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AD7982 Datasheet(PDF) 18 Page - Analog Devices |
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AD7982 Datasheet(HTML) 18 Page - Analog Devices |
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18 / 24 page 
AD8475 Rev. B | Page 18 of 24 INPUT VOLTAGE RANGE The AD8475 can measure input voltages that are larger than the supply rails. The internal gain and feedback resistors form a divider, which reduces the input voltage seen by the internal input nodes of the amplifier. The largest voltage that can be measured is constrained by the capability of the amplifier’s internal summing nodes. This voltage is defined by the input voltage and the ratio between the feedback and the gain resistors. Figure 51 shows the voltage at the internal summing nodes of the amplifier, defined by the input voltage and internal resistor network. If VN is grounded, the expression shown in the figure reduces to ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + + = = P MINUS PLUS V RG RF VOCM RG RF RG V V 2 1 The internal amplifier of the AD8475 has rail-to-rail inputs. To obtain accurate measurements with minimal distortion, the voltage at the internal inputs of the amplifier must stay below +VS − 1 V and above −VS. For example, with VS = 5 V in a G = 0.4 configuration, the AD8475 can measure an input as high as ±12.5 V and maintain its excellent distortion performance. The AD8475 provides overvoltage protection for excessive input voltages beyond the supply rails. Integrated ESD protection diodes at the inputs prevent damage to the AD8475 up to +VS + 10.5 V and −VS − 16 V. DRIVING THE AD8475 Care should be taken to drive the AD8475 with a low impedance source: for example, another amplifier. Source resistance can unbalance the resistor ratios and, therefore, significantly degrade the gain accuracy and common-mode rejection of the AD8475. For the best performance, source impedance to the AD8475 input terminals should be kept below 0.1 Ω. Refer to the DC Precision section for details on the critical role of resistor ratios in the precision of the AD8475. POWER SUPPLIES The AD8475 operates over a wide range of supply voltages. It can be powered on a single supply as low as 3 V and as high as 10 V. The AD8475 can also operate on dual supplies from ±1.5 V up to ±5 V A stable dc voltage should be used to power the AD8475. Note that noise on the supply pins can adversely affect performance. For more information, see the PSRR performance curve in Figure 13. Place a bypass capacitor of 0.1 μF between each supply pin and ground, as close as possible to each supply pin. Use a tantalum capacitor of 10 μF between each supply and ground. It can be farther away from the supply pins and, typically, it can be shared by other precision integrated circuits. RF RF RG RG VON VOP VOCM VP VN VN RF + RG RF VP − VN RG RF VOCM RF + RG RG + + 2 1 Figure 51. Voltages at the Internal Op Amp Inputs of the AD8475 |
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