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university:courses:engineering_discovery:lab_3 [31 Mar 2016 19:30] Jonathan Pearsonuniversity: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.
  
-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
 +
 +**Return to [[university:courses:engineering_discovery|Engineering Discovery Index]]**
university/courses/engineering_discovery/lab_3.txt · Last modified: 03 Jan 2018 19:41 by Doug Mercer