10
FN7386.6
July 7, 2009
EL5156 Product Description
The EL5156, EL5157, EL5256, and EL5257 are wide
bandwidth, single or dual supply, low power and low offset
voltage feedback operational amplifiers. Both amplifiers are
internally compensated for closed loop gain of +1 or greater.
Connected in voltage follower mode and driving a 500Ω
load, the -3dB bandwidth is about 610MHz. Driving a 150Ω
load and a gain of 2, the bandwidth is about 180MHz while
maintaining a 600V/µs slew rate. The EL5156 and EL5256
are available with a power-down pin to reduce power to
17µA typically while the amplifier is disabled.
Input, Output and Supply Voltage Range
The EL5156 and EL5157 families have been designed to
operate with supply voltage from 5V to 12V. That means for
single supply application, the supply voltage is from 5V to
12V. For split supplies application, the supply voltage is from
±2.5V to ±5V. The amplifiers have an input common mode
voltage range from 1.5V above the negative supply (VS- pin)
to 1.5V below the positive supply (VS+ pin). If the input
signal is outside the above specified range, it will cause the
output signal to be distorted.
The outputs of the EL5156 and EL5157 families can swing
from -4V to 4V for V
S
= ±5V. As the load resistance becomes
lower, the output swing is lower. If the load resistor is 500Ω,
the output swing is about -4V at a 4V supply. If the load
resistor is 150Ω, the output swing is from -3.5V to 3.5V.
Choice of Feedback Resistor and Gain Bandwidth
Product
For applications that require a gain of +1, no feedback
resistor is required. Just short the output pin to the inverting
input pin. For gains greater than +1, the feedback resistor
forms a pole with the parasitic capacitance at the inverting
input. As this pole becomes smaller, the amplifier's phase
margin is reduced. This causes ringing in the time domain
and peaking in the frequency domain. Therefore, R
F
can't be
very big for optimum performance. If a large value of R
F
must be used, a small capacitor in the few Pico farad range
in parallel with R
F
can help to reduce the ringing and
peaking at the expense of reducing the bandwidth.
For gain of +1, R
F
= 0 is optimum. For the gains other than
+1, optimum response is obtained with R
F
between 500Ω to
750Ω.
The EL5156 and EL5157 families have a gain bandwidth
product of 210MHz. For gains ≥5, its bandwidth can be
predicted by Equation 1:
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 is especially difficult when driving a standard video load
of 150Ω, because of the change in output current with DC
level. The dG and dP for these families are about 0.006%
and 0.04%, while driving 150Ω at a gain of 2. Driving high
impedance loads would give a similar or better dG and dP
performance.
Driving Capacitive Loads and Cables
The EL5156 and EL5157 families can drive 27pF loads in
parallel with 500Ω with less than 5dB of peaking at gain of
+1. If less peaking is desired in applications, a small series
resistor (usually between 5Ω to 50Ω) can be placed in series
with the output to eliminate most peaking. However, this will
reduce the gain slightly. If the gain setting is greater than 1,
the gain resistor R
G
can then be chosen to make up for any
gain loss which may be created by the additional series
resistor at the output.
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, a back-termination series resistor at the
amplifier's output will isolate the amplifier from the cable and
allow extensive capacitive drive. However, other applications
may have high capacitive loads without a back-termination
resistor. Again, a small series resistor at the output can help
to reduce peaking.
Disable/Power-Down
The EL5156 and EL5256 can be disabled and their output
placed in a high impedance state. The turn-off time is about
330ns and the turn-on time is about 130ns. When disabled,
the amplifier's supply current is reduced to 17µA typically,
thereby effectively eliminating the power consumption. The
amplifier's power-down can be controlled by standard TTL or
CMOS signal levels at the ENABLE pin. The applied logic
signal is relative to VS- pin. Letting the ENABLE pin float or
applying a signal that is less than 0.8V above V
S
- will enable
the amplifier. The amplifier will be disabled when the signal
at ENABLE pin is above V
S
+ - 1.5V.
Output Drive Capability
The EL5156 and EL5157 families do not have internal short
circuit protection circuitry. They have a typical short circuit
current of 95mA and 70mA. If the output is shorted
indefinitely, the power dissipation could easily overheat the
die or the current could eventually compromise metal
integrity. Maximum reliability is maintained if the output
current never exceeds ±40mA. This limit is set by the design
of the internal metal interconnect. Note that in transient
applications, the part is robust.
Power Dissipation
With the high output drive capability of the EL5152 and
EL5153 families, it is possible to exceed the +125°C
absolute maximum junction temperature under certain load
current conditions. Therefore, it is important to calculate the
maximum junction temperature for an application to
Gain BW 210MHz=×
(EQ. 1)
EL5156, EL5157, EL5256, EL5257
