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EE 435 Lect 22 Spring 2010

EE 435 Lect 22 Spring 2010 - EE 435 Lecture 22 Offset...

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EE 435 Lecture 22 Offset Voltages
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Offset Voltage Definition: The input-referred offset voltage is the differential dc input voltage that must be applied to obtain the desired output when V ic is the quiescent common-mode input voltage. Note: V OFF is usually related to the output offset voltage by the expression OUTOFF OFF D V V = A Note: V OFF is dependent upon V ICQ although this dependence is usually quite weak and often not specified .• Review from last lecture .• • •
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Systematic Offset Voltage Random Offset Voltage Offset Voltage ICQ OUT Two types of offset voltage: After fabrication it is impossible (difficult) to distinguish between the systematic offset and the random offset in any individual op amp Measurements of offset voltages for a large number of devices will provide mechanism for identifying systematic offset and statistical Characteristics of the random offset voltage .• Review from last lecture .• • •
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Offset Voltage V OS Can be modeled as a dc voltage source in series with the input .• Review from last lecture .• • •
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Offset Voltage Effects can be reduced or eliminated by adding equal amplitude opposite Dc signal (many ways to do this) Widely used in offset-critical applications Comes at considerable effort and expense OS OS Prefer to have designer make V OS small in the first place .• Review from last lecture .• • •
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Offset Voltage Distribution Pdf of zero-mean Gaussian distribution Percent between: ± σ 68.3% ±2 σ 95.5% ±3 σ 99.73% .• Review from last lecture .• • •
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Source of Random Offset Voltages Determine the offset voltage – i.e. value of V X needed to obtain desired output m V g A = - R 2 V DD V SS R R+ Δ R M 1 M 2 V OUT I T V X T X V I -1 V = Δ R A 2 T T T X EB m m T EB I I I 2 Δ R Δ R Δ R V = Δ R = V g R 2 g R I /V R R = = X EB Δ R V = V R .• Review from last lecture .• • •
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Source of Random Offset Voltages The random offset voltage is almost entirely that of the input stage in most op amps M 1 M 2 M 3 M 4 V 2 V X V S I T M 1 M 2 M 3 M 4 V DD V 1 V 2 V X V S I T V 1 V DD V OUT V OUT
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