Sample and Holds & Voltage References 4/8/2019

S&H performance parameters Sampling pedestal or hold step Signal feed-through Speed Droop rate Aperture jitter or aperture uncertainty 4/8/2019

Basics of MOS S&Hs The simplest S&H: 4/8/2019

Basics of MOS S&Hs Reducing the impact of charge injection: 4/8/2019

Basics of MOS S&Hs Non-zero rise and fall times: 4/8/2019

A S&H with feedback 4/8/2019

A track and hold with enhanced slewing behavior 4/8/2019

Another S&H circuit 4/8/2019

Another S&H circuit 4/8/2019

A high speed S&H 4/8/2019

Another S&H 4/8/2019

Another S&H 4/8/2019

An accurate but not very fast 4/8/2019

A S&H with increased accuracy 4/8/2019

Another T/H 4/8/2019

A S&H with low pass filter 4/8/2019

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 4/8/2019

Basics of bandgap voltage reference The bandgap reference: 4/8/2019

Basics of bandgap voltage reference Example: If the current density ratio is 10:1 then: DVBE=59.5mV @ 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. 4/8/2019

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: 4/8/2019

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. 4/8/2019

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: 4/8/2019

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. 4/8/2019

BJT bandgap references 4/8/2019

BJT bandgap references Note that all currents are proportional to absolute temperature (PTAT) (assuming R2 is temperature independent): 4/8/2019

BJT bandgap references A bipolar bandgap with a voltage larger than 1.24V: 4/8/2019

CMOS bandgap references In an n-well (p-well) CMOS process vertical pnp (npn) transistors can be implemented: 4/8/2019

CMOS bandgap references 4/8/2019

CMOS bandgap references Typically the BJTs are taken to have the same sizes but R1 and R3 are different: 4/8/2019

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 4/8/2019

Sources of error SC circuits can reduce the impact of offset. 4/8/2019