10
FN7105.5
August 11, 2015
Description of Operation and Application
Information
Product Description
The EL8200, EL8201 and EL8401 are wide bandwidth,
single supply, low power and rail-to-rail output voltage
feedback operational amplifiers. The amplifiers are internally
compensated for closed loop gain of +1 of greater.
Connected in voltage follower mode and driving a 1k load,
they have a -3dB bandwidth of 200MHz. Driving a 150
load, the bandwidth is about 130MHz while maintaining a
200V/us slew rate. The EL8200 is available with a power
down pin to reduce power to 30µA typically while the
amplifier is disabled.
Input, Output and Supply Voltage Range
The EL8200, EL8201 and EL8401 have been designed to
operate with a single supply voltage from 3V to 5.0V. Split
supplies can also be used as long as their total voltage is
within 3V to 5.0V. The amplifiers have an input common
mode voltage range from 0.15V below the negative supply
(V
S
- pin) to within 1.5V of the positive supply (V
S
+ pin). If the
input signal is outside the above specified range, it will cause
the output signal to be distorted.
The output of the EL8200, EL8201 and EL8401 can swing
rail to rail. As the load resistance becomes lower, the ability
to drive close to each rail is reduced. For the load resistor
1k, the output swing is about 4.9V at a 5V supply. For the
load resistor 150, the output swing is about 4.6V.
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
has
some maximum value that should not be exceeded 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.
As far as the output stage of the amplifier is concerned, the
output stage is also a gain stage with the load. R
F
and 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
also has a minimum value that should not
be exceeded for optimum performance. For gain of +1, R
F
=0
is optimum. For the gains other than +1, optimum response
is obtained with R
F
between 300 to 1k.
The EL8200, EL8201 and EL8401 have a gain bandwidth
product of 100MHz. For gains 5, its bandwidth can be
predicted by the following equation:
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 150, because the change in output current with DC level.
Special circuitry has been incorporated in the EL8200,
EL8201 and EL8401 to reduce the variation of the output
impedance with the current output. This results in dG and dP
specifications of 0.03% and 0.05, while driving 150 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 EL8200, EL8201 and EL8401 can drive 10pF loads in
parallel with 1k with less than 5dB of peaking at gain of +1.
If less peaking is desired in applications, a small series
resistor (usually between 5 to 50) 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 EL8200 can be disabled and placed its output in a high
impedance state. The turn off time for each channel is about
25ns and the turn on time is about 200ns. When disabled,
the amplifier’s supply current is reduced to 30µA 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 V
S
- 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 2V above V
S
-.
Output Drive Capability
The EL8200, EL8201 and EL8401 do not have internal short
circuit protection circuitry. They have a typical short circuit
current of 70mA sourcing and 140mA sinking for the output
is connected to half way between the rails with a 10
resistor. If the output is shorted indefinitely, the power
EL8200, EL8201, EL8401
