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AD524 Datasheet(PDF) 13 Page - Analog Devices |
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AD524 Datasheet(HTML) 13 Page - Analog Devices |
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13 / 20 page  Preliminary Technical Data AD8426 Rev. PrD | Page 13 of 20 HIDDEN PADDLE BOTTOM VIEW EXPOSED LEAD FRAME TABS NOTES 1. EXPOSED LEAD FRAME TABS AT THE FOUR CORNERS OF THE PACKAGE ARE INTERNALLY CONNECTED TO +VS. REFER TO THE OUTLINE DIMENSIONS PAGE, FOR FURTHER INFORMATION ON PACKAGE AVAILABILITY. Figure 10. Hidden Paddle Package, Bottom View 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, the 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 component 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 11, a 10 μF capacitor can be used farther away from the part. In most cases, it can be shared by other precision integrated circuits. AD8426 +VS +IN –IN LOAD REF 0.1µF 10µF 0.1µF 10µF –VS VOUT Figure 11. Supply Decoupling, REF, and Output Referred to Local Ground References The output voltage of the AD8426 is developed with respect to the potential on the reference terminal. Care should be taken to tie REF to the appropriate local ground. This should also help minimize crosstalk between the two channels. INPUT BIAS CURRENT RETURN PATH The input bias current of the AD8426 must have a return path to ground. When the source, such as a thermocouple, cannot provide a return current path, one should be created, as shown in Figure 12. THERMOCOUPLE +VS REF –VS AD8426 CAPACITIVELY COUPLED +VS REF C C –VS AD8426 TRANSFORMER +VS REF –VS AD8426 INCORRECT CAPACITIVELY COUPLED +VS REF C R R C –VS AD8426 1 fHIGH-PASS = 2πRC THERMOCOUPLE +VS REF –VS 10MΩ AD8426 TRANSFORMER +VS REF –VS AD8426 CORRECT Figure 12. Creating an Input Bias Current Return Path INPUT PROTECTION The AD8426 has very robust inputs and typically does not need additional input protection. Input voltages can be up to 40 V from the opposite supply rail. For example, with a +5 V positive supply and a −8 V negative supply, the part can safely withstand voltages from −35 V to +32 V. Unlike some other instrumentation amplifiers, the part can handle large differen- tial input voltages even when the part is in high gain. The rest of the AD8426 terminals should be kept within the supplies. All terminals of the AD8426 are protected against ESD. |
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Similar Description - AD524 |
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