ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

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Agenda - ABCs of ADCs• What’s an ADC?• Review of Definitions• Sources of Distortion and Noise• Common Design Mistakes• High Speed ADCs at National2ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray2Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved
What Is an ADC?• Mixed-Signal Device– Analog Input– Digital Output• May be Considered to be a Divider– Output says: Input is What Fraction of V REF ?– Output = 2 n x G x A IN / V REF• n = # of Output Bits (Resolution)• G = Gain Factor (usually “1”)• A IN = Analog Input Voltage (or Current)• V REF (I REF )= Reference Voltage (or Current)3October 2001Because the Analog-to-Digital Converter (A/D Converter or ADC) has both analog and digitalfunctions, it is a mixed-signal device. Many of us consider the ADC to be a mysterious device.It can, however, be considered very simply to be the instrument that it is: a device thatprovides an output that digitally represents the input voltage or current level.Notice I said voltage or current. Most ADCs convert an input voltage to a digital word, but thetrue definition of an ADC does include the possibility of an input current.An ADC has an analog reference voltage or current against which the analog input iscompared. The digital output word tells us what fraction of the reference voltage or current isthe input voltage or current. So, basically, the ADC is a divider.The Input/Output transfer function is given by the formula indicated here. If you have seen thisformula before, you probably did not see the “G” term (gain factor). This is because wegenerally consider this to be unity. However, National Semiconductor has introduced ADCswith other gain factors, so it is important to understand that this factor is present.____________________________________________________________________________PLEASE NOTE: The discussion here assumes an ADC with a binary output. Some of thestatements here would be modified slightly for Offset Binary or 2’s Complement outputs.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray3Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved
Page 1: ABCs of ADCsAnalog-to-Digital Conve
Page 5 and 6: Least Significant Bit (LSB)andMost
Page 7 and 8: Quantization Error111Digital Output
Page 9 and 10: Offset Error111OUTPUT CODE110101100
Page 11 and 12: Gain Error (Full-Scale GainError)Ga
Page 13 and 14: DNL1111101010.3 LSB;DNL = -0.7Ideal
Page 15 and 16: Integral Non-Linearity (INL)orInteg
Page 17 and 18: Total Unadjusted Error101OUTPUT COD
Page 19 and 20: THD - Total HarmonicDistortionAnalo
Page 21 and 22: 1Signal-to-Noise and Distortion(SIN
Page 23 and 24: ENOB - Effective Number OfBits• E
Page 25 and 26: Input Dynamic RangeDynamic Range is
Page 27 and 28: Sources of Noiseand DistortionABCs
Page 29 and 30: Inadequate Supply Bypassing• Nois
Page 31 and 32: V A - DR Decoupling• ADC Outputs
Page 33 and 34: Output to Input Coupling• Output
Page 35 and 36: Noisy Components/Circuitry• ADC I
Page 37 and 38: Clock Noise• Clock Can Add Noise
Page 39 and 40: Common Design Mistakes• Inadequat
Page 41 and 42: Noisy Components/Circuitry• ADC I
Page 43 and 44: Better Conditioning Circuitry22 220
Page 45 and 46: Resistor Pack Danger500*V REF +V RE
Page 47 and 48: Clock Noise• Clock Can Add Noise
Page 49 and 50: Excessive Clock Jitter (cont’d)No
Page 51 and 52: Treating the Clock LineAs a Trace
Page 53 and 54:
A Better Reference CircuitLM4040-4.
Page 55 and 56:
High Speed ADCs FromNational: 10-bi
Page 57 and 58:
High Speed ADCs FromNational: 14 &
Page 59 and 60:
Other General Purpose ADCsFrom Nati
Page 61 and 62:
ADC Web Site• Site: www.national.
Page 63 and 64:
Absolute Maximum Ratings - Voltages
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ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

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