EL5166 Intersil Corporation, EL5166 Datasheet - Page 7

• Manufacturer Part Number: EL5166
• Description: 1.4GHz Current Feedback Amplifiers with Enable
• Manufacturer: Intersil Corporation

inductance. Use of sockets, particularly for the SO package,
should be avoided if possible. Sockets add parasitic
inductance and capacitance which will result in additional
peaking and overshoot.
Disable/Power-Down
The EL5166 amplifier can be disabled placing its output in a
high impedance state. When disabled, the amplifier supply
current is reduced to 13µA. The EL5166 is disabled when its
CE pin is pulled up to within 1V of the positive supply.
Similarly, the amplifier is enabled by floating or pulling its CE
pin to at least 3V below the positive supply. For ±5V supply,
this means that an EL5166 amplifier will be enabled when
CE is 2V or less, and disabled when CE is above 4V.
Although the logic levels are not standard TTL, this choice of
logic voltages allows the EL5166 to be enabled by tying CE
to ground, even in 5V single supply applications. The CE pin
can be driven from CMOS outputs.
Capacitance at the Inverting Input
Any manufacturer’s high-speed voltage- or current-feedback
amplifier can be affected by stray capacitance at the
inverting input. For inverting gains, this parasitic capacitance
has little effect because the inverting input is a virtual
ground. But for non-inverting gains, this capacitance (in
conjunction with the feedback and gain resistors) creates a
pole in the feedback path of the amplifier. This pole, if low
enough in frequency, has the same destabilizing effect as a
zero in the forward open-loop response. The use of large
value feedback and gain resistors exacerbates the problem
by further lowering the pole frequency (increasing the
possibility of oscillation).
The EL5166 and EL5167 frequency responses are
optimized with the resistor values in Figure 9. With the high
bandwidth of these amplifiers, these resistor values might
cause stability problems when combined with parasitic
capacitance, thus ground plane is not recommended around
the inverting input pin of the amplifier.
Feedback Resistor Values
The EL5166 and EL5167 have been designed and specified
at a gain of +2 with R
feedback resistor gives 800MHz of -3dB bandwidth at A
with about 0.5dB of peaking. Since the EL5166 and EL5167
are current-feedback amplifiers, it is also possible to change
the value of R
of Frequency Response for Various R
and peaking can be easily modified by varying the value of
the feedback resistor.
Because the EL5166 and EL5167 are current-feedback
amplifiers, their gain-bandwidth product is not a constant for
different closed-loop gains. This feature actually allows the
EL5166 and EL5167 to maintain reasonable constant -3dB
bandwidth for different gains. As gain is increased,
bandwidth decreases slightly while stability increases. Since
the loop stability is improving with higher closed-loop gains,
F
to get more bandwidth. As seen in the curve
F
approximately 392Ω. This value of
7
F
and R
G
, bandwidth
EL5166, EL5167
V
= 2
it becomes possible to reduce the value of R
specified 250Ω and still retain stability, resulting in only a
slight loss of bandwidth with increased closed-loop gain.
Supply Voltage Range and Single-Supply
Operation
The EL5166 and EL5167 have been designed to operate
with supply voltages having a span of greater than 5V and
less than 10V. In practical terms, this means that the EL5166
and EL5167 will operate on dual supplies ranging from
±2.5V to ±5V. With single-supply, they will operate from 5V to
10V.
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
EL5166 and EL5167 have an input range which extends to
within 1.8V of either supply. So, for example, on ±5V
supplies, the EL5166 and EL5167 have an input range
which spans ±3.2V. The output range of the EL5166 and
EL5167 is also quite large, extending to within 1V of the
supply rail. On a ±5V supply, the output is therefore capable
of swinging from -4V to +4V.
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. Previously, good differential gain
could only be achieved by running high idle currents
through the output transistors (to reduce variations in
output impedance.) These currents were typically
comparable to the entire 8.5mA supply current of each
EL5166 and EL5167 amplifier. Special circuitry has been
incorporated in the EL5166 and EL5167 to reduce the
variation of output impedance with current output. This
results in dG and dP specifications of 0.01% and 0.03°,
while driving 150Ω at a gain of 2.
Output Drive Capability
In spite of their low 8.5mA of supply current, the EL5166 and
EL5167 are capable of providing a minimum of ±110mA of
output current. With so much output drive, the EL5166 and
EL5167 are capable of driving 50Ω loads to both rails,
making them 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 EL5166 and EL5167 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
F
below the