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Chapter10
Operational Amplifier Applications
Microelectronic Circuit Design
Richard C. Jaeger
Travis N. Blalock
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Chapter Goals
• Continue study of methods to determine transfer functions
of circuits containing op amps.
• Introduction to active filters and switched capacitor circuits
• Explore digital-to-analog converter specifications and
basic circuit implementations.
• Study analog-to-digital converter specifications and
implementations.
• Explore applications of op amps in nonlinear circuits, such
as precision rectifiers.
• Provide examples of multivibrator circuits employing
positive feedback.
• Demonstrate use of ac analysis capability of SPICE.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: Low-pass (Transfer Function)
The transfer function is: G G
1 2
CC
Vo(s)
1 2
A ( s) 

LP
Vs(s) 2 G  G G G
s s 1 2  1 2
C
CC
1
1 2
In standard form,
s2
A ( s) 

LP
• Op amp is voltage follower with
s2  s o  o2
Q
unity gain over a wide range of
C RR
1
o 
1 2
frequencies.
Q 1
RR CC
R R
C
1
2
1
2
• Uses positive feedback through C1 at
2 1 2
frequencies above dc to realize
complex poles without inductors.
• Feedback network provides dc path
for amplifier’s input bias currents.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Often, circuits are designed with C1 =
C2 = C.
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Active Filters: Low-pass (Frequency
Response)
For Q=0.71,magnitude response is
maximally flat (Butterworth Filter:
Maximum bandwidth without
peaking)
For Q>0.71, response shows
undesired peaking.
For Q<0.71: Filter’s bandwidth
capability is wasted.
At <<o, filter has unity gain.
At >>o,response exhibits twopole roll-off at 40dB/decade.
At =o, gain of filter =Q.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Sensitivity, S represents fractional
change in parameter, P due to a given
fractional change in value of Z.
Sensitivity of with respect to R and
C is:
S  S  1
R C
2
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Active Filters: Low-pass (Example)
• Problem: Design second-order low-pass filter with
maximally flat response.
• Given data: fH = 5 kHZ
• Analysis:C1 = 2C2 = 2C and R1 = R2 = R.
Q 1
2
2oC
1/oC is the reactance of C at o, R is 30% smaller than this value. Thus
impedance level of filter is set by C. If impedance level is too low, op amp
will not be able to supply current required to drive feedback network.
1
At 5 kHz, for a 0.01 mF capacitor, 1 
 3180W
oC 104 (10 8 )
3180W
R
 2250W
2
Final values: = R1 = R2 = 2.26kW, C1 = 0.02 mF, C2 = 0.01 mF
R
1
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: High-pass with Gain
(Transfer Function)
The transfer function is:
1
s2


A ( s) 
o

HP
RC
s2  s o  o2
Q
1



R C C
R C 

Q   1 1 2  (1 K ) 2 2 
 R
CC
R C 

2
1 2
1 1

•
•
Voltage follower in low-pass
filter replaced by non-inverting
amplifier with gain K, which
gives an added degree of
freedom in design.
dc paths for both op amp input
bias currents through R2 and
feedback resistors.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
For R1 = R2 = R and C1 = C2 = C,
1
o  1
Q

RC
3 K
For K=3, Q is infinite, poles are on
j axis causing sinusoidal
oscillations. K>3 causes instability
due to right-half plane poles.
1  K  3
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Active Filters: High-pass with Gain
(Frequency Response)
• For Q=0.71,magnitude response is maximally flat
(Butterworth Filter response).
• Amplifier gain is constant at >o, the lower cutoff
frequency of the filter.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: Band-pass (Transfer
Function)
Uses inverting op amp and its full loop gain
(ideally infinite).
V (s)
sC V (s)   o
2 1
R
2
 


G V   sC  C   G V (s)  sC Vo(s)
1
th th   1 2  th  1
R RC
Vo(s)
so
3
2 2
A ( s) 

BP
R  R R C s2  s o   2
V ( s)
1 3 1 1
th
o
Q
o 
1
R
CC
1 2
2
R R C C Q
th 2 1 2
R C1  C2
th
For C1 = C2 = C,
R
1
2 BW  2
o 
Q
C R R
R
R C
th 2
th
2
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: Band-pass (Frequency
Response)
• Response peaks at o and gain at center frequency is 2Q2.
• At <<o or >>o, filter response corresponds to
single-pole high-pass or low-pass filter changing at a rate
of 20dB/decade.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: Tow-Thomas Biquad
General biquadratic transfer function
to represent low-pass, high-pass,
band-pass, all-pass and notch filters:
a s2  a s  a
0
T ( s)  2  1
s2  s o  o2
Q
In Tow-Thomas biquad, first op amp
is a multi-input integrator and third
op amp is simply an inverter.

1
1 
1

V ( s)  
Vs ( s) 
V
(
s
)
V ( s)


bp
bp
lp
 sR C
sR C
sRC 
1
2
1
V ( s)  
V ( s)
lp
sRC bp
so
A ( s)   K

bp
s2  s o  o2
Q



R 
R BW  1
1

K  
o 
Q 2
R C
R
R 
RC
2
 1
o 2
A ( s)   K

lp
s2  s o  o2
Q
Thus, center frequency, Q
and gain can each be
adjusted independently.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: Complete Tow-Thomas
Biquad
• The Tow-Thomas Biquad can achieve all filter functions
with addition of extra passive components
as shown.


C 

R 


s
1
1
2

s  1     3  
 RR C 2
RR
 C  C  R



Vo(s)
5
 1

4
Av (s) 



Vs(s)


1
2
 R  1
s  s 

2C 2
 R  RC
R


 2
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Active Filters: Tow-Thomas Biquad
(Example)
•
•
•
•
Problem: Design band-pass filter using Tow-Thomas circuit
Given data: fo = 5 kHZ, BW = 200 Hz, midband gain =20
Unknowns: R, R1, R2, R3, C
Analysis: Q  fo 10
BW
Input resistance to the filter is set by R1.At the center frequency,
X  1  R  2R
C oC
1
Also, first op amp must supply ac signal current to parallel combination of
R, R2, C, second op amp must drive parallel combination of R3, C third must
drive R3 in parallel with R. If we choose C = 2700 pF,
R R
1
R 10R  294kW
R  2  14.7kW
R
 29.4kW
2
1 20 2
4000C
R3 can be chosen arbitrarily as long as it doesn’t load down second and
third op amps. R3 =49.9 kW
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Magnitude Scaling
• Magnitude of filter impedances may all be increased or
decreased by a magnitude scaling factor KM, without changing
o or Q of the filter.
• To scale the magnitude of the impedance of the filter elements:
R'  K
M
C
C' 
R
K
M
1
Z '
K Z
M C
C C '
Applying magnitude scaling to low-pass filter:
1
o '
K R  K
M 1 M
Q' 
C
1
K
M
C
2
K
M
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
C C
1
2
K
M M
R 
2K

1
o
RR CC
1 2 1 2
R  K R 
C
RR
M 1 M 2 
1 2 Q
1

K R K R
R

C 1 R2
M 1
M 2
2
K
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Frequency Scaling
• Cutoff or center frequencies of filter may be scaled by a
frequency scaling factor KF, without changing Q of the
filter if each capacitor value is divided by KF and resistor
values areR' left
 R unchanged.
C
C' 
K
F
Applying frequency scaling to low-pass filter:
K
1
F
o ' 

 K o
F
C C
RR CC
1
2
1
2
RR 1 2
1 2K K
F F
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Q' 
C
1
C
K
R R2
RR
1 
1 2 Q
F
1
R

R
R

C
C 1 R2
2 1 2
2
K
F
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Switched-Capacitor Circuits
• Switched-capacitor (SC) circuits eliminate resistors in
filters by replacing them with capacitors and switches.
• Resulting filters are discrete-time or sampled-data
equivalents of continuous-time filters discussed so far.
• Provide additional flexibility not readily available in
continuous-time form, such as inversion of signal
polarity without using an amplifier.
• SC circuits are compatible with high density MOS IC
processes.
• SC circuits provide low-pass filters and CMOS Ics for
signal processing and communications applications.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
SC Integrator
In phase 1, input voltage is sampled and
output is constant.In phase 2, output changes
to reflect sampled information in phase 1.
Charge stored in phase 1 is: Q1  C1Vs
Vs vs  n 1T  Voltage stored on C1 at


end of sampling interval
Q  C vo Change in charge stored
2
2
on C2 in phase 2.
C
vo   1 Vs
C
2
Output voltage at end of nth clock cycle is:
C
vo nT   vo (n 1)T  1 vs (n 1)T 
C
2
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Equivalence Between SC Integrator and
Continuous Time Integrator
Consider total charge Qs flowing from source vs through resistor R
during clock period T.
Vs
Qs  IT  T
R
Equating this charge to charge stored on C1
Vs
T  C Vs
1
R
R
T
1

C f C
1 C 1
fC is clock frequency.
For a capacitance of 1 pF and switching frequency of 100 kHz,
equivalent resistance is 10 MW which is too large for IC realization.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Noninverting SC Integrator
In phase 1, input voltage is sampled and
output is constant. In phase 2, output
changes to reflect sampled information in
phase 1.
C
vo   1 Vs
C
2
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Stray-Insensitive SC Circuits
In phase 1, source is connected to summing
junction of op amp, charge C1Vs is delivered
to C2.Node 1 is driven by and node 2 is kept
at zero. v   C1 V
o
C s
2
In phase 2, source is disconnected, output is
constant and C1 is totally discharged. Any
stray capacitances at nodes 1 or 2 don’t
introduce errors into charge transfer process.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Switched-Capacitor Band-Pass Filter
R T
R T
th C
2 C
3
4
C C
1 C3C4
o 
 fc 3 4
T CC
CC
1 2
1 2
C
CC
3
1 2
Q
C C C
4
1 2
Center frequency is tunable just by changing
clock frequency, Q is independent of
frequency.But, SC filters are sampled-data
systems, hence, f  f c due to sampling theorem.
2
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Switched-Capacitor Tow-Thomas
Biquad
• Ability of SC circuits to change polarities without an
amplifier eliminate one op amp in the SC implementation.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Digital-to-Analog (D/A) Converters:
Fundamentals
• In a DAC, an n-bit binary input word (b1,b2,…bn) is combined
with reference voltage VREF to give output of the DAC.
b 1,0
i
vo V (b 21  b 2 2  ...  bn 2 n ) V
FS 1
2
OS
• Full-scale voltage VFS is related to VREF of the converter
V by KV
REF
FS
where K determines converter gain commonly set to 1.
• VOS, the offset voltage of the converter characterizes the DAC
output when the digital input code is zero.Offset voltage is
normally adjusted to zero.
• The smallest voltage change at DAC output occurs when the
LSB bn in the digital word changes from a 0 to 1 is also called
resolution.
V
 2 nV
LSB
FS
• b1, the MSB has a weight of one-half VFS.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
D/A Converter Specifications: Offset
and Gain Errors
• Maximum output of ideal converter is
•
•
•
•
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
always 1 LSB smaller than VFS.
For shown ideal DAC characteristic,
0.875 VFS corresponds to maximum
output code of 111.
Gain error of converter represents
deviation of slope of converter
transfer function from that of
corresponding ideal DAC.
Shown ideal DAC has been calibrated
so that VOS =0 and 1 LSB is VFS /8.
Offset voltage is output of converter
for zero binary input code.
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D/A Converter Specifications: Linearity
Errors
• Overall linearity error is
magnitude of largest error that
occurs.Good converter has
linearity error<0.5 LSB
• Differential linearity error is
magnitude of maximum
difference between each output
step of converter and ideal step
size of 1 LSB.
• Integral linearity error or
• Integral linearity error for a
linearity error measures
given binary input is the sum
deviation of actual converter
(integral) of differential
output from straight line fitted to
linearity errors for inputs up
converter output voltages,
through the given input.
specified as a fraction of LSB.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
D/A Converter Specifications:
Monotonicity
• If the output of the DAC does
not increase in a monotonic
manner if the input code is
increased, the DAC is said to
be nonmonotonic.
• It is possible for a monotonic
converter to have a differential
linearity error >1 LSB but, a
nonmonotonic converter
always has a differential
linearity error > 1 LSB
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Weighted-Resistor DAC
Drawbacks:
• Need to have accurate
resistor ratios over a wide
range of resistor values.
• Switches in series with
resistors require zero offset
voltage and low onBinary input data controls the switches.
resistance.
Successive resistors are progressively
• Current drawn form
weighted by a factor of 2 producing:
reference varies with input
vo V
(b 21  b 2 2  ...  bn 2 n )
pattern causing change in
REF 1
2
voltage drop in Thevenin
Linearity errors arise due to improper
equivalent source resistance
resistor ratios, op amp offset voltage adds
of reference leading to datato VOS of converter.
dependent errors called
superposition errors.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
R-2R Ladder
•
• Avoids weighted-resistor DAC
problem of wide range of resistor
values.
• Well-suited to IC realization as it
requires matching of only two
resistor values, R and 2R.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
The contribution of
each bit is reduced by a
factor of 2 going from
MSB to LSB
Drawbacks:
• Requires switches with
low on-resistance and
zero offset voltage.
• Current drawn from
reference varies
depending on input data
pattern.
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Inverted R-2R Ladder
• Currents in ladder and reference are independent of
digital input.
• Complementary currents are available at output of
inverted ladder.
• Switches still need to have low on-resistance to minimise
errors.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Inherently Monotonic DAC
• Analog switch tree connects
desired tap to input of an op
amp operating as a voltage
follower.
• Each tap on resistor network is
forced to produce voltage grater
than or equal to the taps below it
, forcing the output to increase
monotonically as the digital
input code increases.
• An 8-bit version requires 256
equal-valued resistors, 510
switches and additional
decoding logic.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Switched-Capacitor DACs
• Since circuits consist only of
capacitors and switches, static
power dissipation occurs only in
the op amps.
• Dynamic switching losses occur.
• When switch changes state,
current impulses charge/discharge
network capacitos, changing
voltage on feedback capacitor by
an amount corresponding to bit
weight of switch that changed
state.
• Circuits represent direct SC
analogs of weighted-resistor and
R-2R ladder DACs.Consume
very less power.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
DACs in Bipolar Technology
Bipolar transistors aren’t good voltage switches due to their
inherent offset voltage in saturation, but, are very good
current sources and switches.
•
Currents switched into summing junction, supplied through RF,
determine output voltage of DAC.
• VBE of current-source transistors must be same for proper weighting
of current sources, requiring equal current densities in the
transistors. Thus, area of each transistor is raised by factor of 2
from LSB to MSB.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
DACs in Bipolar Technology (contd.)
Several methods can be used to
overcome problems due to unmanageable
transistor and resistor ratios in bipolar
implementations of DACs- splitting
current sources into groups with proper
ladder termination, using R-2R ladder to
generate weighted current sources,
driving R-2R ladder by equal-value
current sources.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Reference Current Circuitry for Bipolar
Implementations of DACs
An op amp is used to set the current
in the reference transistor.
Bipolar transistor and resistor ratio
matching determine currents in rest
of current-source network.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Analog -to-Digital (A/D) Converters:
Fundamentals
• ADC takes unknown continuous analog input signal, mostly
voltage vX and converts it into n-bit binary number, which is a
binary fraction representing ratio between unknown input
voltageVandfull-scale
voltage
KV
REF
FS
• For given output code, we know that value of input voltage lies
within a 1-LSB quantization interval.
Quantization error occurs due
to initial underestimation and
then overestimation of input
voltage by output code.
Ideal ADC should pick values of
bits in binary word to minimise
magnitude of quantization error:
v  v  (b 21  b 2 2  ...  bn 2 n )V
X
1
2
FS
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
A/D Converter Specifications: Offset
and Gain Errors
• Differential linearity error is
•
difference between actual code step
width and ideal step size of 1 LSB.
• ADC with differential linearity error<
1LSB has no missing codes.
• Gain error is deviation of slope of
converter transfer function from that
of corresponding ideal ADC.
• Shown ADC characteristic has offset
Integral linearity error or
error of 0.5LSB first transition
linearity error is deviation of code occurs at a 0.5LSB higher voltage.
transition points from their ideal
• Good ADC is monotonic with
positions. Integral linearity error is linearity error<0.5LSB and no
sum of differential linearity errors missing codes over full temperature
for individual steps.
range.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
A/D Conversion Techniques
• If vX > vREF, output voltage is high corresponding to logic 1.
• If vX < vREF, output voltage is high corresponding to logic 0.
vREF is time-dependent reference voltage, varied till unknown
input is determined within quantization error of converter.
Ideally ADC logic chooses bi so that
n
i V
v
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
V
 b 2
X FS i 1 i
 FS
2n 1
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Counting ADC
n-bit DAC used to generate any one of 2n
outputs by applying appropriate digital input
word. vX determined by sequentially
comparing it to each possible DAC output.
• Maximum conversion time occurs for fullscale input signal requiring 2n clock periods.
• Binary value in counter is smallest DAC
voltage larger than unknown input, not the
DAC output closest to unknown input.
• If input varies, binary output is accurate
representation of input signal value at the
instant the comparator changes state.
• Requires minimum amount of hardware,
inexpensive to implement.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Successive Approximation ADC
•Binary search used by SAL to determine vX.
•n-bit conversion needs n clock periods. Speed limited
by time taken by DAC output to settle within a fraction
of an LSB of VFS and by comparator to respond to input
signals differing by small amounts.
•Slowly varying input signals not changing by more than
0.5 LSB (VFS /2n+1 ) during conversion time (TT = nTC)
are acceptable.
•For a sinusoidal input signal with p-p amplitude= VFS,
fo 
fc
2n2n
•To avoid this frequency limitation, high speed sampleand-hold circuit is used ahead of the successive
approximation ADC.
•Very popular ADC with fast conversion times, used in
8- to 16- bit converters
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Single-Ramp (Single-Slope) ADC
•Reference voltage varies linearly with a welldefines slope from slightly below 0 to above VFS
v
N
v  KNT  X  n if K= VFS / 2nTC.
X
C V
2
FS
•Maximum conversion time occurs for fullscale input signal requiring 2n clock periods.
•Counter output is value of vX at the time endof-conversion signal occurs.
•Ramp voltage can be generated using an
integrator connected to a constant reference
voltage.
•Dependence of ramp’s slope on RC product
which is susceptible to changes due to
temperature variations or aging is a limitation of
this ADC.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Dual-Ramp (Dual-Slope) ADC
T  2nT
1
C
T  NT
2
C
T
T T
1
1
1 1 2
 v x (t )dt 
VREF (t )dt
RC 0
RC T
1
T
vx
N

 2 n
T 2
V
REF 1
•Absolute values of R and C don’t affect
operation.
•Digital output word gives average value of
vX during first integration phase.
•Conversion time is given by:
T  (2n  N )T  2n1T
T
C
C
•Can be used to get resolutions exceeding 20
bits but at lower conversion rates.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Dual-Ramp (Dual-Slope) ADC (contd.)
• Integrator in dual-ramp ADC has the
shown normalised transfer function.
• Sinusoidal inputs with frequencies that
are exact multiples of 1/T1, have
integrals of zero and don’t appear at
integrator output. This property is
called normal-mode rejection.
• Recent dual-slope ADCs include extra
integration phases for automatic offset
elimination.
• Triple ramp ADC uses coarse and fine
down ramps to improve speed by
factor of 2n/2 for n-bit dual-ramp
converter.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Parallel or Flash ADC
• Unknown input simultaneously compared
to 7 different reference voltages (3-bit
converter). Logic network encodes
comparator outputs into 3-bit binary
output representing quantized value of
input voltage.
• Very fast speed ( up to 108-109
conversions/sec), limited only by delays of
comparators and logic network.
• Output continuously reflects input delayed
by comparator and logic network.
• Requires 2n-1 comparators and reference
voltages for n-bit conversion. Used for
resolutions up to 10 bits.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Delta Sigma ADCs
Feedback loop attempts to force
integrator output to zero.
V
V
Called oversampled because internal
ADC samples integrator output at 16
to 512 times Nyquist rate.Digital
filter produces higher resolution.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
 MT

C
X  RC


 NT
 V

C

REF  RC


V

(2m  2 N )
  REF
m
X
2 

 (M


 V

REF 


 N )T 
C 0

RC

If M=2m
N/M is average value of binary bit
stream at output.LSB is VREF= 2m
Effective resolution is determined by
the time for which the output is
averaged.
Converter operation is considerably
complex for time-varying input
signals.
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Delta Sigma ADCs: SC Implementation
• Continuous-time integrator can be
replaced by SC integrator for low-power
operation.
• Charge proportional to input is added to
integrator output at each sample time and
charge given by CVREF is added or
subtracted at each sample depending on
control sequence applied to switches.
• Delta-Sigma ADCs are inherently linear
due to the 1-bit DAC.
• SC integrator suffers lesser from jitter
than continuous-time integrator as long as
the clock interval is long enough for
complete charge transfer to finish
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Nonlinear Circuit Applications:
Precision Half-Wave Rectifier
• For vS >0, vO = vS, i>0, diode is forward•
•
•
•
vO is rectified replica of vS
without loss of voltage drop
as in diode rectifier circuit.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
biased and feedback loop is closed.
Rectification is perfect even for small
input voltages..
For vS <0,diode is cutoff, i=0, vO=0.
Primary sources of error are gain error
and offset error due to nonideal op amp.
For negative input voltages, output
voltage v1 is saturated at negative limit.
Large negative voltages across input can
destroy unprotected op amps.
Response time of circuit is slowed down
due to slow recovery of internal circuits
from saturation.
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Nonlinear Circuit Applications: NonSaturating Precision Half-Wave
Rectifier
• For vS >0, v1 is negative (one
diode-drop below zero), D2 is
forward biased, current in R2 is
zero, vO = 0, D1 is reverse biased.
Feedback loop is closed through
D2.
• For vS <0, v1 is one diode-drop
above output voltage, diode D1
turns on, D2 is off. Circuit
behaves as inverting amplifier
with gain - R2 / R1. Feedback
loop is closed through D1 and R2.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Nonlinear Circuit Applications: AC
Voltmeter
For a sinusoidal input of amplitude
VM and frequency o, output is a
rectified sine wave given by its
Fourier series. If cutoff frequency
of low-pass filter c << o, output
consists primarily of dc voltage
component.
R  R V 
vo  4  2 M 
R R

3 1

Half-wave rectifier is combined
with low-pass filter to form basic
ac voltmeter.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Voltmeter range can be adjusted
through the 4 resistors.
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Circuits with Positive Feedback:
Comparator
• For inputs>VREF,output saturates at VCC.
• For inputs<VREF,output saturates at -VEE.
• Amplifiers built for use as comparators
can handle saturation at the voltage
extremes without incurring excessive
internal time delays.
• For noisy inputs, multiple transitions
may occur as input signal crosses
reference level.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Schmitt Trigger
• Schmitt trigger uses positive feedback
and is bistable.
• For positive output
voltages,VREF=bVCC. For positive
output voltages,VREF=-bVEE.
Reference level changes when output
changes state.
• Voltage transfer characteristic exhibits
hysteresis and doesn’t respond to
noise voltage magnitude smaller than
the difference between the 2 threshold
levels set by the reference voltage
Vn  b V  (V )   b (V V )
EE

2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
CC
EE

CC
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Astable Multivibrator
• Uses positive and negative
feedback to generate
rectangular output.
• Output voltage switches
periodically between VCC and
-VEE.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
For symmetrical power supplies,
output of circuit is square wave with
period T
1 b
T  T  T  2RC ln
1 2
1 b
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Astable Multivibrator (contd.)
• Astable multivibrator can be used to generate square, triangular
and sine wave outputs as shown at frequencies up to few MHz.
• Frequency is varied by changing R3 or C3, C3 is changed in
decade steps, R3 may be varied continuously using
potentiometer.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Monostable Multivibrator or One Shot
• Operates with one stable state,
generates single pulse of known
duration on application of trigger
signal.
• D1 couples trigger signal into circuit,
clamping diode D2 limits negative
voltage excursion on capacitor C.
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Monostable Multivibrator (contd.)
• Output of circuit consists of positive pulse with fixed


duration T given by
 V

1  D 
V



CC

T  RC ln 
1 b
• For well-defined pulse, circuit should not be triggered till
voltages on all nodes return to their quiescent steady-state
values.


• Recovery time (return of circuit
V

 CC 
1 b 

to state before trigger pulse was
V



 EE 
applied) is given by:
Tr  RC ln
1
2 Microelettronica – Circuiti integrati analogici 2/ed
Richard C. Jaeger, Travis N. Blalock







V
D
V
EE







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