Texas Instruments | AN-87 Comparing the High Speed Comparators (Rev. B) | Application notes | Texas Instruments AN-87 Comparing the High Speed Comparators (Rev. B) Application notes

Texas Instruments AN-87 Comparing the High Speed Comparators (Rev. B) Application notes
Application Report
SNOA658B – June 1973 – Revised April 2013
AN-87 Comparing the High Speed Comparators
.....................................................................................................................................................
ABSTRACT
This application report compares the Texas Instruments high speed comparators to similar devices from
other manufacturers.
1
2
3
4
5
Contents
Introduction ..................................................................................................................
Speed .........................................................................................................................
Input Parameters ............................................................................................................
Other Performance Areas ..................................................................................................
Applications ..................................................................................................................
2
3
3
4
7
List of Figures
1
Delay vs Overdrive.......................................................................................................... 3
2
Delay vs Temperature ...................................................................................................... 3
3
Offset Temperature Coefficient
4
LM161 Common Mode Range ............................................................................................ 4
5
LM161 Schematic Diagram ................................................................................................ 5
6
LM160 Schematic Diagram ................................................................................................ 6
7
Peak Detector ............................................................................................................... 7
8
High Speed 3-bit A to D Converter ....................................................................................... 8
9
Direct Interfacing to ECL ................................................................................................... 9
10
Level Detector with Hysteresis ............................................................................................ 9
...........................................................................................
4
List of Tables
1
LM360/μA760C Comparison (0°C ≤ TA ≤ +70°C, V+ = +5.0V, V− = −5.0V) .......................................... 2
2
LM261/NE529 Comparison (0°C ≤ TA ≤ +70°C, V+ = +10V, V− = −10V, VCC = +5.0V) ............................. 2
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AN-87 Comparing the High Speed Comparators
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1
Introduction
1
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Introduction
Several integrated circuit voltage comparators exist which were designed with high speed and
complementary TTL outputs as the main objectives. The more common applications for these devices are
high speed analog to digital (A to D) converters, tape and disk-file read channels, fast zero-crossing
detectors, and high speed differential line receivers.
The product philosophy at Texas Instruments was to create pin-for-pin replacement circuits that could be
considered as second-sources to the other comparators, while simultaneously containing the
improvements necessary to make a more optimum device for the intended usage. Optimized parameters
include speed, input accuracy and impedance, supply voltage range, fanout, and reliability. The
LM160/LM260/LM360 are replacement devices for the μA760, while the LM161/LM261/LM361 replace the
SE/NE529. Table 1 and Table 2 compare the critical parameters of the Texas Instruments commercial
range devices to their respective counterparts.
Table 1. LM360/μA760C Comparison (0°C ≤ TA ≤ +70°C, V+ = +5.0V, V− = −5.0V)
LM360
μA760C
Input Offset Voltage
5.0
6.0
mV max
Input Offset Current
3.0
7.5
μA max
μA max
Parameter
Units
Input Bias Current
20
60
Input Capacitance
4.0
8.0
pF typ
Input Impedance
17
5.0
kΩ typ @ 1 MHz 25°C
Differential Voltage Range
±5.0
±5.0
V typ
Common Mode Voltage Range
±4.0
±4.0
V typ
Gain
3.0
3.0
V/mV typ 25°
Fanout
4.0
2.0
74 Series TTL Loads
(1) 30 mVp-p 10 MHz Sinewave in
25
30
ns max 25°
(2) 2.0 Vp-p 10 MHz Sinewave in
20
25
ns max 25°
(3) 100 mV Step + 5.0 mV Overdrive
14
22
ns typ 25°
Propagation Delays:
Table 2. LM261/NE529 Comparison (0°C ≤ TA ≤ +70°C, V+ = +10V, V− = −10V, VCC = +5.0V)
Parameter
2
LM261
NE529
Input Offset Voltage
3.0
10
mV max
Input Offset Current
3.0
15
μA max
Input Bias Current
20
50
μA max
Input Impedance
17
5.0
kΩ typ @ 1 MHz 25°C
Differential Voltage Range
±5.0
±5.0
V typ
Common Mode Voltage Range
±6.0
±6.0
V typ
Gain
3.0
4.0
V/mV typ 25°
Fanout
4.0
6.0
74 Series TTL Loads
Propagation Delay - 50 mV Overdrive
20
22
ns max 25°
AN-87 Comparing the High Speed Comparators
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Units
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Speed
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2
Speed
Throughout the universe the subject of speed must be approached with caution; the same holds true here.
Speed (propagation delay time) is a function of the measurement technique. The earlier “standard” of
using a 100 mV input step with 5.0 mV overdrive has given way to seemingly endless variations. To be
meaningful, speed comparisons must be made with identical conditions. It is for this reason that the speed
conditions specified for the Texas Instruments parts are the same as those of the parts replaced.
Probably the most impressive speed characteristic of the six Texas Instruments parts is the fact that
propagation delay is essentially independent of input overdrive (Figure 1); a highly desirable characteristic
in A to D applications. Their delay typically varies only 3 ns for overdrive variations of 5.0 mV to 500 mV,
whereas the other parts have a corresponding delay variation of two to one. As can be seen in Table 1
and Table 2, the Texas Instruments parts have an improved maximum delay specification. Further, the 20
ns maximum delay is meaningful since it is specified with a representative load: a 2.0 kΩ resistor to +5.0V
and 15 pF total load capacitance. Figure 2 shows typical delay variation with temperature.
Figure 1. Delay vs Overdrive
3
Figure 2. Delay vs Temperature
Input Parameters
The A to D, level detector, and line receiver applications of these devices require good input accuracy and
impedance. In all these cases the differential input voltage is relatively large, resulting in a complete switch
of input bias current as the input signal traverses the reference voltage level. This effect can give rise to
reduced gain and threshold inaccuracy, dependent on input source impedances and comparator input bias
currents. Table 1 and Table 2 show that the Texas Instruments parts have a substantially lower maximum
bias current to ease this problem. This was done without resorting to Darlington input stages whose price
is higher offset voltages and longer delay times. The lower bias currents also raise input resistance in the
threshold region. Lower input capacitance and higher input resistance result in higher input impedance at
high frequencies.
Even with low source impedances, input accuracy is still dependent on offset voltage. Since none of the
devices under discussion has internal offset null capability, ultimate accuracy was improved by designing
and specifying lower maximum offset voltage. Refer to Figure 3 for typical offset voltage drift with
temperature.
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Other Performance Areas
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Figure 3. Offset Temperature Coefficient
4
Other Performance Areas
In the case of the LM160/LM260/LM360, fanout was doubled over the previous device. For the
LM161/LM261/LM361, operating supply voltage range was extended to ±15V op amp supplies which are
often readily available where such a comparator is used. Figure 4 reveals the common mode range of the
latter device.
Figure 4. LM161 Common Mode Range
The performance improvements previously mentioned were a result of circuit design (Figure 5 and
Figure 6) and device processing. Schottky clamping, which can give rise to reliability problems, was not
used. Gold doping, which results in processing dependent speeds and low transistor beta, was not used.
Instead a non-gold-doped process with high breakdown voltage, high beta, and high fT (≈1.5 GHz) was
selected which produced remarkably consistent performance independent of normal process variation.
The higher breakdown voltage allows the LM161/LM261/LM361 to operate on ±15V supplies and results
in lower transistor capacitance; higher beta provides lower input bias currents; and higher fT helps reduce
propagation time.
4
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Other Performance Areas
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Figure 5. LM161 Schematic Diagram
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Other Performance Areas
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Figure 6. LM160 Schematic Diagram
6
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Applications
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5
Applications
Typical applications have been mentioned previously. The LM160 and LM161 may be combined as in
Figure 7 to create a fast, accurate peak detector for use in tape and disk-file read channels. A 3-bit A to D
converter with 21 ns typical conversion time is shown in Figure 8. Although primarily intended for
interfacing to TTL logic, direct connection may be made to ECL logic from the LM161 by the technique
shown in Figure 9. When used this way the common mode range is shifted from that of the TTL
configuration. Finally level detectors or line receivers may be implemented with hysteresis in the transfer
characteristic as seen in Figure 10.
Figure 7. Peak Detector
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Applications
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Figure 8. High Speed 3-bit A to D Converter
8
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Applications
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Figure 9. Direct Interfacing to ECL
(1)
Figure 10. Level Detector with Hysteresis
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Copyright © 1973–2013, Texas Instruments Incorporated
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