el2386 Intersil Corporation, el2386 Datasheet - Page 9

• Manufacturer Part Number: el2386
• Description: 250mhz Triple Current Feedback Amplifier With Disable
• Manufacturer: Intersil Corporation

Applications Information
Product Description
The EL2386 is a current-feedback operational amplifier that
offers a wide -3dB bandwidth of 250MHz, a low supply
current of 3mA per amplifier and the ability to power down to
0mA. It also features high output current drive. The EL2386
can output 55mA per amplifier. The EL2386 works with
supply voltages ranging from a single 3V to ±6V, and it is
also capable of swinging to within 1V of either supply on the
input and the output. Because of its current-feedback
topology, the EL2386 does not have the normal gain-
bandwidth product associated with voltage-feedback
operational amplifiers. This allows its -3dB bandwidth to
remain relatively constant as closed-loop gain is increased.
This combination of high bandwidth and low power, together
with aggressive pricing make the EL2386 the ideal choice for
many low-power/high-bandwidth applications such as
portable computing, HDSL, and video processing.
For single and dual applications, consider the
EL2186/EL2286. For single, dual and quad applications
without disable, consider the EL2180, EL2280, or EL2480,
all in industry standard pin outs. The EL2180 also is
available in the tiny SOT-23 package, which is 28% the size
of an SO8 package. For lower power applications where
speed is still a concern, consider the EL2170/EL2176 family
which also comes in similar single, dual and quad
configurations with 70MHz of bandwidth while consuming
1mA of supply current per amplifier.
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. Pin 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 4.7µF tantalum capacitor in parallel with a
0.1µF capacitor has been shown to work well when placed at
each supply pin. For single supply operation, where pin 3
(V
tantalum capacitor in parallel with a 0.1µF ceramic capacitor
across pins 14 and 3 will suffice.
For good AC performance, parasitic capacitance should be
kept to a minimum especially at the inverting input (see the
Capacitance at the Inverting Input section). Ground plane
construction should be used, but it should be removed from
the area near the inverting input to minimize any stray
capacitance at that node. 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 parasitic
inductance and capacitance which will result in some
additional peaking and overshoot.
S
-
) is connected to the ground plane, a single 4.7µF
9
EL2386
Disable/Power-Down
The EL2386 amplifier can be disabled, placing its output in a
high-impedance state. When disabled, the supply current is
reduced to 0mA. The EL2386 is disabled when its ENABLE
pin is floating or pulled up to within 0.5V of the positive
supply. Similarly, the amplifier is enabled by pulling its
ENABLE pin at least 3V below the positive supply. For ±5V
supplies, this means that an EL2386 amplifier will be
enabled when ENABLE is at 2V or less, and disabled when
ENABLE is above 4.5V. Although the logic levels are not
standard TTL, this choice of logic voltages allows the
EL2386 to be enabled by tying ENABLE to ground, even in
+3V single-supply applications. The ENABLE pin can be
driven from CMOS outputs or open-collector TTL.
When enabled, supply current does vary somewhat with the
voltage applied at ENABLE. For example, with the supply
voltages of the EL2186 at ±5V, if ENABLE is tied to -5V
(rather than ground) the supply current will increase about
15% to 3.45mA.
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 further exacerbates the
problem by further lowering the pole frequency.
The experienced user with a large amount of PC board
layout experience may find in rare cases that the EL2386
has less bandwidth than expected. In this case, the inverting
input may have less parasitic capacitance than expected.
The reduction of feedback resistor values (or the addition of
a very small amount of external capacitance at the inverting
input, e. g. 0.5pF) will increase bandwidth as desired. Please
see the curves for Frequency Response for Various R
R
Feedback Resistor Values
The EL2386 has been designed and specified at gains of +1
and +2 with R
250MHz of -3dB bandwidth at A
peaking, and 180MHz of -3dB bandwidth at A
about 0.1dB of peaking. Since the EL2386 is a current-
feedback amplifier, it is also possible to change the value of
R
Frequency Response For Various R
and peaking can be easily modified by varying the value of
the feedback resistor.
Because the EL2386 is a current-feedback amplifier, its
gain-bandwidth product is not a constant for different closed-
G
F
, and Frequency Response for Various C
to get more bandwidth. As seen in the curve of
F
= 750Ω. This value of feedback resistor gives
V
= +1 with about 2.5dB of
F
and R
IN
G
, bandwidth
V
-.
= +2 with
F
and