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AD8553 データシート(PDF) 16 Page - Analog Devices |
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AD8553 データシート(HTML) 16 Page - Analog Devices |
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16 / 24 page  AD8222 Rev. 0 | Page 16 of 24 REFERENCE TERMINAL The output voltage of the AD8222 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 AD8222 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 best performance, source impedance to the REF terminal should be kept below 1 Ω. As shown in Figure 44, the reference terminal, REF, is at one end of a 10 kΩ resistor. Additional impedance at the REF terminal adds to this 10 kΩ resistor and results in amplification of the signal connected to the positive input. The amplification from the additional RREF can be computed by ( ) REF REF R R + + kΩ 20 kΩ 10 2 Only the positive signal path is amplified; the negative path is unaffected. This uneven amplification degrades the amplifier’s CMRR. INCORRECT AD8222 VREF CORRECT AD8222 OP2177 + – VREF CORRECT AD8222 AD8222 + – VREF Figure 45. Driving the Reference Pin LAYOUT The AD8222 is a high precision device. To ensure optimum performance at the PC board level, care must be taken in the design of the board layout. The AD8222 pinout is arranged in a logical manner to aid in this task. Package Considerations The AD8222 comes in a 4 mm × 4 mm LFCSP. Beware of blindly copying the footprint from another 4 mm × 4 mm LFCSP part; it may not have the same thermal pad size and leads. Refer to the Outline Dimensions section to verify that the PCB symbol has the correct dimensions. Space between the leads and thermal pad should be kept as wide as possible for the best bias current performance. Thermal Pad The AD8222’s 4 mm × 4 mm LFCSP comes with a thermal pad. This pad is connected internally to −VS. The pad can either be left unconnected or connected to the negative supply rail. To preserve maximum pin compatibility with future dual instrumentation amplifiers, leave the pad unconnected . This can be done by not soldering the paddle at all or by soldering the part to a landing that is a not connected to any other net. For high vibration applications, a landing is recommended. Because the AD8222 dissipates little power, heat dissipation is rarely an issue. If improved heat dissipation is desired (for example, when driving heavy loads), connect the thermal pad to the negative supply rail. For the best heat dissipation performance, the negative supply rail should be a plane in the board. See the section for thermal coefficients with and without the pad soldered. Common-Mode Rejection over Frequency The AD8222 has a higher CMRR over frequency than typical in-amps, which gives it greater immunity to disturbances, such as line noise and its associated harmonics. A well-implemented layout is required to maintain this high performance. Input source impedances should be matched closely. Source resistance should be placed close to the inputs so that it interacts with as little parasitic capacitance as possible. Parasitics at the RGx pins can also affect CMRR over frequency. The PCB should be laid out so that the parasitic capacitances at each pin match. Traces from the gain setting resistor to the RGx pins should be kept short to minimize parasitic inductance. Reference Errors introduced at the reference terminal feed directly to the output. Care should be taken to tie REF to the appropriate local ground. Power Supplies A stable dc voltage should be used to power the instrumentation amplifier. Noise on the supply pins can adversely affect performance. The AD8222 has two positive supply pins (Pin 5 and Pin 16) and two negative supply pins (Pin 8 and Pin 13). While the part functions with only one pin from each supply pair connected, both pins should be connected for specified performance and optimum reliability. |
同様の部品番号 - AD8553 |
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同様の説明 - AD8553 |
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