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AD8226ARZ-RL データシート(PDF) 10 Page - Analog Devices |
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AD8226ARZ-RL データシート(HTML) 10 Page - Analog Devices |
10 / 16 page AD8226 Preliminary Technical Data Rev. PrA | Page 10 of 16 REFERENCE TERMINAL The output voltage of the AD8226 is developed with respect to the potential on the reference terminal. This is useful when the output signal needs to be offset to a precise midsupply level. For example, a voltage source can be tied to the REF pin to level- shift the output so that the AD8226 can drive a single-supply ADC. The REF pin is protected with ESD diodes and should not exceed either +VS or −VS by more than 0.3 V. For the best performance, source impedance to the REF terminal should be kept below 2 Ω. As shown in Figure 3, the reference terminal, REF, is at one end of a 50 kΩ resistor. Additional impedance at the REF terminal adds to this 50 kΩ resistor and results in amplification of the signal connected to the positive input. The amplification from the additional RREF can be computed by 2(50 kΩ + RREF)/100 kΩ + RREF. Only the positive signal path is amplified; the negative path is unaffected. This uneven amplification degrades CMRR. INCORRECT V CORRECT AD8226 OP1177 + – V REF AD8226 REF Figure 4. Driving the Reference Pin INPUT VOLTAGE RANGE The three op amp architecture of the AD8226 applies gain in the first stage before removing common-mode voltage in the difference amplifier stage. In addition, the input transistors in the first stage shift the common mode voltage up one diode drop (about 650 mV.) Therefore, internal nodes between the first and second stages (nodes 1 and 2 in Figure 3) experience a combination of gained signal, common-mode signal, and 650 mV. This combined signal can be limited by the voltage supplies even when the individual input and output signals are not. Figure XX through Figure XX show the allowable common-mode input voltage ranges for various output voltages and supply voltages. The following formulas can also be used to understand how the reference voltage (VREF), common mode input voltage (VCM), and differential input voltage (VDIFF) interact. These two formulas, along with the input range specifications in Table 1 and Table 3, set the boundaries where the part operates with best performance. V 9. 0 2 ) )( ( V 4. 0 − + < + < − − S CM DIFF S V V GAIN V V V V V V GAIN V S REF CM DIFF .6 1 2 2 ) )( ( − + < + + The common-mode input range shifts upwards with temper- ature. At cold temperatures, the part requires an extra 200 mV of headroom from the positive supply, and operation near the negative supply has more margin. Conversely, hot temperatures require less headroom from the positive supply, but are the worst- case conditions for input voltages near the negative supply. LAYOUT To ensure optimum performance of the AD8226 at the PCB level, care must be taken in the design of the board layout. The AD8226 pins are arranged in a logical manner to aid in this task. 8 7 6 5 1 2 3 4 –IN RG RG +VS VOUT REF –VS +IN TOP VIEW (Not to Scale) AD8226 Figure 5. Pinout Diagram Common-Mode Rejection Ratio over Frequency Poor layout can cause some of the common-mode signals to be converted to differential signals before reaching the in-amp. Such conversions occur when one input path has a frequency response that is different from the other. To keep CMRR across frequency high, input source impedance and capacitance of each path should be closely matched. Additional source resistance in the input path (for example, for input protection) should be placed close to the in-amp inputs, which minimizes their interaction with parasitic capacitance from the PCB traces. Parasitic capacitance at the gain setting pins can also affect CMRR over frequency. If the board design has a component at the gain setting pins (for example, a switch or jumper), the part should be chosen so that the parasitic capacitance is as small as possible. Power Supplies A stable dc voltage should be used to power the instrumenta- tion amplifier. Noise on the supply pins can adversely affect performance. A 0.1 µF capacitor should be placed as close as possible to each supply pin. As shown in Figure 6, a 10 µF tantalum capacitor can be used farther away from the part. In most cases, it can be shared by other precision integrated circuits. |
同様の部品番号 - AD8226ARZ-RL |
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同様の説明 - AD8226ARZ-RL |
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