EL4430C EL4431C
Video Instrumentation Amplifiers
Applications Information
The EL4430 and EL4431 are designed to convert
a fully differential input to a single-ended output
It has two sets of inputs one which is connected
to the signal and does not respond to its com-
mon-mode level and another which is used to
complete a feedback loop with the output Here is
a typical connection
stray capacitance should be at least 200 MHz
typical strays of 3 pF thus require a feedback im-
pedance of 270X or less Two 510X resistors are
acceptable for a gain of 2 300X and 2700X make
a good gain-of-10 divider Alternatively a small
capacitor across R
F
can be used to create more of
a frequency-compensated divider The value of
the capacitor should scale with the parasitic ca-
pacitance at the FB terminal input It is also
practical to place small capacitors across both the
feedback resistors (whose values maintain the de-
sired gain) to swamp out parasitics For instance
two 10 pF capacitors (for a gain of 2) across equal
divider resistors will dominate parasitic effects
and allow a higher divider resistance
Input Connections
4430 – 2
The gain of the feedback divider is H The trans-
fer function of the part is
V
OUT
e
A
O
c
((V
IN
a
)
b
(V
IN
b
)
a
(V
REF
b
V
FB
))
V
FB
is connected to V
OUT
through a feedback
network so V
FB
e
H
c
V
OUT
A
O
is the open-
loop gain of the amplifier and is about 600 for
the EL4430 and EL4431 The large value of A
O
drives
(V
IN
a
)
b
(V
IN
b
)
a
(V
REF
b
V
FB
)
Rearranging and substituting for V
FB
V
OUT
e
((V
IN
a
)
b
(V
IN
b
)
a
V
REF
) H
Thus the output is equal to the difference of the
V
IN
’s and offset by V
REF
all gained up by the
feedback divider ratio The input impedance of
the FB terminal (equal to R
IN
of the input termi-
nals) is in parallel with an R
G
and raises circuit
gain slightly
The EL4430 is stable for a gain of 1 (a direct
connection between V
OUT
and FB) or more and
the EL4431 for gains of 2 or more It is important
to keep the feedback divider’s impedance at the
FB terminal low so that stray capacitance does
not diminish the loop’s phase margin The pole
caused by the parallel of resistors R
F
and R
G
and
x
0
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 4 of
unshielded wiring or about 6 of unterminated
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 approximately
51X to de-Q the inputs
Signal Amplitudes
Signal input common-mode voltage must be be-
tween (V
b
)
a
3V and (V
a
)
b
3V to ensure linear-
ity Additionally the differential voltage on any
input stage must be limited to
g
6V to prevent
damage The differential signal range is
g
2V in
the EL4430 and EL4431 The input range is sub-
stantially constant with temperature
The Ground Pin
The ground pin draws only 6mA maximum DC
current and may be biased anywhere between
(V
b
)
a
2 5V and (V
a
)
b
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 CMRR over frequen-
cy and if connected to a potential other than
ground it must be bypassed
8
ELANTEC [ ELANTEC SEMICONDUCTOR ]
