Input Connections
Applications Information Ð Contd.
Thus the output is equal to the difference of the
The input transistors can be driven from resistive
and capacitive sources, but are capable of oscilla-
tion when presented with an inductive input. It
takes about 80nH of series inductance to make
the inputs actually oscillate, equivalent to four
V
IN
by V
’s times the difference of V and offset
GAIN’S
, all gained up by the feedback divider
REF
ratio. The EL4452 is stable for a divider ratio of
(/10, and the divider may be set for higher output
gain, although with the traditional loss of band-
width.
inches of unshielded wiring or 6 of unterminat-
×
ed input transmission line. The oscillation has a
characteristic frequency of 500 MHz. Often plac-
ing one’s finger (via a metal probe) or an oscillo-
scope probe on the input will kill the oscillation.
Normal high-frequency construction obviates
any such problems, where the input source is rea-
sonably close to the input. If this is not possible,
one can insert series resistors of around 51X to
de-Q the inputs.
It is important to keep the feedback divider’s im-
pedance at the FB terminal low so that stray ca-
pacitance does not diminish the loop’s phase
margin. The pole caused by the parallel imped-
ance of the feedback resistors and stray capaci-
tance should be at least 130 MHz; typical strays
of 3 pF thus require a feedback impedance of
400X or less. Alternatively, a small capacitor
Signal Amplitudes
Signal input common-mode voltage must be be-
across R can be used to create more of a fre-
F
quency-compensated divider. The value of the ca-
pacitor should scale with the parasitic capaci-
tance at the FB input. It is also practical to place
small capacitors across both the feedback and the
gain resistors (whose values maintain the desired
gain) to swamp out parasitics. For instance, a
b
a
a b
) 2.5V to ensure
linearity. Additionally, the differential voltage on
tween (V
)
2.5V and (V
g
any input stage must be limited to 6V to pre-
vent damage. The differential signal range is
g
0.5V in the EL4452. The input range is sub-
stantially constant with temperature.
3 pF capacitor across R and 27 pF to ground
F
will dominate parasitic effects in a (/10 divider
The Ground Pin
The ground pin draws only 6 mA maximum DC
current, and may be biased anywhere between
and allow a higher divider resistance.
The REF pin can be used as the output’s ground
reference, for DC offsetting of the output, or it
can be used to sum in another signal.
b a
)
a b
(V
2.5V and (V
) 3.5V. The ground pin is
connected to the IC’s substrate and frequency
compensation components. It serves as a shield
within the IC and enhances input stage CMRR
and feedthrough over frequency, and if connected
to a potential other than ground, it must be by-
passed.
Gain-Control Characteristics
The quantity V
in the above equations is
2, even though the exter-
GAIN
GAIN
s
s
bounded as 0
V
nally applied voltages exceed this range. Actual-
ly, the gain transfer function around 0 and 2V is
‘‘soft’’; that is, the gain does not clip abruptly
Power Supplies
The EL4452 operates with power supplies from
g g
3V to 15V. The supplies may be of different
voltages as long as the requirements of the
ground pin are observed (see the Ground Pin sec-
tion). The supplies should be bypassed close to
the device with short leads. 4.7 mF tantalum ca-
pacitors are very good, and no smaller bypasses
need be placed in parallel. Capacitors as small as
0.01 mF can be used if small load currents flow.
below the 0%-V
voltage nor above the
level. An overdrive of 0.3V must be
GAIN
100%-V
GAIN
applied to V
to obtain truly 0% or 100%.
levels cannot
GAIN
Because the 0%- or 100%- V
GAIN
be precisely determined, they are extrapolated
from two points measured inside the slope of the
gain transfer curve. Generally, an applied V
GAIN
b
range of 0.5V to 2.5V will assure the full nu-
merical span of 0
a
s
s
V
GAIN
2.
a
5V can be used, where the ground pin is con-
The gain control has a small-signal bandwidth
equal to the V channel bandwidth, and over-
load recovery resolves in about 20 nsec.
Single-polarity supplies, such as
a
nected to
12V with
IN
a b
5V and V to ground. The inputs
8
ELANTEC [ ELANTEC SEMICONDUCTOR ]
