Applications for the NE521/522

AND8176/D
Applications for the
NE521/522
ON Semiconductor
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APPLICATION NOTE
Comparators
Voltage comparators are high gain differential input--logic
output devices. They are specifically designed for
open--loop operation with a minimum of delay time.
Although variations of the comparator are used in a host of
applications, all uses depend upon the basic transfer
function. Device operation is simply a change of output
voltage dependent upon whether the signal input is above or
below the threshold input.
Comparator inputs are customarily marked with plus or
minus signs to indicate their polarity. For example, the
circuit of Figure 1 produces a logic 1 level when the
non--inverting input is more positive than the reference
voltage.
VREF
RIN
VSIGNAL
-+
than linear. Hence, input offset voltage is defined for
comparators as the DC voltage required at the input to force
the output to the logic threshold of ensuing devices (1.2 V
for TTL).
Input Offset Current
Imbalances of input bias current arise from small
variances of the junction geometry of the differential input
amplifier. As for op amps, the imbalance is referred to as
input offset current.
Bias Current
As with op amps the input structure of comparators is
usually a differential bipolar stage. Input bias current is the
average of the two input currents.
Common--Mode Range
LOGIC
OUTPUT
When specifying voltage comparators, one of the key
parameters is common--mode range, which is defined as the
range of voltages over which both inputs can be varied
simultaneously without abnormal output voltage transitions
or device degradation. This parameter must be kept
uppermost in the designer’s mind because the reference and
signal voltages become common--mode signals at threshold.
All ranges of input signals thus must be within the
common--mode range of the input amplifier.
RIN
Figure 1. Basic Comparator Circuit
Definitions
Many similarities exist between operational amplifiers
and the amplifier section of voltage comparators. In fact, op
amps can be used to implement the comparator function at
low frequencies.
Thus, the characteristic definitions presented here are
similar to those reviewed for op amps.
Voltage Gain
Specifications of voltage gain refer to the overall gain of
the device, the bulk of which occurs in the amplifier section.
In general, higher gains would be advantageous for
resolving smaller input signals. Of course, the propagation
delay suffers due to the more severe saturation of the
transistors. Typical gains for TTL output devices are set for
5000 V/V. This gain provides 5.0 V of output swing with
1.0 mV input signal change for reasonable accuracy, but
does not contribute severely to the overload recovery delay.
Input Offset Voltage
As with operational amplifiers, the non--ideal comparator
possesses some offset voltage. The definition differs slightly
in that the output structure of comparators is digital rather
© Semiconductor Components Industries, LLC, 2005
November, 2005 -- Rev. 0
1
Publication Order Number:
AND8176/D
AND8176/D
OUTPUT VOLTAGE (V)
Propagation Delay
INPUT VOLTAGE (mV)
Voltage comparisons of analog signals with a reference
voltage usually require that the operation take as little time
as possible. Long delays in the comparator cause a pulse
position error at the output since the analog signal in the
meantime has changed value. At low frequencies the delay
is of small consequence, but at higher frequencies, transit
time becomes intolerable. Design of voltage comparator
devices includes, as a prime goal, the minimizing of transit
times.
Propagation delay testing is done under worst--case
conditions. The recovery from saturation varies, depending
upon the initial state of the amplifier and the overdrive.
Worst--case conditions begin by applying a 100 mV signal
on the reference terminal. With no signal applied, the
amplifier is in saturation in one direction. A step input pulse
on the signal line of 100 mV ±VOS will bring the amplifier
to a threshold level. Propagation delay at this point is
undefined since the output has not switched.
To attain output switching, a small overdrive is necessary.
Propagation delay is tested in a configuration such as
Figure 2. The input is a step function of 100 mV plus a
specified excess or overdrive signal. This causes the
amplifier to be exercised from saturation in one direction to
saturation in the other for worst--case propagation delay.
Note that larger overdrive reduces delay time as can be seen
in Figure 3. An overdrive of 5 mV causes 12 ns delay,
whereas a 100 mV overdrive improves transit time to only
6 ns.
If the measurement were made without initial saturation
(less than 100 mV/V threshold) the delay time would be less,
due to the decreased storage times of unsaturated transistors.
100mV PLUS
OVERDRIVE
--
100mV
20mV
2
5mV
THRESHOLD
1
10mV
0
OVERDRIVE
100
50
0
5
10
15
20
25
30
TIME -- nS
Figure 3. Response Time for NE521 Comparator
for Various Input Overdrives
State--of--the--Art
Comparator design has always been optimized for four
basic parameters. They are:
1. High Speed
2. Wide Input Voltage Range
3. Low Input Current
4. Good Resolution
Unfortunately, these four parameters are not compatible.
For instance, gain and input current can be improved by
using thinner diffusions for higher beta, but only at the
expense of input voltage range. Higher gain also means
higher saturation for an increase in delay time. So it becomes
obvious that older comparators such as the 710 were
designed with the best compromises in mind using standard
processing.
One method of improving overall response adds gold
doping to the processing flow. The gold dopant causes a
decrease in minority carrier lifetime which aids the
recombination process and shortens the saturation recovery
time. Unfortunately, the transistor beta is adversely affected
by gold, causing slightly higher bias and offset currents.
It was not until the advent of the Schottky clamp that a vast
improvement in speed without input degradation was
possible. A very familiar term in the semiconductor
industry, the Schottky Barrier Diode’s (SBD) location is
illustrated in Figure 4.
-+
3
0
+
SIGNAL
VS = +5V
TA = 25oC
4
VTH
100mV
Figure 2. Propagation Delay Test Setup
Figure 4. Schottky Clamped Transistor
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Comparing the Comparators
Presently available comparator ICs range from the ultra
fast SE/NE521 to the general purpose comparator fashioned
from an inexpensive op amp. Selection of the device
depends upon the application in which it will be used. Speed
of conversion is often of primary importance to minimize
pulse position errors of high frequency signals. At other
times the requirements are much less stringent, allowing the
use of a general purpose comparator. A handy reference
guide to the major parameters is summarized in Table 1. The
necessary parameters can be chosen to select the proper
device.
A general description of the comparator devices is
included here to familiarize the user with available devices
and their advantages.
The Schottky clamped transistor is formed by paralleling
the Schottky diode with the base--collector junction of the
NPN transistor. Without the clamp, as base drive is increased
the collector voltage falls until hard saturation occurs. At
this point the collector voltage is very near the emitter
voltage, and stored charges in the junctions causes slow
recovery from saturation after base drive has been removed.
The forward voltage drop of the Schottky diode is 0.4 V--less
than the forward drop of silicon diodes. This difference in
forward drop is used by placing the diode across the
transistor base--collector junction.
The Schottky diode becomes forward--biased when the
collector voltage falls 0.4 V below the base voltage. Excess
base drive is then shunted into the collector circuit,
prohibiting the transistor from reaching classic saturation.
With almost no stored charge in either the SBD or the
transistor, there is a large reduction in storage time. Thus,
transistor switching time is significantly reduced.
A cross sectional area of the Schottky diode is shown in
Figure 5.
SIS2
ISOLATION
P+
N
NE521/522 Comparators
Processed with state--of--the--art Schottky barrier diodes,
the NE521/522 series devices provide good input
characteristics while providing the fastest analog--to--TTL
conversion. Total delay from input to output is typically 6 ns
with a guaranteed speed of 12 ns. Additional features of this
device include the dual configuration and individual output
strobes to simplify system logic. The NE522, although
sacrificing some speed, features open--collector outputs for
party line or wired--OR configurations for additional system
flexibility.
COLLECTOR CONTACT
EMITTER CONTACT
BASE CONTACT
ISOLATION
N+
N+
P
N+
P
P+
P TYPE SUBSTRATE
SCHOTTKY DIODE
GUARD RING
Figure 5. Schottky Clamped Transistor Geometry
Table 1. Comparator Selection Guide (Parameters are based on min/max limits at 25°C as defined in the individual data sheet.)
Prop Delay
(ns)
VOS
(mV)
IOS
(mA)
IBIAS
(mA)
Gain
CMR (V)
NE521
12
7.5
5
20
5000
±3
Dual, very fast, standard supplies TTL compatible,
individual and common strobe.
NE522
15
7.5
5
20
5000
±3
Same as NE521 plus open--collector outputs for additional decoding.
LM311
200
7.5
0.05
0.25
200 k
V+ -- 1.5 V
High common--mode input range, +5 V to ±15 V supply, strobe input, open--collector output.
LM319
80
8
0.2
1.2
40 k
±5
Low input bias, dual, +5 V to ±15 V supply, open--collector output.
LM333
1300
2
0.05
0.25
200 k
V+ -- 1.5 V
Low input bias, dual, +5 V to ±15 V supply, open--collector output.
LM393
1300
2
0.05
0.25
200 k
V+ -- 1.5 V
Same as LM339 but dual.
Device
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Benefits
AND8176/D
Applications
High--speed comparators are capable of making logic
decisions in less than 10ns. They are easily applied and
possess good input and power supply noise rejection. As
with all linear ICs, however, some preliminary steps should
be taken in their use.
provide a means of utilizing the Schottky gate for other
system logic functions.
If the strobe inputs are not used, they should be connected
to the output of a logic gate that is always high, or to the +5 V
supply through a 5 to 10 kΩ resistor. They should never be
tied directly to the +5 V supply as the relatively minor
spiking on the supply may damage these inputs.
General Precautions
Common--Mode Signals
Layout
Manufacturers specify the maximum voltage range over
which the inputs may be taken. In addition, the maximum
differential voltage that may be safely applied to the inputs
is specified. In the case of the NE521 comparator, the
differential voltage is restricted to less than ±6 V, with a
common--mode of ±5 V. That these two quantities interact
cannot be overlooked. For instance, with both inputs at ±4 V
the common--mode restriction is satisfied. If VREF is now
left at +4 V the signal input may not be taken more than 2 V
below ground because the differential signal becomes 6 V.
It is important to observe this maximum rating since
exceeding the differential input voltage limit and drawing
excessive current in breaking down the emitter--base
junctions of the input transistors could cause gross
degradation in the input offset current and bias current
parameters.
It is also important to note that response time is specified
for a common--mode voltage of zero and may degrade when
the common--mode voltage approaches the common--mode
specification limits.
Exceeding the absolute maximum positive input voltage
limit of the device will saturate the input transistor and
possibly cause damage through excessive current. However,
even if the current is limited to a reasonable value so that the
device is not damaged, erratic operation can result.
The comparator is capable of resolving sub--millivolt
signals. To prevent unwanted signals from appearing at
signal ports, good physical layout is required. For any
high--speed design, ground planes should be used to guard
against ground loops and other sources of spurious signals.
At high frequencies, hidden signal paths become dominant.
Distributed capacitance is a particular nuisance. If care is not
taken to isolate output from input, distributed capacitance
can couple a few millivolts into the input, causing
oscillation.
Another source of spurious signals is ground current.
Input structures are relatively high impedance while the gate
structures of comparators run with large signal and ground
currents. If this gate ground current is allowed to pass near
the input signal path, the small impedances of the ground
circuit will cause millivolt changes in reference or signal
voltages producing errors, sustained oscillation, ringing, or
excessive VOS. A ground plane arranged such that output
currents do not flow near input areas is highly
recommended.
Power Supplies
Another general precaution that should always be
exercised is power supply bypassing. As mentioned, the
name of the game is speed. Very high--speed gates are used
to produce the desired output logic levels. Maximizing
response speed also requires higher current levels, giving
rise to power supply noise. For this reason, good power
supply bypassing very close to the device itself is always
mandatory. A tantalum capacitor of 1 to 10 mF in parallel
with 500 to 1000 pF will prove effective in most cases. Lead
lengths should be as short as physically possible to preserve
low impedances at high frequency.
Input Impedance
The differential bias and offset currents of comparators
are minimized by design. As was pointed out for op amps,
the input resistance seen by both inputs should be equal. This
reduces to a minimum the contribution of offset current to
threshold error. Unbalanced input impedance also adds to
the offset error due to the difference in voltage drop across
the input resistances.
Unused Inputs
Basic Applications
The basic comparator circuit and its transfer function were
presented by Figure 1.
When the input exceeds the reference voltage, the output
switches either positive or negative, depending on how the
inputs are connected.
The vast majority of specific applications involve only the
basic configuration with a change of reference voltage.
A to D converters are realized by applying the signal to one
terminal and the voltage derived from a ladder network to
the other. Limit detectors are likewise made from only the
very basic circuit. Both are only a small deviation from the
basic level detector.
Some currently available comparators such as the NE521
and NE522 are dual devices. Most often both sections of
these devices will be utilized. Should a system utilize one
device, the unused inputs should be biased in a known
condition. The high gain--bandwidth may otherwise cause
oscillations in the unused comparator section. A low
impedance should be provided from both unused inputs to
ground. A resistor of relatively high impedance may then be
used to supply a differential input on the order of 100 mV to
insure the comparator assumes a known state.
If the inverting input is tied to the positive differential
voltage the gate output will be low. The strobe inputs then
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Hysteresis
Hysteresis occurs because a small portion of the “one”
level output voltage is fed back in phase and added to the
input signal. This feedback aids the signal in crossing the
threshold. When the signal returns to the threshold, the
positive feedback must be overcome by the signal before
switching can occur. The switching process is then assured
and oscillations cannot occur. The threshold “dead zone”
created by this method, illustrated in Figure 7, prevents
output chatter with signals having slow and erratic zero
crossings.
As shown in Figure 6, the voltage feedback is calculated
from the expression:
E
⋅ RIN
VHYST = OUT
RIN + RF
Normally saturated high or low, the amplifiers used in
voltage comparators are seldom held in their threshold
region.
They possess high gain--bandwidth products and are not
compensated to preserve switching speed. Therefore, if the
compared voltages remain at or near the threshold for long
periods of time, the comparator may oscillate or respond to
noise pulses. For instance, this is a common problem with
successive approximation D to A converters where the
differential voltage seen by the comparator becomes
successively smaller until noise signals cause indecision. To
avoid this oscillation in the linear range, hysteresis can be
employed from output to input. Figure 6 defines the
arrangement. Both positive and negative feedback is
provided by RIN and RF.
RIN
where EOUT is the gate high output voltage.
The hysteresis voltage is bounded by the common--mode
range and the ability of the gate to source the current
required by the feedback network. If symmetrical hysteresis
is desired, an additional inverting gate is required if the
comparator does not have differential outputs. The NE521
and NE522 devices will require the inverter. Care should be
taken in the selection of the inverter that propagation delay
is minimum, especially for very high--speed comparators
such as the NE521.
RF
VIN
EOUT
RIN
RF
VTH
NOTE:
Line Receiver
Retrieving signals which have been transmitted over long
cables in the presence of high electrical noise is a perfect
application for differential comparators. Such systems as
automated production lines and large computer systems
must transmit high frequency digital signals over long
distances.
If the twisted--pair of the system is driven differentially
from ground, the signals can be reclaimed easily via a
differential line receiver.
Since the electrical noise imposed upon a pair of wires
takes the form of a common--mode signal, the very high
common--mode rejection of the NE521/522 makes the unit
ideal for differential line receivers. Figure 8 depicts the
simple schematic arrangement. The NE521 is used as a
differential amplifier having a logic level output. Because
common--mode signals are rejected, noise on the cable
disappears and only the desired differential signal remains.
Figure 9 illustrates the NE521 response to the 200 mVP--P 10
MHz differential signal. In Figure 10 the same signal has
been buried in 5 VP--P of 1 MHz common--mode “noise.”
E
⋅ RIN
VHYST = OUT
RIN + RF
Figure 6. Level Detector With Hysteresis
OUTPUT
1V/cm
INPUT (10mV/cm)
Figure 7. 0 V Level Detector With ±10 mV
Hysteresis
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AND8176/D
STROBE
--5 V
EOUT
EIN
R = 1/2
CHARACTERISTIC LINE
IMPEDANCE
R
RL
1/2 522
+
--
R
EOUT
R
ENABLE
UNLATCH
R
Figure 8. Line Receiver
1/2 522
+
--
The circuit suffers no degradation of signal. If desired,
several NE522 comparators may be “wire--ORed,” or a
latched output can be built as shown in Figure 8.
The NE521 comparator has the advantage of wider
bandwidth to permit higher data rates.
EOUT VOLTS
--5 V
OUTPUT
2V/DIV.
TRANSFER CURVE
4
3
2
1
0
--VTH
0
EIN
+VTH
Figure 11. Double--Ended Limit Detector
INPUT
100mV/DIV.
20MHz
Each half of the NE522 is referenced to the desired upper
or lower voltage limit producing the desired transfer curve
shown. Taking advantage of the dual configuration and the
open--collectors of the NE522 minimizes external
components and connections.
HORIZ. 100ns/DIV.
Crystal Oscillator
Figure 9. Line Receiver Response
Any device with a reasonable gain can be made to oscillate
by applying positive feedback in controlled amounts. The
NE521 will lend itself to crystal control easily, provided the
crystal is used in its fundamental mode. Figure 12 shows a
typical oscillator circuit.
OUTPUT
2V/DIV.
+
1/2 522
-INPUT
2V/DIV.
1MHz
COMMON
MODE
10pF
R
HORIZ. 100ns/DIV.
----
Figure 10. Response During Common--Mode
Noise
RADJ
CRYSTAL
(FUNDAMENTAL
MODE)
Figure 12. Crystal Oscillator
The crystal is operated in its series--resonant mode,
providing the necessary feedback through the capacitor to
the input of the NE521. The resistor RADJ is used to control
the amount of feedback for symmetry. Oscillations will start
whenever a circuit disturbance such as turning on the power
supplies occurs. The NE521 will oscillate up to 70 MHz.
Double--Ended Limit (Window) Detector
Many system designs require that it be known when a
signal level lies between two limits. This function is easily
accomplished with a single NE522 package. The schematic
and transfer curve of the circuit is shown in Figure 11.
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AND8176/D
However, crystals with frequencies higher than about
20 MHz are usually operated in one of their overtones. To
build an oscillator for a specific overtone requires tuned
circuits in addition to the crystal to provide the necessary
mode suppression. If the spurious modes are not tuned out,
the crystal will oscillate at the fundamental frequency.
Higher frequency oscillators could be realized using input
and output mode suppression or tuning. The NE522 is
especially desirable since the open--collector topology
allows the output to be collector--tuned readily.
VIN
R
2R
2R
2R
2R
Analog--to--Digital Converter
There are many types of A to D converter designs, each
having its own merits. However, where speed of conversion
is of prime interest, the multi--threshold conversion type is
used exclusively. It is apparent from Figure 13 that the
conversion speed of this design is the sum of the delay
through the comparator and the decoding gates.
20
(LSB)
-+
2R
2R
2R
5
521
21
-+
-+
-+
521
22
(MSB)
-+
521
-+
-+
521
-+
2R R/4
4
3
OUTPUT
1 V/cm
2
VREF
(3 Vmax)
1
OVER
RANGE
Figure 14. 3--Bit Parallel A to D Converter
0
3
Reference voltages for each bit are developed from a
precision resistor ladder network. Values of R and 2R are
chosen so that the threshold is one half of the least significant
bit. This assures maximum accuracy of ± bit.
It is apparent from the schematic that the individual strobe
line and duality features of the NE521 have greatly reduced
the cost and complexity of the design. The speed of the
converter is graphically illustrated by the photo of Figure 13.
All 3--bit outputs have settled and are true a mere 15 ns after
the input step of 3V has arrived. The output is usually
strobed into a register only after a certain time has elapsed
to insure that all data has arrived.
2
1
INPUT
1 V/cm 0
0
5 10 15 20 25 30 35 40
HORIZ. 5ns/cm
Figure 13. Parallel A to D Response
The sacrifices which must be made to obtain speed are the
number of components, bit accuracy and cost. The number
of comparators needed for an N--bit converter is 2n--1.
Although the NE521 provides two comparators per
package, the length of parallel converters is usually limited
to less than 4 bits. Accuracy of multi--threshold A to D
converters also suffers since the integrity of each bit is
dependent upon comparator threshold accuracy.
The implementation of a 3--bit parallel A to D converter
is shown in Figure 14 with a 3--bit digital equivalent of an
analog input shown in Figure 13.
Sense Amplifiers
Closely related to the comparator is the sense amplifier.
Signals derived from the many sources, such as transducers,
are not of sufficient amplitude to be compatible with
subsequent logic. It then becomes necessary to amplify and
convert the signal to TTL levels, which is the responsibility
of the sense amplifier.
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AND8176/D
Some transducers produce an output current. It remains,
then, for the user to convert these currents to TTL levels. A
terminating resistor from the drain to ground provides a
voltage output proportional to the current and the resistor
size. Larger signals can be produced by larger resistors; but
in practice, resistors larger than 1kΩ are avoided because of
increasing access time. Distributed capacitance forms a time
constant with this output resistance causing slow rise and fall
times when the resistor is large, adding to the access time.
Virtually any voltage comparator or sense amplifier can
be used. Since total time is the sum of all delays, the sense
amplifier is most often the fastest available.
ON Semiconductors comparators NE521 and NE522 are
ideal in this application because of low input offset voltages
and very fast response. Using these Schottky clamped
comparators significantly reduces the total cycle time of the
memory.
Design of the sense amplifier network depends upon the
transducer used and the input characteristics of the sense
amplifier. The significant specifications are given in
Table 2.
Consideration must first be given to the differential input
voltage requirements of the sense amplifier. The required
reference voltage is calculated from the relationship:
VREF ≤ (IT − IB) R1 − VDIFF
Where IT is the transducer output current, IB is sense
amplifier bias current and VDIFF is minimum differential
voltage to switch the sense amplifier.
In large systems, noise coupled into the sense lines by
stray capacitance can be very troublesome. Judicious layout
patterns with sense lines as short as possible will help, but
will not always be sufficient. One method of eliminating
noise is to use a balance sense line as shown in Figure 15.
A dummy line should be run parallel to the actual sense
line in as close proximity as possible. One end is connected
to the sense amplifier at the VREF point while the other end
is left open. The normal sense line is connected as usual.
Electrical noise imposed upon the pair of sense lines takes
the form of a common--mode signal and will be rejected by
the sense amplifier. Signal currents in the sense line, on the
other hand, form differential signals at the sense amp,
causing the output to switch.
Table 2. Important Sense Amplifier Parameters
DEVICE
VOS
(mV)
IB
(μA)
VIN (MIN)
(mV)
SPEED (ns)
(VIN=100mV)
GAIN
521
10
40
15
12
5000
522
10
40
15
15
5000
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BIT M
BIT I
PC 1
CE 1
RW 1
NC
NC
PC 2
CE 2
RW 2
PC N
CE N
RW N
A1
A2
A3
A4
A5
A6
A7
A8
A9
V+
NE
R 1/2 522
R
DATA OUT
Figure 15. Balanced Sense Line to Reduce Noise
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V+
NE
R 1/2 522
R
AND8176/D
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