1995 Elantec, Inc.
EL
2257C/EL
2357C
General Description
The EL2257C/EL2357C are supply op amps. Prior single supply op
amps have generally been limited to bandwidths and slew rates 1/4
that of the EL2257C/EL2357C. The 125 MHz bandwidth, 275 V/s
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 appli-
cations. 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 com-
pletely to ground without saturating. The outputs can also drive to
within 1.2V of the top rail. The EL2257C/EL2357C will output
100 mA and will operate with single supply voltages as low as 2.7V,
making them ideal for portable, low power applications.
The EL2257C/EL2357C have a high speed disable feature. Applying a
low logic level to all ENABLE pins reduces the supply current to 0 A
within 50 ns. Each amplifier has its own ENABLE pin. This is useful
for both multiplexing and reducing power consumption.
The EL2257C/EL2357C also have an output voltage clamp feature.
This clamp is a fast recovery (<7 ns) 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 EL2257C/EL2357C are available in plastic DIP and SOIC pack-
ages. Both parts operate over the industrial temperature range of -40C
t o + 8 5 C . F o r s i n g l e a m p l i f i e r a p p l i c a t i o n s , s e e t h e
EL2150C/EL2157C. For space saving, industry standard pin out dual
and quad applications, see the EL2250C/EL2450C.
Connection Diagrams
Top View
Top View
Features
Specified for +3V, +5V, or 5V
Applications
Power Down to 0 A
Output Voltage Clamp
Large Input Common Mode Range
0V < V
CM
< V
S
- 1.2V
Output Swings to Ground Without
Saturating
-3 dB Bandwidth = 125 MHz
0.1 dB Bandwidth = 30 MHz
Low Supply Current = 5 mA
Slew Rate = 275 V/s
Low Offset Voltage = 4 mV max
Output Current = 100 mA
High Open Loop Gain = 80 dB
Differential Gain = 0.05%
Differential Phase = 0.05
Applications
Video Amplifier
PCMCIA Applications
A/D Driver
Line Driver
Portable Computers
High Speed Communications
RGB Printer, FAX, Scanner
Applications
Broadcast Equipment
Active Filtering
Multiplexing
Ordering Information
Part No.
Temp. Range
Package
Outline #
EL2257CN
-40C to +85C
14 Pin PDIP
MDP0031
EL2257CS
-40C to +85C
14 Pin SOIC
MDP0027
EL2357CN
-40C to +85C
16 Pin PDIP
MDP0031
EL2357CS
-40C to +85C
16 Pin SOIC
MDP0027
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
Jan
uary 5, 2000
2
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
Absolute Maximum Ratings
(T
A
= 25 C)
Supply Voltage between V
S
and GND
12.6V
Input Voltage (IN+, IN-, ENABLE, CLAMP)
GND0.3V, V
S
+0.3V
Differential Input Voltage
6V
Maximum Output Current
90 mA
Output Short Circuit Duration
(see note
[1]
DC Electrical Characteristics)
Power Dissipation
See Curves
Storage Temperature Range
-65C to +150C
Ambient Operating Temperature Range
-40C to +85C
Operating Junction Temperature
150C
Important Note:
All parameters having Min/Max specifications are guaranteed. The Test Level column indicates the specific device testing actually performed during
production and Quality inspection. Elantec performs most electrical tests using modern high-speed automatic test equipment, specifically the LTX77
Series system. Unless otherwise noted, all tests are pulsed tests, therefor T
J
= T
C
= T
A
.
Test Level
Test Procedure
I
100% production tested and QA sample tested per QA test plan QCX0002.
II
100% production tested at T
A
= 25C and QA sample tested at T
A
= 25C, T
MAX
and T
MIN
per QA test plan QCX0002.
III
QA sample tested per QA test plan QCX0002.
IV
Parameter is guaranteed (but not tested) by Design and Characterization Data.
V
Parameter is typical value at T
A
= 25C for information purposes only.
DC Electrical Characteristics
V
S
=+5V, GND=0V, T
A
=25C, V
CM
=1.5V, V
OUT
=1.5V, V
CLAMP
=+5V, V
ENABLE
=+5V, unless otherwise specified.
Parameter
Description
Test Conditions
Min
Typ
Max
Test
Level
Units
V
OS
Offset Voltage
EL2257C
-4
4
I
mV
EL2357C
-6
6
I
mV
TCV
OS
Offset Voltage Temperature Coefficient
Measured from Tmin to Tmax
10
V
V/C
IB
Input Bias Current
V
IN
=0V
-5.5
-10
I
A
I
OS
Input Offset Current
V
IN
=0V
-1100
150
+1100
I
nA
TCI
OS
Input Bias Current Temperature Coefficient
Measured from Tmin to Tmax
50
V
nA/C
PSRR
Power Supply Rejection Ratio
V
S
=V
ENABLE
=+2.7V to +12V,
V
CLAMP
=OPEN
45
70
I
dB
CMRR
Common Mode Rejection Ratio
VCM=0V to +3.8V
50
65
I
dB
VCM=0V to +3.0V
55
70
I
dB
CMIR
Common Mode Input Range
0
V
S
-1.2
I
V
R
IN
Input Resistance
Common Mode
1
2
I
M
C
IN
Input Capacitance
SOIC Package
1
V
pF
PDIP Package
1.5
V
pF
R
OUT
Output Resistance
Av=+1
40
V
m
I
S,ON
Supply Current - Enabled (per amplifier)
V
S
=V
CLAMP
=+12V, V
ENABLE
=+12V
5
6.5
I
mA
I
S,OFF
Supply Current - Shut Down (per amplifier)
V
S
=V
CLAMP
=+10V, V
ENABLE
=+0.5V
0
50
I
A
V
S
=V
CLAMP
=+12V, V
ENABLE
=+0.5V
5
V
A
PSOR
Power Supply Operating Range
2.7
12.0
I
V
AVOL
Open Loop Gain
V
S
=V
CLAMP
=+12V, V
OUT
=+2V to +9V,
R
L
=1 k
to GND
65
80
I
dB
V
OUT
=+1.5V to +3.5V, R
L
=1 k
to GND
70
V
dB
V
OUT
=+1.5V to +3.5V, R
L
=150
to GND
60
V
dB
3
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
V
OP
Positive Output Voltage Swing
V
S
=+12V, A
V
=+1, R
L
=1 k
to 0V
10.8
V
V
V
S
=+12V, A
V
=+1, R
L
=150
to 0V
9.6
10.0
I
V
V
S
=5V, A
V
=+1, R
L
=1 k
to 0V
4.0
V
V
V
S
=5V, A
V
=+1, R
L
=150
to 0V
3.4
3.8
I
V
V
S
=+3V, A
V
=+1, R
L
=150
to 0V
1.8
1.95
I
V
V
ON
Negative Output Voltage Swing
V
S
=+12V, A
V
=+1, R
L
=150
to 0V
5.5
8
I
mV
V
S
=5V, A
V
=+1, R
L
=1 k
to 0V
-4.0
V
V
V
S
=5V, A
V
=+1, R
L
=150
to 0V
-3.7
-3.4
I
V
I
OUT
Output Current
[1]
V
S
=5V, A
V
=+1, R
L
=10
to 0V
75
100
I
mA
V
S
=5V, A
V
=+1, R
L
=50
to 0V
60
V
mA
I
OUT,OFF
Output Current, Disabled
V
ENABLE
=+0.5V
0
20
I
A
V
IH-EN
ENABLE pin Voltage for Power Up
Relative to GND Pin
2.0
I
V
V
IL-EN
ENABLE pin Voltage for Shut Down
Relative to GND Pin
0.5
I
V
I
IH-EN
ENABLE pin Input Current-High
[2]
V
S
=V
CLAMP
=+12V, V
ENABLE
=+12V
340
410
I
A
I
IL-EN
ENABLE pin Input Current-Low
[2]
V
S
=V
CLAMP
=+12V, V
ENABLE
=+0.5V
0
1
I
A
V
OR-CL
Voltage Clamp Operating Range
[3]
Relative to GND Pin
1.2
V
OP
I
V
V
ACC-CL
CLAMP Accuracy
[4]
V
IN
=+4V, R
L
=1 k
to GND
V
CLAMP
=+1.5V and +3.5V
-250
100
250
I
mV
I
IH-CL
CLAMP pin Input Current - High
V
S
=V
CLAMP
=+12V
12
25
I
A
I
IL-CL
CLAMP pin Input Current - Low / Per
Amplifier
V
S
=+12V, V
CLAMP
=+1.2V
-30
-15
I
A
1.
Internal short circuit protection circuitry has been built into the EL2257C/EL2357C. See the Applications section.
2.
If the disable feature is not desired, tie the ENABLE pins to the V
S
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 V
OP
. Applying a Voltage higher than V
OP
inactivates the clamp. If the clamp feature is not desired, either tie the CLAMP pin to the V
S
pin, or simply let the CLAMP pin float.
4.
The clamp accuracy is affected by V
IN
and R
L
. See the Typical Curves Section and the Clamp Accuracy vs. V
IN
and R
L
curve.
DC Electrical Characteristics (Continued)
V
S
=+5V, GND=0V, T
A
=25C, V
CM
=1.5V, V
OUT
=1.5V, V
CLAMP
=+5V, V
ENABLE
=+5V, unless otherwise specified.
Parameter
Description
Test Conditions
Min
Typ
Max
Test
Level
Units
4
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
Closed Loop AC Electrical Characteristics
V
S
=+5V, GND=0V, T
A
=25C, V
CM
=+1.5V, V
OUT
=+1.5V, V
CLAMP
=+5V, V
ENABLE
=+5V, A
V
=+1, RF=0
, RL=150
to GND pin,
unless otherwise specified
[1]
Parameter
Description
Test Conditions
Min
Typ
Max
Test
Level
Units
BW
-3 dB Bandwidth (Vout=400 mVp-p)
V
S
=+5V, A
V
=+1, R
F
=0
125
V
MHz
V
S
=+5V, A
V
=-1, R
F
=500
60
V
MHz
V
S
=+5V, A
V
=+2, R
F
=500
60
V
MHz
V
S
=+5V, A
V
=+10, R
F
=500
6
V
MHz
V
S
=+12V, A
V
=+1, R
F
=0
150
V
MHz
V
S
=+3V, A
V
=+1, R
F
=0
100
V
MHz
BW
0.1 dB Bandwidth (Vout=400 mVp-p)
V
S
=+12V, A
V
=+1, R
F
=0
25
V
MHz
V
S
=+5V, A
V
=+1, R
F
=0
30
V
MHz
V
S
=+3V, A
V
=+1, R
F
=0
20
V
MHz
GBWP
Gain Bandwidth Product
V
S
=+12V, @ A
V
=+10
60
V
MHz
PM
Phase Margin
R
L
=1 k
, CL=6 pF
55
V
SR
Slew Rate
V
S
=+10V, R
L
=150
, Vout=0V to +6V
200
275
I
V/s
V
S
=+5V, R
L
=150
, Vout=0V to +3V
300
V
V/s
t
R
,t
F
Rise Time, Fall Time
0.1V Step
2.8
V
ns
OS
Overshoot
0.1V Step
10
V
%
t
PD
Propagation Delay
0.1V step
3.2
V
ns
t
S
0.1% Settling Time
V
S
=5V, R
L
=500
, A
V
=+1, V
OUT
=3V
40
V
ns
0.01% Settling Time
V
S
=5V, R
L
=500
, A
V
=+1, V
OUT
=3V
75
V
ns
dG
Differential Gain
[2]
A
V
=+2, R
F
=1 k
0.05
V
%
dP
Differential Phase
[2]
A
V
=+2, R
F
=1 k
0.05
V
e
N
Input Noise Voltage
f=10 kHz
48
V
nV/H
z
i
N
Input Noise Current
f=10 kHz
1.25
V
pA/H
z
t
DIS
Disable Time
[3]
50
V
ns
t
EN
Enable Time
[3]
25
V
ns
t
CL
Clamp Overload Recovery
7
V
ns
1.
All AC tests are performed on a "warmed up" part, except slew rate, which is pulse tested.
2.
Standard NTSC signal = 286 mVp-p, f=3.58 MHz, as V
IN
is swept from 0.6V to 1.314V. R
L
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.
5
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
Typical Performance Curves
Non-Inverting
Frequency Response (Gain)
Non-Inverting
Frequency Response (Phase)
3 dB Bandwidth vs
Temperature for
Non-Inverting Gains
3 dB Bandwidth vs
Temperature for
Inverting Gains
Inverting Frequency
Response (Phase)
Inverting Frequency
Response (Gain)
Frequency Response
for Various R
L
Frequency Response
for Various C
L
Non-Inverting
Frequency Response vs
Common Mode Voltage
6
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
3 dB Bandwidth vs
Supply Voltage for
Non-Inverting Gains
Frequency Response for
Various Supply Voltages,
A
V
= + 1
PSSR and CMRR
vs Frequency
PSRR and CMRR vs
Die Temperature
Frequency Response for
Various Supply Voltages,
A
V
= + 2
3 dB Bandwidth vs Supply
Voltage for Inverting Gains
Open Loop Gain and
Phase vs Frequency
Open Loop Voltage Gain
vs Die Temperature
Closed Loop Output
Impedance vs Frequency
7
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
Large Signal Step Response,
V
S
= +3V
Large Signal Step Response,
V
S
= +5V
Large Signal Step Response,
V
S
= +12V
Small Signal Step Response
Large Signal Step Response,
V
S
= 5V
Slew Rate vs Temperature
Settling Time vs
Settling Accuracy
Voltage and Current Noise
vs Frequency
8
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
Differential Gain for
Single Supply Operation
Differential Phase for
Single Supply Operation
Differential Gain and Phase
for Dual Supply Operation
2nd and 3rd Harmonic
Distortion vs Frequency
2nd and 3rd Harmonic
Distortion vs Frequency
2nd and 3rd Harmonic
Distortion vs Frequency
Output Voltage Swing vs
Frequency for THD < 0.1%
Output Voltage Swing vs
Frequency for Unlimited
Distortion
Output Current
vs Die Temperature
9
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
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,
R
L
= 150
to GND
Negative Output Voltage
Swing vs Die Temperature,
R
L
= 150
to GND
Clamp Accuracy
vs Die Temperature
10
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
IClamp Accuracy
R
L
= 150
Clamp Accuracy
R
L
= 1 k
Clamp Accuracy
R
L
= 10 k
Enable Response for a
Family of DC Inputs
Disable Response for a
Family of DC Inputs
Disable/Enable Response for a
Family of Sine Waves
OFF Isolation
11
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
14-Lead Plastic DIP
Maximum Power Dissipation
vs Ambient Temperature
EL2257
Channel to Channel
Isolation vs Frequency
EL2357
Channel to Channel
Isolation vs Frequency
14-Lead Plastic SO
Maximum Power Dissipation
vs Ambient Temperature
16-Lead Plastic DIP
Maximum Power Dissipation
vs Ambient Temperature
16-Lead Plastic SO
Maximum Power Dissipation
vs Ambient Temperature
12
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
Simplified Schematic (One Channel)
13
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
Applications Information
Product Description
The EL2257C/EL2357C's, connected in voltage fol-
lower mode, -3 dB bandwidth is 125 MHz while
maintaining a 275 V/s slew rate. With an input and out-
put common mode range that includes ground, these
amplifiers were optimized for single supply operation,
but will also accept dual supplies. They operate 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 sup-
ply, 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, caus-
ing distortion and recovery delays. However, special
circuitry built into the EL2257C/EL2357C 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.
Lead 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 4.7 F tantalum capac-
itor in parallel with a 0.1 F ceramic capacitor has been
shown to work well when placed at each supply pin. For
single supply operation, where the GND pin is con-
nected to the ground plane, a single 4.7 F tantalum
capacitor in parallel with a 0.1 F ceramic capacitor
from the V
S+
pin to the GND pin will suffice.
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
package should be avoided if possible. Sockets add par-
asitic inductance and capacitance which will result in
some additional peaking and overshoot.
Supply Voltage Range and Single-Supply
Operation
The EL2257C/EL2357C have 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
EL2257C/EL2357C will operate on dual supplies rang-
ing from 1.35V to 6V. With a single-supply, the
EL2257C/EL2357C will operate from +2.7V to +12V.
Performance has been optimized for a single +5V
supply.
Pins 11 and 4 (14 and 3) are the power supply pins on
the EL2257C (EL2357C). The positive power supply is
connected to pin 11 (14). When used in single supply
mode, pin 4 (3) is connected to ground. When used in
dual supply mode, the negative power supply is con-
nected to pin 4 (3).
As supply voltages continue to decrease, it becomes nec-
essary to provide input and output voltage ranges that
can get as close as possible to the supply voltages. The
EL2257C/EL2357C have 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 EL2257C/EL2357C have an input range
which spans from 0V to 3.8V.
The output range of the EL2257C/EL2357C is also quite
large. It includes the negative rail, and extends to within
1V of the top supply rail with a 1 k
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, R
F
The feedback resistor forms a pole with the input capac-
itance. As this pole becomes larger, phase margin is
reduced. This increases ringing in the time domain and
peaking in the frequency domain. Therefore, R
F
has
some maximum value which should not be exceeded for
optimum performance. If a large value of R
F
must be
used, a small capacitor in the few picofarad range in par-
allel with R
F
can help to reduce this ringing and peaking
at the expense of reducing the bandwidth.
14
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
As far as the output stage of the amplifier is concerned,
R
F
+ R
G
appear in parallel with R
L
for gains other than
+1. As this combination gets smaller, the bandwidth
falls off. Consequently, R
F
has a minimum value that
should not be exceeded for optimum performance.
For A
V
= +1, R
F
= 0
is optimum. For A
V
= -1 or +2
(noise gain of 2), optimum response is obtained with R
F
between 500
and 1 k
. For A
V
= -4 or +5 (noise gain
of 5), keep R
F
between 2 k
and 10 k
.
Video Performance
For good video performance, an amplifier is required to
maintain the same output impedance and the same fre-
quency response as DC levels are changed at the output.
This can be difficult when driving a standard video load
of 150
, because of the change in output current with
DC level. Differential Gain and Differential Phase for
the EL2257C/EL2357C 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 con-
figuration. This results in dG and dP specifications of
0.05% and 0.05 while driving 150
at a gain of +2.
Setting the black level to other values, although accept-
able, will compromise peak performance. For example,
looking at the single supply dG and dP curves for
R
L
=150
, if the output black level clamp is reduced
f r o m 1 . 2 V t o 0 . 6 V d G / d P w i l l i n c r e a s e f r o m
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 EL2257C/EL2357C,
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 1 k
, if the input black level is 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 5 mA of supply current,
the EL2257C/EL2357C are capable of providing 100
mA of output current into a 10
load, or 60 mA into
50
. With this large output current capability, a 50
load can be driven to 3V with V
S
= 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 resis-
tor will de-couple the EL2257C/EL2357C from the
cable and allow extensive capacitive drive. However,
other applications may have high capacitive loads with-
out a back-termination resistor. In these applications, a
small series resistor (usually between 5
and 50
) can
be placed in series with the output to eliminate most
peaking. The gain resistor (R
G
) 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
Each amplifier in the EL2257C/EL2357C can be indi-
vidually disabled, placing each output in a high-
impedance state. The disable or enable action takes only
about 40 ns. When all amplifiers are disabled, the total
supply current is reduced to 0 mA, thereby eliminating
all power consumption by the EL2257C/EL2357C. The
EL2257C/EL2357C 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 EL2257C/EL2357C. If the power-down feature is
not desired, connect all ENABLE pins to the V
S+
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.
15
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
Output Voltage Clamp
The EL2257C/EL2357C amplifiers have an output volt-
age clamp. This clamping action is fast, being activated
almost instantaneously, and being deactivated in < 7 ns,
and prevents the output voltage from going above the
preset clamp voltage. This can be very helpful when the
EL2257C/EL2357C are 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 EL2257C/EL2357C is connected in a
gain of 2, and +3V DC is applied to the CLAMP pin, any
voltage higher than +1.5V at the inputs will be clamped
and +3V will be seen at the output. Each amplifier of the
EL2257C have their own CLAMP pin, so individual
clamp levels may be set, whereas a single CLAMP pin
controls the clamp level of the EL2357C.
Figure 1 below is the EL2257C with each amplifier
unity gain connected. Amplifier A is being driven by a 3
Vp-p sinewave and has 2.25V applied to CLAMPA,
while amplifier B is driven by a 3 Vp-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 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 sig-
nal is clamped to the voltage applied to the CLAMP pin
almost instantaneously. The output recovers from the
clamped mode within 57 ns, 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 11 ns.
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 mil-
livolts. Be sure to allow for these inaccuracies when
choosing the clamp voltage. Curves of Clamp Accuracy
vs. V
CLAMP
and V
IN
for 3 values of R
L
are included in
the Typical Performance Curves Section.
Figure 1.
Figure 2.
Figure 3.
16
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
Unlike amplifiers that clamp at the input and are there-
fore limited to non-inverting applications only, the
EL2257C/EL2357C output clamp architecture works for
both inverting and non-inverting gain applications.
There is also no maximum voltage difference limitation
between V
IN
and V
CLAMP
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, V
OP
. Forcing the CLAMP pin much
below +1.2V can saturate transistors and should there-
fore be avoided. Forcing the CLAMP pin above V
OP
simply de-activates the CLAMP feature. In other words,
one cannot expect to clamp any voltage higher than what
the EL2257C/EL2357C 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 V
S+
pin.
EL2257C/EL2357C Comparator Application
The EL2257C/EL2357C can be used as a very fast, sin-
gle 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 EL2257C/EL2357C, the maximum output voltage
can be clamped, thus preventing saturation. Figure 4 is
amplifier A of an EL2257C implemented as a compara-
tor. 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, 10 MHz triangular wave is applied,
while Figure 6 is a graph of propagation delay vs. over-
drive as a square wave is presented at the input.
Figure 4.
Figure 5.
Figure 6.
Propagation Delay vs Overdrive
EL2257/EL2357 as a Comparator
17
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
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 use-
ful 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 EL2257C. Figure 8 shows the com-
plete input video signal applied at the input, as well as
the output signal with the negative going sync pulse
removed.
Multiplexing with the EL2257C/EL2357C
The ENABLE pins on the EL2257C/EL2357C allow for
multiplexing applications. Figure 9 shows an EL2357C
with all 3 outputs tied together, driving a back termi-
nated 75
video load. Three sinewaves of varying
amplitudes and frequencies are applied to the three
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 V
OUT
. Switching is complete in about 50
ns. Notice the outputs are tied directly together. Decou-
pling resistors at each output are not necessary. In fact,
adding them approximately doubles the switching time
to 100 ns.
Short Circuit Current Limit
The EL2257C/EL2357C have internal short circuit pro-
tection circuitry that protect it in the event of its output
being shorted to either supply rail. This limit is set to
around 100 mA nominally and reduces with increasing
junction temperature. It is intended to handle temporary
shorts. If an output is shorted indefinitely, the power dis-
sipation could easily increase such that the part will be
destroyed. Maximum reliability is maintained if the out-
put current never exceeds 90 mA. A heat sink may be
Figure 7.
Figure 8.
Figure 9.
Figure 10.
18
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
required to keep the junction temperature below abso-
lute maximum when an output is shorted indefinitely.
Power Dissipation
W i t h t h e h i g h o u t p u t d r i v e c a p a b i l i t y o f t h e
EL2257C/EL2357C, it is possible to exceed the 150C
Absolute Maximum junction temperature under certain
load current conditions. Therefore, it is important to cal-
culate the maximum junction temperature for the
application to determine if power-supply voltages, load
conditions, or package type need to be modified for the
EL2257C/EL2357C to remain in the safe operating area.
The maximum power dissipation allowed in a package is
determined by:
where:
T
JMAX
= Maximum Junction Temperature
T
AMAX
= Maximum Ambient Temperature
JA
= Thermal Resistance of the Package
PD
MAX
= 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:
where:
N = Number of amplifiers
V
S
= Total Supply Voltage
I
SMAX
= Maximum Supply Current per amplifier
V
OUT
= Maximum Output Voltage of the Application
R
L
= Load Resistance tied to Ground
If we set the two PD
MAX
equations, [1] and [2], equal to
each other, and solve for V
S
, we can get a family of
curves for various loads and output voltages according
to:
Figures 11 and 12 below show total single supply volt-
age V
S
vs. R
L
for various output voltage swings for the
PDIP and SOIC packages. The curves assume WORST
CASE conditions of T
A
= +85C and I
S
= 6.5 mA per
amplifier.
PD
MAX
T
JM AX
T
AMAX
JA
----------------------------------------------
=
PD
MAX
N
V
S
I
SM AX
V
S
V
OUT
(
)
V
OUT
R
L
----------------
+
=
V
S
R
L
T
JM AX
T
AMAX
(
)
N
JA
-----------------------------------------------------------------
V
OUT
(
)
2
+
I
S
R
L
(
)
V
OUT
+
-----------------------------------------------------------------------------------------------
=
EL2257 Single Supply Voltage
vs. R
Load
for Various
V
OUT
and Packages
Figure 11.
EL2357 Single Supply Voltage
vs. R
Load
for Various
V
OUT
and Packages
Figure 12.
19
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
EL
2257C/EL
2357C
EL2257C/EL2357C 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 250A
i2 7 11 250A
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
*
* Models
*
20
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
.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
21
EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
E
L
2257C/E
L2357C
General Disclaimer
Specifications contained in this data sheet are in effect as of the publication date shown. Elantec, Inc. reserves the right to make changes in the cir-
cuitry or specifications contained herein at any time without notice. Elantec, Inc. assumes no responsibility for the use of any circuits described
herein and makes no representations that they are free from patent infringement.
WARNING - Life Support Policy
Elantec, Inc. products are not authorized for and should not be used
within Life Support Systems without the specific written consent of
Elantec, Inc. Life Support systems are equipment intended to sup-
port or sustain life and whose failure to perform when properly used
in accordance with instructions provided can be reasonably
expected to result in significant personal injury or death. Users con-
templating application of Elantec, Inc. Products in Life Support
Systems are requested to contact Elantec, Inc. factory headquarters
to establish suitable terms & conditions for these applications. Elan-
tec, Inc.'s warranty is limited to replacement of defective
components and does not cover injury to persons or property or
other consequential damages.
Elantec, Inc.
1996 Tarob Court
Milpitas, CA 95035
Telephone: (408) 945-1323
(800) 333-6314
Fax:
(408) 945-9305
European Office: 44-71-482-4596
Jan
uary 5, 2000
Printed in U.S.A.
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