EL2280 Intersil Corporation, EL2280 Datasheet - Page 11

• Manufacturer Part Number: EL2280
• Description: 250MHz/3mA Current Mode Feedback Amplifiers
• Manufacturer: Intersil Corporation

Applications Information
Product Description
The EL2180, EL2280, and EL2480 are current-feedback
operational amplifiers that offer a wide -3dB bandwidth of
250MHz and a low supply current of 3mA per amplifier. All of
these products also feature high output current drive. The
EL2180 can output 100mA, while the EL2280 and the
EL2480 can output 55mA per amplifier. The EL2180,
EL2280, and EL2480 work with supply voltages ranging from
a single 3V to ±6V, and they are also capable of swinging to
within 1V of either supply on the input and the output.
Because of their current-feedback topology, the EL2180,
EL2280, and EL2480 do not have the normal gain-
bandwidth product associated with voltage-feedback
operational amplifiers. This allows their -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 EL2180, EL2280, and
EL2480 the ideal choice for many low-power/high-bandwidth
applications such as portable computing, HDSL, and video
processing.
For applications where board space is extremely critical, the
EL2180 is available in the tiny 5-pin SOT-23 package, which
has a footprint 28% the size of an 8-pin SO. The EL2180,
EL2280, and EL2480 are each also available in industry
standard pinouts in PDIP and SO packages.
For single, dual and triple applications with disable, consider
the EL2186 (8-pin single), EL2286 (14-pin dual) and EL2386
(16-pin triple). If lower power is required, refer to the
EL2170/EL2176 family which provides singles, duals, and
quads 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. Lead 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 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
11
EL2180, EL2280, EL2480
inductance and capacitance which will result in some
additional peaking and overshoot.
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 EL2180,
EL2280, and EL2480 have less bandwidth 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 EL2180, EL2280, and EL2480 have been designed and
specified at gains of +1 and +2 with R
SO packages and R
values of feedback resistors give 250MHz of -3dB bandwidth
at A
bandwidth at A
SOT23-5 package is characterized with a smaller value of
feedback resistor, for a given bandwidth, to compensate for
lower parasitics within both the package itself and the printed
circuit board where it will be placed. Since the EL2180,
EL2280, and EL2480 are current-feedback amplifiers, it is
also possible to change the value of R
bandwidth. As seen in the curve of Frequency Response For
Various R
modified by varying the value of the feedback resistor.
Because the EL2180, EL2280, and EL2480 are current-
feedback amplifiers, their gain-bandwidth product is not a
constant for different closed-loop gains. This feature actually
allows the EL2180, EL2280, and EL2480 to maintain about
the same -3dB bandwidth, regardless of closed-loop gain.
However, as closed-loop gain is increased, bandwidth
decreases slightly while stability increases. Since the loop
stability is improving with higher closed-loop gains, it
becomes possible to reduce the value of R
specified 560Ω and 750Ω and still retain stability, resulting in
only a slight loss of bandwidth with increased closed-loop
gain.
G
, and Frequency Response for Various C
V
= +1 with about 2.5dB of peaking, and 180MHz of -3dB
F
and R
V
= +2 with about 0.1dB of peaking. The
G
, bandwidth and peaking can be easily
F
= 560Ω in SOT23-5 package. These
F
F
= 750Ω in PDIP and
to get more
F
IN
below the
-.
F
and