1. Sample and Holds & Voltage References
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2. S&H performance parameters
Sampling pedestal or hold step
Signal feed-through
Speed
Droop rate
Aperture jitter or aperture uncertainty
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3. Basics of MOS S&Hs
The simplest S&H:
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4. Basics of MOS S&Hs
Reducing the impact of charge injection:
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5. Basics of MOS S&Hs
Non-zero rise and fall times:
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6. A S&H with feedback
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7. A track and hold with enhanced slewing behavior
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8. Another S&H circuit
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9. Another S&H circuit
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10. A high speed S&H
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11. Another S&H
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12. Another S&H
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13. An accurate but not very fast
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14. A S&H with increased accuracy
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15. Another T/H
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16. A S&H with low pass filter
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17. Basics of bandgap voltage reference
Making a stable reference voltage:
Using zener diode
Using the difference between the threshold voltages of enhancement and depletion transistors
Cancelling the negative TC of a pn junction with a positive TC from a PTAT
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18. Basics of bandgap voltage reference
The bandgap reference:
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19. Basics of bandgap voltage reference
Example: If the current density ratio is 10:1 then:
300OK
@ 301OK: DVBE=59.5*301/300=59.7mV
So, TCDVBE=0.198mV/OK
Hence, in order to cancel the TC of a diode voltage drop, DVBE should be amplified by a factor of 10.
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20. Basics of bandgap voltage reference
We assume that in a bandgap circuit the junction currents are proportional to temperature, i.e. Ji/Ji0=T/T0.
The reference voltage obtained from a bandgap is:
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21. Basics of bandgap voltage reference
Setting the above equation to zero at a reference temperature T0 leads to:
At T=300OK and for m=2.3, Vref,0=1.24V.
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22. Basics of bandgap voltage reference
Note that Vref is independent of the ratio of the current density, however, K should be chosen accordingly:
The reference voltage at any other temperature is:
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23. Basics of bandgap voltage reference
Example: If a bandgap reference generator is calibrated at 20OC, what would be the TC at 0OC?
In practice TC is about 4 to 10 times larger.
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24. BJT bandgap references
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25. BJT bandgap references
Note that all currents are proportional to absolute temperature (PTAT) (assuming R2 is temperature independent):
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26. BJT bandgap references
A bipolar bandgap with a voltage larger than 1.24V:
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27. CMOS bandgap references
In an n-well (p-well) CMOS process vertical pnp (npn) transistors can be implemented:
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28. CMOS bandgap references
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29. CMOS bandgap references
Typically the BJTs are taken to have the same sizes but R1 and R3 are different:
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30. Sources of error Sources of error in a bandgap reference:
Non-zero TC of resistors
Inherent TC of bandgap voltage:
TC of VG0:
Opamp offset
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31. Sources of error
SC circuits can reduce the impact of offset.
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