EE 122Precision of passive devicesD. W. Parent1Voltage Divider ExampleVoltage Divider Example•Design a circuit that will generate a 5 voltDesign a circuit that will generate a 5 volt reference DC reference from a 10 volt DC power supplypower supply. • Assume that is this application the input impedance of that will use this signal isimpedance of that will use this signal is 100 Meg Ω.2Do this for a quizDo this for a quiz•10 minutes10 minutes3A solutionA solution•Minimized current flowMinimized current flow.• 100 load Meg does not affect circuit 4VREF will vary with variance in the resistors•If we use resistors that are plus or minusIf we use resistors that are plus or minus 10% VREF can vary by5Another QuizAnother Quiz•If we use resistors that are plus or minusIf we use resistors that are plus or minus 0.01% how much can VREF can vary?6Another QuizAnother Quiz•If we use resistors that are plus or minusIf we use resistors that are plus or minus 0.01% how much can VREF can vary?7All resistors are not created equal.•Resistor specifications include:Resistor specifications include:– Through hole, surface mount– Variable/Fixed (tolerance)– Temperature stability– Lead free RoHS compliant–Power– Composition– Fire resistanceLow inductance–Low inductance– Dimensions– Packaging–Stability over time– http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=66690&k=resistors8Choosing you first resistor can be tricky.•Let’s try to find a 1MegΩresistorLet s try to find a 1Meg Ωresistor.• http://search.digikey.com/scripts/DkSearch/dksus dll?lang=en&WT medium=cpc&WT/dksus.dll?lang=en&WT.medium=cpc&WT.campaign=1053&site=US&WT.term=resistors&keywords=resistors&WT content=texttors&keywords=resistors&WT.content=text&WT.source=google&cshift_ck=2287948299cs502000711&WT srch=199cs502000711&WT.srch=19Choose through hole resistorsChoose through hole resistors•Choose 1MegΩlead freeRoHScompliantChoose 1Meg Ω, lead free, RoHScompliant61000 choices188 choices1061000 choices188 choicesChoose 0 01% ToleranceChoose 0.01% Tolerance•Two choices left, $23 and $18 each.Two choices left, $23 and $18 each.• Do we really need 0.01% tolerance?•Business decision Do we need this precisionBusiness decision. Do we need this precision for the design to work and can we sell our design at a profit?gp11Low pass filterLow pass filter12What do we need to do/have to test this circuit?13What do we need to do/have to test this circuit?•PlanPlan– How to test•Sweep frequency and measureVoutvsVin?•Sweep frequency and measure VoutvsVin?• Step Response Impulse response• Find out what we expect to see!p• Input vector (frequency, amplitude)• Corner Frequencies • Lab Equipment–Scope, resistor, capacitor, probes power p, , p ,p psupply, board1410% variance10% varianceWe could get the corner Frequencies viaExcel as well.15Impedance meter results for 10MRi10MegΩResistor•Resistors have capacitance andResistors have capacitance and inductance16Equivalent circuit for a capacitorEquivalent circuit for a capacitor•Equivalent circuit generated from a AgilentEquivalent circuit generated from a Agilent 4194A17OPAMP Open Loop GainOPAMP Open Loop
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