DATASHEET

EL2257
®
Data Sheet
February 28, 2002
FN7063
125MHz Single Supply, Clamping Op Amp
Features
The EL2257 is a single supply op amp.
Prior single supply op amps have
generally been limited to bandwidths
and slew rates one-fourth of that of the EL2257. The
125MHz bandwidth, 275V/µs slew rate and 0.05%/0.05°
differential gain/differential phase makes this part ideal for
single or dual supply video speed applications. With its
voltage feedback architecture, this amplifier can accept
reactive feedback networks, allowing them to be used in
analog filtering applications. The inputs can sense signals
below the bottom supply rail and as high as 1.2V below the
top rail. Connecting the load resistor to ground and operating
from a single supply, the outputs swing completely to ground
without saturating. The outputs can also drive to within 1.2V
of the top rail. The EL2257 will output ±100mA and will
operate with single supply voltages as low as 2.7V, making
them ideal for portable, low power applications.
• Specified for +3V, +5V, or ± 5V applications
The EL2257 has a high speed disable feature. Applying a
low logic level to all ENABLE pins reduces the supply current
to 0µA within 50ns. Each amplifier has its own ENABLE pin.
This is useful for both multiplexing and reducing power
consumption.
The EL2257 also has an output voltage clamp feature. This
clamp is a fast recovery (< 7ns) output clamp that prevents
the output voltage from going above the preset clamp
voltage. This feature is desirable for A/D applications, as A/D
converters can require long times to recover if overdriven.
The EL2257 is available in 14-pin SO (0.150") and 14-pin
PDIP packages and operates over the industrial temperature
range of -40°C to +85°C. For single amplifier applications,
see the EL2150 and EL2157. For space-saving, industrystandard pinout dual and quad applications, see the EL2250
and EL2450.
TAPE &
REEL
PKG. NO.
EL2257CS
14-Pin SO (0.150")
-
MDP0027
EL2257CS-T7
14-Pin SO (0.150")
7”
MDP0027
EL2257CS-T13
14-Pin SO (0.150")
13”
MDP0027
14-Pin PDIP
-
MDP0031
1
• Large input common-mode range 0V < VCM < VS - 1.2V
• Output swings to ground without saturating
• -3dB bandwidth = 125MHz
• ±0.1dB bandwidth = 30MHz
• Low supply current = 5mA
• Slew rate = 275V/µs
• Low offset voltage = 4mV max
• Output current = ±100mA
• High open loop gain = 80dB
• Differential gain = 0.05%
• Differential phase = 0.05°
Applications
• Video amplifiers
• PCMCIA applications
• A/D drivers
• Line drivers
• Portable computers
• High speed communications
• RGB printer, fax, scanner applications
• Broadcast equipment
• Active filtering
Pinout
PACKAGE
EL2257CN
• Output voltage clamp
• Multiplexing
Ordering Information
PART NUMBER
• Power-down to 0µA
EL2257
[14-PIN SO (0.150”), PDIP]
TOP VIEW
INA+
1
+
-
14
INA-
13
OUTA
CLAMPA
2
ENABLEA
3
12
NC
GND
4
11
VS+
ENABLEB
5
10
NC
9
OUTB
8
INB-
CLAMPB
6
INB+
7
+
-
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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All other trademarks mentioned are the property of their respective owners.
EL2257
Absolute Maximum Ratings (TA = 25°C)
Supply Voltage between VS and GND . . . . . . . . . . . . . . . . . . . 12.6V
Input Voltage (IN+, IN-, ENABLE, CLAMP) . . . GND–0.3V, VS+0.3V
Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .±6V
Maximum Output Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90mA
Output Short Circuit Duration. . . . . . . . . . . . . . . . See Note 1 page 3
Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . See curves
Storage Temperature Range . . . . . . . . . . . . . . . . . .-65°C to +150°C
Ambient Operating Temperature Range . . . . . . . . . .-40°C to +85°C
Operating Junction Temperature . . . . . . . . . . . . . . . . . . . . . . +150°C
CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed. Typical values are for information purposes only. Unless otherwise noted, all tests
are at the specified temperature and are pulsed tests, therefore: TJ = TC = TA
DC Electrical Specifications
VS = +5V, GND = 0V, TA = 25°C, VCM = 1.5V, VOUT = 1.5V, VCLAMP = +5V, VENABLE = +5V, unless
otherwise specified.
PARAMETER
DESCRIPTION
CONDITIONS
VOS
Offset Voltage
TCVOS
Offset Voltage Temperature
Coefficient
Measured from TMIN to TMAX
IB
Input Bias Current
VIN = 0V
IOS
Input Offset Current
VIN = 0V
TCIOS
Input Bias Current Temperature Measured from TMIN to TMAX
Coefficient
PSRR
Power Supply Rejection Ratio
CMRR
MIN
TYP
-4
MAX
UNIT
4
mV
10
-1100
µV/°C
-5.5
-10
µA
150
+1100
nA
50
nA/°C
45
70
dB
Common-mode Rejection Ratio VCM = 0V to +3.8V
50
65
dB
VCM = 0V to +3.0V
55
70
dB
VS = VENABLE = 2.7V to 12V, VCLAMP = OPEN
CMIR
Common-mode Input Range
0
RIN
Input Resistance
Common-mode
CIN
Input Capacitance
SO (0.150") package
1
VS-1.2
V
2
MΩ
1
pF
PDIP package
1.5
pF
mΩ
ROUT
Output Resistance
AV = +1
40
ISON
Supply Current - Enabled (per
amplifier)
VS = VCLAMP = 12V, VENABLE = 12V
5
6.5
mA
ISOFF
Supply Current - Shut-down (per VS = VCLAMP = 10V, VENABLE = 0.5V
amplifier)
VS = VCLAMP = 12V, VENABLE = 0.5V
0
50
µA
PSOR
Power Supply Operating Range
AVOL
Open Loop Gain
VOP
Positive Output Voltage Swing
5
2.7
VS = VCLAMP = 12V, VOUT = 2V to 9V, RL = 1kΩ to GND
65
2
V
dB
VOUT = 1.5V to 3.5V, RL = 1kΩ to GND
70
dB
VOUT = 1.5V to 3.5V, RL = 150Ω to GND
60
dB
10.8
V
10.0
V
4.0
V
VS = 12V, AV = 1, RL = 1kΩ to 0V
9.6
VS = ±5V, AV = 1, RL = 1kΩ to 0V
Negative Output Voltage Swing
12.0
80
VS = 12V, AV = 1, RL = 150Ω to 0V
VON
µA
VS = ±5V, AV = 1, RL = 150Ω to 0V
3.4
3.8
V
VS = 3V, AV = 1, RL = 150Ω to 0V
1.8
1.95
V
VS = 12V, AV = 1, RL = 150Ω to 0V
5.5
VS = ±5V, AV = 1, RL = 1kΩ to 0V
-4.0
VS = ±5V, AV = 1, RL = 150Ω to 0V
-3.7
8
mV
V
-3.4
V
EL2257
DC Electrical Specifications
VS = +5V, GND = 0V, TA = 25°C, VCM = 1.5V, VOUT = 1.5V, VCLAMP = +5V, VENABLE = +5V, unless
otherwise specified. (Continued)
PARAMETER
IOUT
DESCRIPTION
Output Current (Note 1)
CONDITIONS
VS = ±5V, AV = 1, RL = 10Ω to 0V
MIN
TYP
±75
±100
mA
±60
mA
VS = ±5V, AV = 1, RL = 50Ω to 0V
IOUT,OFF
Output Current - Disabled
VIH-EN
ENABLE Pin Voltage for Power- Relative to GND pin
up
VIL-EN
ENABLE Pin Voltage for Shutdown
Relative to GND pin
IIH-EN
ENABLE Pin Input Current High(Note 2)
VS = VCLAMP = 12V, VENABLE = 12V
IIL-EN
ENABLE Pin Input Current - Low VS = VCLAMP = 12V, VENABLE = 0.5V
(Note 2)
VOR-CL
Voltage Clamp Operating
Range (Note 3)
Relative to GND pin
VACC-CL
CLAMP Accuracy (Note 4)
VIN = 4V, RL = 1kΩ to GND, VCLAMP = 1.5V and 3.5V
IIH-CL
CLAMP Pin Input Current - High VS = VCLAMP = 12V
IIL-CL
CLAMP Pin Input Current - Low
(per amp)
VENABLE = 0.5V
MAX
0
20
2.0
µA
V
0.5
V
340
410
µA
0
1
µA
VOP
V
100
250
mV
12
25
µA
1.2
-250
VS = 12V, VCLAMP = 1.2V
UNIT
-30
-15
µA
NOTES:
1. Internal short circuit protection circuitry has been built into the EL2257. See the Applications section.
2. If the disable feature is not desired, tie the ENABLE pins to the VS pin, or apply a logic high level to the ENABLE pins.
3. The maximum output voltage that can be clamped is limited to the maximum positive output Voltage, or VOP. Applying a voltage higher than VOP
inactivates the clamp. If the clamp feature is not desired, either tie the CLAMP pin to the VS pin, or simply let the CLAMP pin float.
4. The clamp accuracy is affected by VIN and RL. See the Typical Curves Section and the Clamp Accuracy vs VIN and RL curve.
Closed Loop AC Electrical Specifications VS = +5V, GND = 0V, TA = 25°C, VCM = +1.5V, VOUT = +1.5V, VCLAMP = +5V, VENABLE
= +5V, AV = +1, RF = 0Ω, RL = 150Ω to GND pin unless otherwise specified. (Note 1)
PARAMETER
BW
BW
DESCRIPTION
-3dB Bandwidth
(VOUT = 400mVP-P)
±0.1dB Bandwidth
(VOUT = 400mVP-P)
CONDITIONS
MIN
TYP
MAX
UNIT
VS = 5V, AV = 1, RF = 0Ω
125
MHz
VS = 5V, AV = -1, RF = 500Ω
60
MHz
VS = 5V, AV = 2, RF = 500Ω
60
MHz
VS = 5V, AV = 10, RF = 500Ω
6
MHz
VS = 12V, AV = 1, RF = 0Ω
150
MHz
VS = 3V, AV = 1, RF = 0Ω
100
MHz
VS = 12V, AV = 1, RF = 0Ω
25
MHz
VS = 5V, AV = 1, RF = 0Ω
30
MHz
VS = 3V, AV = 1, RF = 0Ω
20
MHz
GBWP
Gain Bandwidth Product
VS = 12V, @ AV = 10
60
MHz
PM
Phase Margin
RL = 1kΩ, CL = 6pF
55
°
SR
Slew Rate
VS = 10V, RL = 150Ω, VOUT = 0V to 6V
275
V/µs
VS = 5V, RL = 150Ω, VOUT = 0V to +3V
300
V/µs
200
tR,tF
Rise Time, Fall Time
±0.1V step
2.8
ns
OS
Overshoot
±0.1V step
10
%
tPD
Propagation Delay
±0.1V step
3.2
ns
3
EL2257
Closed Loop AC Electrical Specifications VS = +5V, GND = 0V, TA = 25°C, VCM = +1.5V, VOUT = +1.5V, VCLAMP = +5V, VENABLE
= +5V, AV = +1, RF = 0Ω, RL = 150Ω to GND pin unless otherwise specified. (Note 1)
PARAMETER
tS
DESCRIPTION
CONDITIONS
MIN
TYP
MAX
UNIT
0.1% Settling Time
VS = ±5V, RL = 500Ω, AV = 1, VOUT = ±3V
40
ns
0.01% Settling Time
VS = ±5V, RL = 500Ω, AV = 1, VOUT = ±3V
75
ns
dG
Differential Gain (Note 2)
AV = 2, RF = 1kΩ
0.05
%
dP
Differential Phase (Note 2)
AV = 2, RF = 1kΩ
0.05
°
eN
Input Noise Voltage
f = 10kHz
48
nV/√Hz
iN
Input Noise Current
f = 10kHz
1.25
pA/√Hz
tDIS
Disable Time (Note 3)
50
ns
tEN
Enable Time (Note 3)
25
ns
tCL
Clamp Overload Recovery
7
ns
NOTES:
1. All AC tests are performed on a “warmed up” part, except slew rate, which is pulse tested.
2. Standard NTSC signal = 286mVP-P, f = 3.58MHz, as VIN is swept from 0.6V to 1.314V. RL is DC coupled.
3. Disable/Enable time is defined as the time from when the logic signal is applied to the ENABLE pin to when the supply current has reached half
its final value.
4
EL2257
Typical Performance Curves
Non-Inverting Frequency
Response (Gain)
Non-Inverting Frequency
Response (Phase)
3dB Bandwidth vs Temperature for
Non-Inverting Gains
Inverting Frequency Response (Gain)
Inverting Frequency Response (Phase)
3dB Bandwidth vs Temperature for
Inverting Gains
Frequency Response for Various RL
Frequency Response for Various CL
Non-Inverting Frequency Response vs
Common Mode Voltage
5
EL2257
Typical Performance Curves (Continued)
3dB Bandwidth vs Supply Voltage for
Non-Inverting Gains
Frequency Response for Various
Supply Voltages, AV = + 1
PSSR and CMRR vs Frequency
3dB Bandwidth vs Supply
Voltage for Inverting Gains
Frequency Response for Various
Supply Voltages, AV = + 2
PSRR and CMRR vs Die
Temperature
Open Loop Gain and Phase vs
Frequency
Open Loop Voltage Gain vs Die
Temperature
Closed Loop Output Impedance
vs Frequency
6
EL2257
Typical Performance Curves (Continued)
Large Signal Step Response, VS = +3V
Large Signal Step Response, VS = +5V
Small Signal Step Response
Slew Rate vs Temperature
7
Large Signal Step Response, VS = +12V
Large Signal Step Response, VS = ±5V
Settling Time vs Settling Accuracy
Voltage and Current Noise vs Frequency
EL2257
Typical Performance Curves (Continued)
Differential Gain for Single
Supply Operation
2nd and 3rd Harmonic
Distortion vs Frequency
Output Voltage Swing vs
Frequency for THD < 0.1%
8
Differential Phase for Single
Supply Operation
2nd and 3rd Harmonic
Distortion vs Frequency
Output Voltage Swing vs Frequency
for Unlimited Distortion
Differential Gain and Phase for
Dual Supply Operation
2nd and 3rd Harmonic
Distortion vs Frequency
Output Current vs Die Temperature
EL2257
Typical Performance Curves (Continued)
Supply Current vs Supply Voltage
(per amplifier)
Supply Current vs Die Temperature
(per amplifier)
Input Resistance vs Die Temperature
Offset Voltage vs Die
Temperature (4 Samples)
Input Bias Current vs Input Voltage
Input Offset Current and Input Bias
Current vs Die Temperature
Positive Output Voltage Swing vs Die
Temperature, RL = 150Ω to GND
Negative Output Voltage Swing vs
Die Temperature, RL = 150Ω to GND
Clamp Accuracy vs Die Temperature
9
EL2257
Typical Performance Curves (Continued)
Clamp Accuracy RL = 150Ω
Clamp Accuracy RL = 1kΩ
Clamp Accuracy RL = 10kΩ
Disable Response for a Family of DC Inputs
Enable Response for a Family of DC Inputs
OFF Isolation
Disable/Enable Response for a Family of Sine Waves
Package Power Dissipation vs Ambient Temperature
JEDEC JESD51-3 Low Effective Thermal Conductivity Test
Board
EL2257 Channel to Channel
Isolation vs Frequency
1.8
1.54W
Power Dissipation (W)
1.6
PD
I
θ
1.4
JA
=
1.2
81
1.042W
SO
1
θJ
0.8
P1
4
°C
/
W
14
(
0.1
50
”)
20
°C
/W
A =1
0.6
0.4
0.2
0
0
25
50
75 85 100
Ambient Temperature (°C)
10
125
150
EL2257
Simplified Schematic (One Channel)
Applications Information
Product Description
The EL2257, connected in voltage follower mode, -3dB
bandwidth is 125MHz while maintaining a 275V/µs slew rate.
With an input and output common mode range that includes
ground, this amplifier was optimized for single supply
operation, but will also accept dual supplies. It operates on a
total supply voltage range as low as 2.7V or up to 12V. This
makes them ideal for +3V applications, especially portable
computers.
While many amplifiers claim to operate on a single supply,
and some can sense ground at their inputs, most fail to truly
drive their outputs to ground. If they do succeed in driving to
ground, the amplifier often saturates, causing distortion and
recovery delays. However, special circuitry built into the
EL2257 allows the output to follow the input signal to ground
without recovery delays.
Power Supply Bypassing And Printed Circuit
Board Layout
As with any high frequency device, good printed circuit board
layout is necessary for optimum performance. Ground plane
construction is highly recommended. Pin lengths should be
as short as possible. The power supply pins must be well
bypassed to reduce the risk of oscillation. The combination
of a 4.7µF tantalum capacitor in parallel with a 0.1µF
ceramic capacitor has been shown to work well when placed
at each supply pin. For single supply operation, where the
GND pin is connected to the ground plane, a single 4.7µF
tantalum capacitor in parallel with a 0.1µF ceramic capacitor
from the VS+ pin to the GND pin will suffice.
11
For good AC performance, parasitic capacitance should be
kept to a minimum. Ground plane construction should be
used. Carbon or Metal-Film resistors are acceptable with the
Metal-Film resistors giving slightly less peaking and
bandwidth because of their additional series inductance.
Use of sockets, particularly for the SO (0.150") package
should be avoided if possible. Sockets add parasitic
inductance and capacitance which will result in some
additional peaking and overshoot.
Supply Voltage Range and Single-Supply
Operation
The EL2257 has been designed to operate with supply
voltages having a span of greater than 2.7V, and less than
12V. In practical terms, this means that the EL2257 will
operate on dual supplies ranging from ±1.35V to ±6V. With a
single-supply, the EL2257 will operate from +2.7V to +12V.
Performance has been optimized for a single +5V supply.
Pins 11 and 4 are the power supply pins on the EL2257. The
positive power supply is connected to pin 11. When used in
single supply mode, pin 4 is connected to ground. When
used in dual supply mode, the negative power supply is
connected to pin 4.
As supply voltages continue to decrease, it becomes
necessary to provide input and output voltage ranges that
can get as close as possible to the supply voltages. The
EL2257 has an input voltage range that includes the
negative supply and extends to within 1.2V of the positive
supply. So, for example, on a single +5V supply, the EL2257
has an input range which spans from 0V to 3.8V.
The output range of the EL2257 is also quite large. It
includes the negative rail, and extends to within 1V of the top
EL2257
supply rail with a 1kΩ load. On a +5V supply, the output is
therefore capable of swinging from 0V to +4V. On split
supplies, the output will swing ±4V. If the load resistor is tied
to the negative rail and split supplies are used, the output
range is extended to the negative rail.
Choice of Feedback Resistor, RF
The feedback resistor forms a pole with the input
capacitance. As this pole becomes larger, phase margin is
reduced. This increases ringing in the time domain and
peaking in the frequency domain. Therefore, RF has some
maximum value which should not be exceeded for optimum
performance. If a large value of RF must be used, a small
capacitor in the few picofarad range in parallel with RF can
help to reduce this ringing and peaking at the expense of
reducing the bandwidth.
As far as the output stage of the amplifier is concerned,
RF + RG appear in parallel with RL for gains other than +1.
As this combination gets smaller, the bandwidth falls off.
Consequently, RF has a minimum value that should not be
exceeded for optimum performance.
For AV = +1, RF = 0Ω is optimum. For AV = -1 or +2 (noise
gain of 2), optimum response is obtained with RF between
500Ω and 1kΩ. For AV = -4 or +5 (noise gain of 5), keep RF
between 2kΩ and 10kΩ.
kept above 1.25V, dG and dP are a respectable 0.03% and
0.03°.
For other biasing conditions see the Differential Gain and
Differential Phase vs. Input Voltage curves.
Output Drive Capability
In spite of their moderately low 5mA of supply current, the
EL2257 is capable of providing ±100mA of output current
into a 10Ω load, or ±60mA into 50Ω. With this large output
current capability, a 50Ω load can be driven to ±3V with
VS = ±5V, making it an excellent choice for driving isolation
transformers in telecommunications applications.
Driving Cables and Capacitive Loads
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, the back-termination series resistor will decouple the EL2257 from the cable and allow extensive
capacitive drive. However, other applications may have high
capacitive loads without a back-termination resistor. In these
applications, a small series resistor (usually between 5Ω and
50Ω) can be placed in series with the output to eliminate
most peaking. The gain resistor (RG) can then be chosen to
make up for any gain loss which may be created by this
additional resistor at the output.
Disable/Power-Down
Video Performance
For good video performance, an amplifier is required to
maintain the same output impedance and the same
frequency response as DC levels are changed at the output.
This can be difficult when driving a standard video load of
150Ω, because of the change in output current with DC level.
Differential Gain and Differential Phase for the EL2257 are
specified with the black level of the output video signal set to
+1.2V. This allows ample room for the sync pulse even in a
gain of +2 configuration. This results in dG and dP
specifications of 0.05% and 0.05° while driving 150Ω at a
gain of +2. Setting the black level to other values, although
acceptable, will compromise peak performance. For
example, looking at the single supply dG and dP curves for
RL = 150Ω, if the output black level clamp is reduced from
1.2V to 0.6V dG/dP will increase from 0.05%/0.05° to
0.08%/0.25°. Note that in a gain of +2 configuration, this is
the lowest black level allowed such that the sync tip doesn’t
go below 0V.
If your application requires that the output goes to ground,
then the output stage of the EL2257, like all other single
supply op amps, requires an external pull down resistor tied
to ground. As mentioned above, the current flowing through
this resistor becomes the DC bias current for the output
stage NPN transistor. As this current approaches zero, the
NPN turns off, and dG and dP will increase. This becomes
more critical as the load resistor is increased in value. While
driving a light load, such as 1kΩ, if the input black level is
12
Each amplifier in the EL2257 can be individually disabled,
placing each output in a high-impedance state. The disable
or enable action takes only about 40ns. When all amplifiers
are disabled, the total supply current is reduced to 0mA,
thereby eliminating all power consumption by the EL2257.
The EL2257 amplifier’s power down can be controlled by
standard CMOS signal levels at each ENABLE pin. The
applied CMOS signal is relative to the GND pin. For
example, if a single +5V supply is used, the logic voltage
levels will be +0.5V and +2.0V. If using dual ±5V supplies,
the logic levels will be -4.5V and -3.0V. Letting all ENABLE
pins float will disable the EL2257. If the power-down feature
is not desired, connect all ENABLE pins to the VS+ pin. The
guaranteed logic levels of +0.5V and +2.0V are not standard
TTL levels of +0.8V and +2.0V, so care must be taken if
standard TTL will be used to drive the ENABLE pins.
Output Voltage Clamp
The EL2257 amplifier has an output voltage clamp. This
clamping action is fast, being activated almost
instantaneously, and being deactivated in < 7ns, and
prevents the output voltage from going above the preset
clamp voltage. This can be very helpful when the EL2257 is
used to drive an A/D converter, as some converters can
require long times to recover if overdriven. The output
voltage remains at the clamp voltage level as long as the
product of the input voltage and the gain setting exceeds the
clamp voltage. For example, if the EL2257 is connected in a
gain of 2, and +3V DC is applied to the CLAMP pin, any
EL2257
voltage higher than +1.5V at the inputs will be clamped and
+3V will be seen at the output. Each amplifier of the EL2257
have their own CLAMP pin, so individual clamp levels may
be set.
Figure 1 below is the EL2257 with each amplifier unity gain
connected. Amplifier A is being driven by a 3VP-P sinewave
and has 2.25V applied to CLAMPA, while amplifier B is
driven by a 3VP-P triangle wave and 1.5V is applied to
CLAMPB. The resulting output waveforms, with their outputs
being clamped is shown in Figure 2.
FIGURE 3.
The clamp accuracy is affected by 1) the CLAMP pin voltage,
2) the input voltage, and 3) the load resistor. Depending
upon the application, the accuracy may be as little as a few
tens of millivolts up to a few hundred millivolts. Be sure to
allow for these inaccuracies when choosing the clamp
voltage. Curves of Clamp Accuracy vs. VCLAMP and VIN for
3 values of RL are included in the Typical Performance
Curves section.
FIGURE 1.
Unlike amplifiers that clamp at the input and are therefore
limited to non-inverting applications only, the EL2257 output
clamp architecture works for both inverting and non-inverting
gain applications. There is also no maximum voltage
difference limitation between VIN and VCLAMP which is
common on input clamped architectures.
The voltage clamp operates for any voltage between +1.2V
above the GND pin, and the minimum output voltage swing,
VOP. Forcing the CLAMP pin much below +1.2V can saturate
transistors and should therefore be avoided. Forcing the
CLAMP pin above VOP simply de-activates the CLAMP
feature. In other words, one cannot expect to clamp any
voltage higher than what the EL2257 can drive to in the first
place. If the clamp feature is not desired, either let the
CLAMP pin float or connect it to the VS+ pin.
EL2257 Comparator Application
FIGURE 2.
Figure 3 shows the output of amplifier A of the same circuit
being driven by a 0.5V to 2.75V square wave as the clamp
voltage is varied from 1.0V to 2.5V, as well as the unclamped
output signal. The rising edge of the signal is clamped to the
voltage applied to the CLAMP pin almost instantaneously.
The output recovers from the clamped mode within 5–7ns,
depending on the clamp voltage. Even when the CLAMP pin
is taken 0.2V below the minimum 1.2V specified, the output
is still clamped and recovers in about 11ns.
13
The EL2257 can be used as a very fast, single supply
comparator by utilizing the clamp feature. Most op amps
used as comparators allow only slow speed operation
because of output saturation issues. However, by applying a
DC voltage to the CLAMP pin of the EL2257, the maximum
output voltage can be clamped, thus preventing saturation.
Figure 4 is amplifier A of an EL2257 implemented as a
comparator. 2.25V DC is applied to the CLAMP pin, as well
as the IN- pin. A differential signal is then applied between
the inputs. Figure 5 shows the output square wave that
results when a ±1V, 10MHz triangular wave is applied, while
Figure 6 is a graph of propagation delay vs. overdrive as a
square wave is presented at the input.
EL2257
VIN
1
75Ω
2
14
+
-
12
4
11
6
FIGURE 5.
8
FIGURE 8.
Multiplexing with the EL2257
The ENABLE pins on the EL2257 allow for multiplexing
applications. Figure 9 shows an EL2257 with both outputs
tied together, driving a back terminated 75Ω video load. Two
sinewaves of varying amplitudes and frequencies are
applied to the two inputs. Logic signals are applied to each of
the ENABLE pins to cycle through turning each of the
amplifiers on, one at a time. Figure 10 shows the resulting
output waveform at VOUT. Switching is complete in about
50ns. Notice the outputs are tied directly together.
Decoupling resistors at each output are not necessary. In
fact, adding them approximately doubles the switching time
to 100ns.
1.3VP-P
1MHz
Video Sync Pulse Remover Application
All CMOS Analog to Digital Converters (A/Ds) have a
parasitic latch-up problem when subjected to negative input
voltage levels. Since the sync tip contains no useful video
information and it is a negative going pulse, we can chop it
off. Figure 7 shows a unity gain connected amplifier A of an
EL2257. Figure 8 shows the complete input video signal
applied at the input, as well as the output signal with the
negative going sync pulse removed.
1
2
CLK
1 of 2
Decoder
13
3
12
4
11
5
10
150Ω
6
7.5VP-P
5MHz
14
+
-
+
-
7
9
8
FIGURE 9.
14
9
FIGURE 7.
FIGURE 4.
FIGURE 6.
+5V
10
+
-
7
Propagation Delay vs Overdrive
EL2257 as a Comparator
13
3
5
VOUT
150Ω
VOUT
0.1µF 4.4µF
EL2257
where:
N = Number of amplifiers
VS = Total supply voltage
ISMAX = Maximum supply current per amplifier
VOUT = Maximum output voltage of the application
RL = Load resistance tied to ground
FIGURE 10.
Short Circuit Current Limit
The EL2257 has internal short circuit protection circuitry that
protect it in the event of its output being shorted to either
supply rail. This limit is set to around 100mA nominally and
reduces with increasing junction temperature. It is intended
to handle temporary shorts. If an output is shorted
indefinitely, the power dissipation could easily increase such
that the part will be destroyed. Maximum reliability is
maintained if the output current never exceeds ±90mA. A
heat sink may be required to keep the junction temperature
below absolute maximum when an output is shorted
indefinitely.
If we set the two PDMAX equations, [1] and [2], equal to each
other, and solve for VS, we can get a family of curves for
various loads and output voltages according to:
R L × ( T JMAX - T AMAX )
2
--------------------------------------------------------------- + ( V OUT )
N × θ JA
V S = ---------------------------------------------------------------------------------------------( I S × R L ) + V OUT
Figure 11 below shows total single supply voltage VS vs. RL
for various output voltage swings for the PDIP and SO
(0.150") packages. The curves assume worst-case
conditions of TA = +85°C and IS = 6.5mA per amplifier.
EL2257 Single Supply Voltage
vs. RLoad for Various
VOUT and Packages
Power Dissipation
With the high output drive capability of the EL2257, it is
possible to exceed the 150°C Absolute Maximum junction
temperature under certain load current conditions.
Therefore, it is important to calculate the maximum junction
temperature for the application to determine if power-supply
voltages, load conditions, or package type need to be
modified for the EL2257 to remain in the safe operating area.
The maximum power dissipation allowed in a package is
determined by:
T JMAX - T AMAX
PD MAX = -------------------------------------------θ JA
where:
TJMAX = Maximum junction temperature
TAMAX = Maximum ambient temperature
θJA = Thermal resistance of the package
PDMAX = Maximum power dissipation in the package
The maximum power dissipation actually produced by an IC
is the total quiescent supply current times the total power
supply voltage, plus the power in the IC due to the load, or:
V OUT
PD MAX = N × V S × I SMAX + ( V S - V OUT ) × ---------------RL
15
FIGURE 11.
EL2257
EL2257 Macromodel (one channel)
* Revision A, October 1995
* Pin numbers reflect a standard single opamp.
* When not being used, the clamp pin, pin 1,
* should be connected to Vsupply, pin 7
* Connections: +input
*
|
-input
*
|
|
+Vsupply
*
|
|
|
-Vsupply
*
|
|
|
|
output
*
|
|
|
|
| clamp
*
|
|
|
|
|
|
.subckt EL2257/el 3 2 7 4 6 1
*
* Input Stage
*
i1 7 10 250µA
i2 7 11 250µA
r1 10 11 4K
q1 12 2 10 qp
q2 13 3 11 qpa
r2 12 4 100
r3 13 4 100
*
* Second Stage & Compensation
*
gm 15 4 13 12 4.6m
r4 15 4 15Meg
c1 15 4 0.36pF
*
* Poles
*
e1 17 4 15 4 1.0
r6 17 25 400
c3 25 4 1pF
r7 25 18 500
c4 18 4 1pF
*
* Connections:IN+IN+IN+IN+IN+IN+IN+INININININ
* Output Stage & Clamp
*
i3 20 4 1.0mA
q3 7 23 20 qn
q4 7 18 19 qn
q5 7 18 21 qn
q6 4 20 22 qp
q7 7 23 18 qn
d1 19 20 da
d2 18 1 da
r8 21 6 2
r9 22 6 2
r10 18 21 10k
r11 7 23 100k
d3 23 24 da
d4 24 4 da
d5 23 18 da
*
* Power Supply Current
*
ips 7 4 3.2mA
16
EL2257
*
* Models
*
.model qn npn(is800e-18 bf150 tf0.02nS)
.model qpa pnp(is810e-18 bf50 tf0.02nS)
.model qp pnp(is800e-18 bf54 tf0.02nS)
.model da d(tt0nS)
.ends
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17