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

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Inadequate ConditioningCircuitry220 220-Input+ADC Input51+5V4.7k2K430 pF-5V42This simple circuit has a nominal gain of +2. The arm of the potentiometer is properlybypassed to prevent the gain from changing with a change of bias setting.The problems here are two: (1) amplifiers do not like being operated at low gain settings andtend to be a little unstable when forced to do so and (2) the input of most sampling ADCs is aswitched capacitor circuit, which produces current (and voltage) transients at the ADC input.Amplifiers do not like to drive such circuits. Either of these conditions lead to ringing, if notoscillation.The 4.7k-Ohm resistor seems a little on the large side, so you might expect that it will addnoise. While this value is larger than we might like to see, it probably is not as much of aproblem as the low gain and the driving of the switched capacitor input of the ADC.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray42Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved
Better Conditioning Circuitry22 220Input-47ADC Input220+22pF6247+5V4302K3.9 nF-5V43This is a much better solution, The amplifier is operated at a gain of about 11.5 (don’t forgetthe effect of the 430-Ohm resistor on gain!) and the input is padded down so that the overallgain from input to output is about 2. The amplifier is happy with the high gain, but take care towatch gain bandwidth requirements.The amplifier is decoupled from the ADC input with a simple RC. The capacitor is generallychosen such that its value is about ten times the ADC input capacitance when the ADC is inthe sample or track mode, then the resistor is chosen such that the pole frequency is thesample rate. When calculating the pole frequency, be sure to include the ADC inputcapacitance when the ADC is in the sample or track mode. This is for converters with inputfrequency less than half the sample rate.For undersampling applications, where the input frequency is higher than the sample rate,genereally we do not add an input capacitor, but we do use a series resistor of about 10 to 22Ohms.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray43Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved
Page 1 and 2: ABCs of ADCsAnalog-to-Digital Conve
Page 3 and 4: What Is an ADC?• Mixed-Signal Dev
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: Noisy Components/Circuitry• ADC I
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

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

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