KICKBACK NOISE ANALYSIS OF LOW POWER COMPARATOR

IJSRD - International Journal for Scientific Research & Development| Vol. 2, Issue 10, 2014 | ISSN (online): 2321-0613
All rights reserved by www.ijsrd.com 452
Kickback Noise Analysis of Low Power Comparator
Rangoli Mittal
M.Tech Student
Department of Electronics and Communication Engineering
Ajay Kumar Garg Engg. College, Ghaziabad
Abstract— The need for ultra low-power, area efficient, and
high speed analog-to-digital converters is pushing toward
the use of dynamic regenerative comparators to maximize
speed and power efficiency. In this report, an analysis on the
delay of the dynamic comparators will be presented. Based
on the presented analysis, a new dynamic comparator is
proposed, where the circuit of a conventional doubletail
comparator is modified for low-power and fast operation
even in small supply voltages. Comparators presented in this
paper designed with 180nm technology file. The power
consumption and delay has been modified with tradeoff in
area.
Key words: comparator, low power
I. INTRODUCTION
COMPARATOR is one of the fundamental building blocks
in most analog-to-digital converters (ADCs). Many high
speed ADCs, such as flash ADCs, require high-speed, low
power comparators with small chip area. High-speed
comparators in ultra deep sub micrometer (UDSM) CMOS
technologies suffer from low supply voltages especially
when considering the fact that threshold voltages of the
devices have not been scaled at the same pace as the supply
voltages of the modern CMOS processes .Hence, designing
high-speed comparators is more challenging when the
supply voltage is smaller. In other words, in a given
technology, to achieve high speed, larger transistors are
required to compensate the reduction of supply voltage,
which also means that more die area and power is needed.
To overcome the problem of low supply voltages
offset cancellation technique and kickback noise reduction
techniques can be used. These techniques will improve the
power consumption but increase the area. So there always
found tradeoffs between area and power. Fig1 shows the
schematic of conventional comparator.
Inn Inp Out
Low Low Low
Low High High
High Low High
High High Low
Table 1: Truth Table Of Comparator
Fig. 1(a): Schematic diagram of conventional dynamic
comparator.
Fig.1(b):Waveforms of Conventional dynamic comparator
II. PROBLEM ASSOCIATED
Offset voltage is a main problem while designing a low
power comparator. Offset cancellation techniques are used
to reduce the effect. The reduction of feature size leads to a
larger mismatch of transistors which leads to offset voltage
generation. Some offset cancellation techniques are:
 Digital calibration technique, which calibrates the
comparator offset through unbalancing the output
load of the comparator using arrays of MOS
capacitors at each output. The number of MOS
capacitors increases proportional to the increase in
the resolution, thus affecting the speed of the
comparator and calibration accuracy.
 Digital offset calibration technique, based on bulk
voltage trimming is presented. However, instead of
using extra power supply and a resistor ladder to
generate the variable bulk voltage, a bulk voltage
generator is proposed which employs simple logic
and binary-scaled capacitor array.
Another main problem is kickback noise. The large
voltage variations on the regeneration nodes are coupled,

(IJSRD/Vol. 2/Issue 10/2014/104)
through the parasitic capacitances of the transistors, to the
input of the comparator. Since the circuit preceding it does
not have zero output impedance, the input voltage is
disturbed, which may degrade the accuracy of the converter.
These disturbances usually called kickback noise.
Techniques for Kickback noise reduction are:
 Neutralization Technique
 Insertion of MOS switches
III. WORKING OF DYNAMIC COMPARATOR
A. Conventional Comparator:
The schematic diagram of the conventional dynamic
comparator widely used in A/D converters, with high input
impedance, rail-to-rail output swing, and no static power
consumption is shown in Fig. 1
B. Double tail dynamic comparator:
A conventional double-tail comparator is shown in Fig. 2 .
This topology has less stacking and therefore can operate at
lower supply voltages compared to the conventional
dynamic comparator. The double tail enables both a large
current in the latching stage and wider Mtail2, for fast
latching independent of the input common-mode voltage
(Vcm), and a small current in the input stage (small Mtail1),
for low offset.
Fig. 2(a): Double Tail Dynamic Comparator
Fig. 2(b): Waveforms obtained for Double Tail Dynamic
Comparator
IV. SIMULATION RESULTS
DELAY POWER Itail
BASIC COMPARATOR 10µs 63µW 0.88µA
DOUBLE TAIL
COMPARATOR
20µs 16.1µW 3.6µA
A. Kickback Noise:
Principally in latched comparators, the large voltage
variations on the regeneration nodes are coupled, through
the parasitic capacitances of the transistors, to the input of
the comparator. Since the circuit preceding it does not have
zero output impedance, the input voltage is disturbed, which
may degrade the accuracy of the converter. This disturbance
is usually called ―kickback noise‖ it has been proved that the
fastest and most power efficient comparators generate more
kickback noise. Fig.3(a) shows the waveforms of Kickback
Noise for Double Tail Comparator. Fig.3(b) shows the
waveforms of Kickback Noise for Cascaded Double Tail
Comparator.
Kickback noise is a very crucialterm in ase of
dynamic comparators. It is notthing but the varations come
in the input node after latching. When complexity of circuit
incresed then kickback noise also increased. There are some
techniques to reduce this but this can not be removed
permanntly from he circuitary. Table 3.3 shows the
Comparison Of Kickback Noises of different comparators.
Table 3.4 shows the analysis of Cascaded Double Tail
Comparator based on different parameters.
B. Comparison Of Kickback Noise:
COMPARATOR’S STYLE KICKBACK NOISE
Conventional comparator 70mV
Double Tail Comparator 43mV
Cascaded Double Tail Comparator 50mV
Fig. 3(a): Kickback Noise for Double Tail Comparator

(IJSRD/Vol. 2/Issue 10/2014/104)
Fig. 3(b): Kickback Noise for Cascaded Double Tail
Comparator
V. CONCLUSION
It is concluded that the change in different cascading
methods result in performance improvement or degradation.
Different methods work on different parameters to give the
tradeoff between theoretical performance evaluation and
simulation or practical implementation..
Delay is always inversly proportional to the tail
current, depends on switching capabi;ity of the device.
REFERENCES
[1] M. Maymandi-Nejad and M. Sachdev, ―1-bit
quantiser with rail to rail input range for sub-1V ∑
modulators,‖ IEEE Electron. Lett., vol. 39,no. 12,
pp. 894–895, Jan. 2003..
[2] A. Nikoozadeh and B. Murmann, ―An analysis of
latched comaprator offset due to load capacitor
mismatch,‖ IEEE Trans. Circuits Syst. II, Exp.
Briefs, vol. 53, no. 12, pp. 1398–1402, Dec. 2006..
[3] P. M. Figueiredo and J. C. Vital, ―Kickback noise
reduction techniques for CMOS latched
comapartors,‖ IEEE Trans. Circuits Syst. II, Exp.
Briefs, vol. 53, no. 7, pp. 541–545, Jul. 2006.
[4] Samaneh Babayan Mashhadi and Reza Lotfi ―
Analysis and Design of a Low Voltage Low Power
Double Tail Comparator‖ IEEE Trans.,Jan.2013.

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