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AD650AD Datasheet(PDF) 16 Page - Analog Devices |
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AD650AD Datasheet(HTML) 16 Page - Analog Devices |
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16 / 21 page  Data Sheet AD650 Rev. E | Page 15 of 20 A second major difference is that the output only sinks 1 mA to the negative supply. There is no pulldown stage at the output other than the 1 mA current source used for the V-to-F conversion. The op amp sources a great deal of current from the positive supply, and it is internally protected by current limiting. The output of the op amp can be driven to within 3 V of the positive supply when it is not sourcing external current. When sourcing 10 mA the output voltage can be driven to within 6 V of the positive supply. A third difference between this op amp and a normal device is that the inverting input, Pin 3, is bias current compensated and the noninverting input is not bias-current compensated. The bias current at the inverting input is nominally zero, but can be as much as 20 nA in either direction. The noninverting input typically has a bias current of 40 nA that always flows into the node (an npn input transistor). Therefore, it is not possible to match input voltage drops due to bias currents by matching input resistors. The op amp has provisions for trimming the input offset voltage. A potentiometer of 20 kΩ is connected from Pin 13 to Pin 14 and the wiper is connected to the positive supply through a 250 kΩ resistor. A potential of about 0.6 V is established across the 250 kΩ resistor, and the 3 μA current is injected into the null pins. It is also possible to null the op amp offset voltage by using only one of the null pins and by using a bipolar current either into or out of the null pin. The amount of current required is very small—typically less than 3 μA. This technique is shown in the Applications section of this data sheet; the autozero circuit uses this technique. The bipolar offset current is activated by connecting a 1.24 kΩ resistor between Pin 4 and the negative supply. The resulting current delivered to the op amp noninverting input is nominally 0.5 mA and has a tolerance of ±10%. This current is then used to provide an offset voltage when Pin 2 is tied to ground through a resistor. The 0.5 mA that appears at Pin 2 is also flowing through the 1.24 kΩ resistor. An external resistor is used to activate the bipolar offset current source to provide the lowest tolerance and temperature drift of the resulting offset voltage. It is possible to use other values of resistance between Pin 4 and −VS to obtain a bipolar offset current different from 0.5 mA. Figure 19 shows the relationship between the bipolar offset current and the value of the resistor used to activate the source. µA Ω 200 500 4000 EXTERNAL RESISTOR 400 600 800 1000 1000 1500 2000 2500 3000 3500 Figure 19. Bipolar Offset Current vs. External Resistor |
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