Lec. 5:
Flash ADC (Part-II) Flash ADC Errors
Lecturer: Hooman Farkhani
Department of Electrical Engineering Islamic Azad University of Najafabad Feb. 2016.
Email: [email protected]
In The Name of Almighty
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Flash ADC sources of Errors
1. Errors related to Reference Ladder
Input-to-reference signal feedthrough
Resistor Mismatch
2. Errors related to the Sample & Hold
Sampling Aperture Error
3. Errors related to Comparators:
Offset Voltage in Comparators
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Errors related to the Reference Ladder
1. Input-to-reference signal feedthrough
To reduce signal-feedthrough Small resistors in ladder But leading to more power consumption
Ref: PhD dissertation of E. Sall
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1. Input-to-reference signal feedthrough
5 Res. Mismatch → Ref. Voltage Error; DNL
Errors related to the Reference Ladder
2. Resistor Mismatch
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Errors related to the Sample & Hold
Sampling Aperture Error
Aperture Delay
Aperture delay in an ADC is the interval between the sampling edge of the clo ck signal and the instant when the sample is taken.
Aperture Jitter
Aperture jitter (tAJ) is the sample-to-sample variation in the aperture delay, as shown in this figure. Typical ADC aperture jitter values are much smaller than those of aperture delay.
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Errors related to Comparators
● Offset Voltage in Comparators
• Ex: 10bit ADC, V
FS=2V → V
LSB=1.9mV
For 3σ(Vos) ≤ 1V
LSB→ σ(Vos) ≤ 0.65mV !
• σ(Vos)=10mV → V
LSB=30mV; Only 6bit ADC!
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Another Example for Comparator Mismatch
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Solution for Comparators Offset:
Use Pre-Amp to reduce Comp Offset by its Gain
Increases the Area & Power!
Pre-amp Offset will contribute too!
→ σ = (σ2Latch/A + σ2Amp)1/2
1. Pre-amp
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Solution for Comparators Offset (Cont.)
2. Averaging
Reduces INL by the Squared Number of Votes!
Reduces DNL by Number of Votes!!
Costs more Power & Area
Use Averaging among Pre-Amps to Reduce its own Offset
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Do a research on averaging technique.
Question 5:
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Refernces
Professor Boris Murmann Course slides 2012, Stanford University- EE315B course
