9
FN7385.6
March 2, 2009
EL5152 Product Description
The EL5152, EL5153, EL5252, and EL5455 are wide
bandwidth, low power, low offset voltage feedback
operational amplifiers capable of operating from a single or
dual power supplies. This family of operational amplifiers are
internally compensated for closed loop gain of +1 or greater.
Connected in voltage follower mode, driving a 500Ω load
members of this amplifier family demonstrate a -3dB
bandwidth of about 270MHz. With the loading set to
accommodate typical video application, 150Ω load and gain
set to +2, bandwidth reduces to about 180MHz with a
600V/µs slew rate. Power down pins on the EL5152 and
EL5252 reduce the already low power demands of this
amplifier family to 17µA typical while the amplifier is
disabled.
Input, Output and Supply Voltage Range
The EL5152 and EL5153 families have been designed to
operate with supply voltage ranging from 5V to 12V. Supply
voltages range from ±2.5V to ±5V for split supply operation.
Of course split supply operation can easily be achieved
using single supplies by splitting off half of the single supply
with a simple voltage divider as illustrated in the application
circuit section.
Input Common Mode Range
These amplifiers have an input common mode voltage
ranging from 1.5V above the negative supply (V
S
- pin) to
1.5V below the positive supply (V
S
+ pin). If the input signal is
driven beyond this range the output signal will exhibit
distortion.
Maximum Output Swing & Load Resistance
The outputs of the EL5152 and EL5153 families maximum
output swing ranges from -4V to 4V for V
S
= ±5V with a load
resistance of 500Ω. Naturally, as the load resistance
becomes lower, the output swing lowers accordingly; for
instance, if the load resistor is 150Ω, the output swing
ranges from -3.5V to 3.5V. This response is a simple
application of Ohms law indicating a lower value resistance
results in greater current demands of the amplifier.
Additionally, the load resistance affects the frequency
response of this family as well as all operational amplifiers,
as clearly indicated by the Gain vs Frequency for Various RL
curves clearly indicate. In the case of the frequency
response reduced bandwidth with decreasing load
resistance is a function of load resistance in conjunction with
the output zero response of the amplifier.
Choosing a Feedback Resistor
A feedback resistor is required to achieve unity gain; simply
short the output pin to the inverting input pin. Gains greater
than +1 require a feedback and gain resistor to set the
desired gain. This gets interesting because the feedback
resistor forms a pole with the parasitic capacitance at the
inverting input. As the feedback resistance increases the
position of the pole shifts in the frequency domain, the
amplifier's phase margin is reduced and the amplifier
becomes less stable. Peaking in the frequency domain and
ringing in the time domain are symptomatic of this shift in
pole location. So we want to keep the feedback resistor as
small as possible. You may want to use a large feedback
resistor for some reason; in this case to compensate the shift
of the pole and maintain stability a small capacitor in the few
Pico farad range in parallel with the feedback resistor is
recommended.
For the gains greater than unity, it has been determined a
feedback resistance ranging from 500Ω to 750Ω provides
optimal response.
Gain Bandwidth Product
The EL5156 and EL5157 families have a gain bandwidth
product of 210MHz for a gain of +5. Bandwidth can be
predicted by the following Equation 1:
Video Performance
For good video performance, an amplifier is required to
maintain the same output impedance and same frequency
response as DC levels are changed at the output; this
characteristic is widely referred to as “diffgain-diffphase”.
Many amplifiers have a difficult time with this especially while
driving standard video loads of 150Ω, as the output current
has a natural tendency to change with DC level. The EL5152
dG and dP for these families is a respectable 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 as the current output demands placed on the
amplifier lessen with increased load.
Driving Capacitive Loads
The EL5152 and EL5153 families can easily drive capacitive
loads as demanding as 27pF in parallel with 500Ω while
holding peaking to within 5dB of peaking at unity gain. Of
course if less peaking is desired, a small series resistor
(usually between 5Ω to 50Ω) can be placed in series with the
output to eliminate most peaking. However, there will be a
small sacrifice of gain which can be recovered by simply
adjusting the value of the gain resistor.
Driving Cables
Both ends of all cables must always be properly terminated;
double termination is absolutely necessary for reflection-free
performance. Additionally, 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.
Gain BW GainBandwidthProduct=×
(EQ. 1
EL5152, EL5153, EL5252, EL5455
