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university:courses:engineering_discovery:lab_3 [31 Mar 2016 19:30] – Jonathan Pearson | university:courses:engineering_discovery:lab_3 [03 Jan 2018 19:41] (current) – [Observations and Conclusions] Doug Mercer |
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The op-amp requires a small input bias current on each input, and these currents are closely, but not perfectly matched. The current flowing through R<sub>T</sub> produces a voltage drop across R<sub>T</sub>, introducing an error into the frequency setting. Compensation resistor, R<sub>C</sub>, a resistor equal in value to R<sub>T</sub>, is added to the non-inverting input (Pin 4) to produce a nearly identical voltage drop is as is across R<sub>T</sub>. This technique minimizes the offset error between the two op-amp inputs, and produces the most accurate output frequency. This is an important consideration in circuits that are used as voltage-to-frequency converters, but not so important in simple LED flashing circuits. When observing the voltage on Pin 4, a jumper wire must be placed across R<sub>C</sub> (shown as dotted line in the schematic) in order to eliminate DC losses that occur due to the M1K input loading the 5 MΩ source resistance. | The op-amp requires a small input bias current on each input, and these currents are closely, but not perfectly matched. The current flowing through R<sub>T</sub> produces a voltage drop across R<sub>T</sub>, introducing an error into the frequency setting. Compensation resistor, R<sub>C</sub>, a resistor equal in value to R<sub>T</sub>, is added to the non-inverting input (Pin 4) to produce a nearly identical voltage drop is as is across R<sub>T</sub>. This technique minimizes the offset error between the two op-amp inputs, and produces the most accurate output frequency. This is an important consideration in circuits that are used as voltage-to-frequency converters, but not so important in simple LED flashing circuits. When observing the voltage on Pin 4, a jumper wire must be placed across R<sub>C</sub> (shown as dotted line in the schematic) in order to eliminate DC losses that occur due to the M1K input loading the 5 MΩ source resistance. |

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In a voltage-feedback op-amp circuit, negative feedback causes the feedback voltage on the inverting input to track the voltage on the non-inverting input. The voltage on Pin 3 is the feedback voltage, and this should track the voltage applied to Pin 4 very closely as long as the output of the emitter follower can follow it and the input common-mode range is not violated.==== Observations and Conclusions ==== | In a voltage-feedback op-amp circuit, negative feedback causes the feedback voltage on the inverting input to track the voltage on the non-inverting input. The voltage on Pin 3 is the feedback voltage, and this should track the voltage applied to Pin 4 very closely as long as the output of the emitter follower can follow it and the input common-mode range is not violated.** ** |

| ==== Observations and Conclusions ==== |

* The AD654 is a voltage-to frequency converter that can be used for many functions, ranging from precise signal transmission to simple LED flashing | * The AD654 is a voltage-to frequency converter that can be used for many functions, ranging from precise signal transmission to simple LED flashing |

* Op-amp circuits require input bias currents, and offset errors can be minimized by matching the resistances that each input bias current flows through | * Op-amp circuits require input bias currents, and offset errors can be minimized by matching the resistances that each input bias current flows through |

* The two voltage-feedback op-amp input voltage levels track each other closely when negative feedback is applied around the op-amp | * The two voltage-feedback op-amp input voltage levels track each other closely when negative feedback is applied around the op-amp |

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