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# Voltage Reference

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• Goal is to minimize sensitivity to temperature, process n voltage Definition: The fractional change in the reference voltage is equal to the sensitivity times the fractional change in the power supply voltage. For example, if the sensitivity is 1, then a 10% change in VDD will cause a 10% change in VREF.
• The voltage divider formed with two resistors has the advantage of simplicity, temperature insensitivity and process variations but the main problem with this circuit is that in order to reduce the power dissipation (i.e., the current through the resistors), the resistors must be made large. Since large resistors require a large area on the die, this voltage divider may not be practical in many cases. One situation where we will use this simple voltage divider is in generating a voltage halfway between VDD and ground, VDDI2, (sometimes called the common-mode voltage of an analog circuit or system). The voltage divider formed between the resistor and the MOSFET can be recognized as the same circuit used for a bias in the current mirror The final reference, a voltage divider between NMOS and PMOS devices, has the advantage that the layout can be small
• ### Voltage Reference

1. 1. Voltage Reference Sources Shankardas Deepti Bharat CGB0911002 VSD 537 M.Sc. [Engg.] in VLSI System Design Module Title: Analog and mixed signal circuit design Module Leader: Mr. Chandramohan P. Date: 2nd July 201202-07-2012 M. S. Ramaiah School of Advanced Studies 1
2. 2. Contents • Introduction • I-V curve for reference sources • Properties • Sensitivity • Types of voltage reference sources • Applications • Conclusion • References02-07-2012 M. S. Ramaiah School of Advanced Studies 2
3. 3. Introduction A Reference Circuit is an independent voltage or current source which has a high degree of precision and stability Requirements of a Reference Circuit •Should be independent of power supply •Should be independent of temperature Figure 1. Nominal values of a •Should be independent of processing variations reference source [Whelan 2009] •Should be independent of noise and other interference •Low area •Easy to Design02-07-2012 M. S. Ramaiah School of Advanced Studies 3
4. 4. I-V curves for ideal references Figure 2. I-V curves for ideal references [Kumar 2009]02-07-2012 M. S. Ramaiah School of Advanced Studies 4
5. 5. Sensitivity •Sensitivity is a measure of dependence of Vref upon a parameter or variable x which influences Vref ∂Vref Vref Vref Vref x ∂Vref ∂Vref ∂x S = ∂x = vref ∂x = S x Vref x x x Where x could be VDD or temperature VREF Ideally, we want S to be zero V DD02-07-2012 M. S. Ramaiah School of Advanced Studies 5
6. 6. Types of voltage references 1. Voltage Dividers – Active 2. MOS Diode Reference 3. PN Junction Diode Reference 4. Gate-Source Threshold Voltage Reference 5. Base-Emitter Reference Circuit02-07-2012 M. S. Ramaiah School of Advanced Studies 6
7. 7. Types of references (…Continued) • Reference voltages can be driven from the power supplies using the resistor and the MOSFET 1 2 3 4 Figure 3. Circuit diagrams of MOSFET – Resistance reference sources [Kumar 2009]02-07-2012 M. S. Ramaiah School of Advanced Studies 7
8. 8. Voltage references using voltage division Figure 5. Voltage references using voltage division [Allen 2009]02-07-2012 M. S. Ramaiah School of Advanced Studies 8
9. 9. MOSFET – Resistance Voltage references Figure 6. MOSFET – Resistance Voltage references [Allen 2009]02-07-2012 M. S. Ramaiah School of Advanced Studies 9
10. 10. Simulation waveforms Figure 4. Simulation waveforms for MOSFET – Resistance reference sources02-07-2012 M. S. Ramaiah School of Advanced Studies 10
11. 11. Applications • Error amplifier, • Comparator, • A/D and D/A converters Figure 7. Typical use of a voltage reference for an ADC [Whelan 2009]02-07-2012 M. S. Ramaiah School of Advanced Studies 11
12. 12. Conclusion • Voltage reference is used to establish absolute measurement accuracy. • Any reference error undermines the overall system accuracy. • Thus the external voltage reference must provide accurately set constant voltage, independent of load changes, temperature, input supply voltage and time.02-07-2012 M. S. Ramaiah School of Advanced Studies 12
13. 13. References • Brendan Whelan (2009) How to Choose a Voltage Reference [online] available from <http://cds.linear.com/docs/LT%20Journal/LTMag -V19N1-03-References-BrendanWhelan.pdf> [01st July 2012] • Jacob Baker, R. (2010) CMOS: Circuit Design, Layout and Simulation. 3rd edn. Wiley-IEEE • Phillip E. Allen., and Douglas R, Holberg. (2002) CMOS Analog Circuit Design. 2nd edn. Oxford University Press • Sougata Kumar (2009) Current Reference and Voltage Reference [online] availabale from <http://conf05.iitkgp.ac.in/avlsi/logf/ summercourse/ 2008/Current_Voltage_reference.pdf> [01st July02-07-2012 2012] M. S. Ramaiah School of Advanced Studies 13
14. 14. Thank You02-07-2012 M. S. Ramaiah School of Advanced Studies 14
15. 15. Remarks Sl. No. Topic Max. marks Marks obtained 1 Quality of slides 5 2 Clarity of subject 5 3 Presentation 5 4 Effort and question handling 5 Total 2002-07-2012 M. S. Ramaiah School of Advanced Studies 15