linearity and resistor stability considerations. The high open-loop gain of the OP220 is very important in achieving high accuracy in the two-op-amp instrumentation amplifier configuration. Gain error can be approximated by: Gain Error = + < 1 1 2 1 02 01 02 A A A AA D D , where AD is the instrumentation amplifier differential gain and A02 is the open-loop gain of op amp A2. This analysis assumes equal values of R1, R2, R3, and R4. For example, consider an OP220 with A02 of 700 V/mV. If the differential gain AD were set to 700, the gain error would be 1/1.001 which is approxi- mately 0.1%. Another effect of finite op amp gain is undesired feedthrough of common-mode input. Defining A01 as the open-loop gain of op amp A1, then the common-mode error (CME) at the output due to this effect will be approximately: CME A A A A V D D CM = + 2 1 1 01 01 For AD/A01, < 1, this simplifies to (2 AD/A01) 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. The OP220 offers a unique combination of excellent dc perfor- mance, wide input range, and low supply current drain that is particularly attractive for instrumentation amplifier design. THREE OP AMP CONFIGURATION A three op amp instrumentation amplifier configuration using the OP220 and OP777 is recommended for applications requiring high accuracy over a wide gain range. This circuit provides excellent CMR over a wide input range. As with the two op amp instrumentation amplifier circuits, tight matching of the two op amps provides a real boost in performance. A2 + VCM + 1/2 VD 1/2 OP220 A1 – VD VCM – 1/2 VD 1/2 OP220 R1 R1 V+ V– R0 A3 OP777 R2 V+ V– R2 R2 V1 VO R2 V2 VO = VD 1 + 2R1 R0 Figure 5. Three Op Amp Instrumentation Amplifier Using OP220 and OP777 A simplified schematic is shown in Figure 2. 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 11 21 2 21 21 2 21 1 21 =- + Ê ËÁ ˆ ¯˜ + =+ Ê ËÁ ˆ ¯˜ + =- = + Ê ËÁ ˆ ¯˜ = The differential gain AD is 1 + 2R1/RO and the common-mode input VCM is rejected. This three op amp instrumentation amplifier configuration using an OP220 at the input and an OP777 at the output provides excellent performance over a wide gain range with very low power consump- tion. A gain range of 1 to 2,000 is practical and CMR of over 120 dB is readily achievable. |
同様の部品番号 - OP220FZ |
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同様の説明 - OP220FZ |
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