EL4450
a metal probe) or an oscilloscope probe on the input will kill
Applications Information
the oscillation. Normal high-frequency construction obviates
any such problems, where the input source is reasonably
close to the input. If this is not possible, one can insert series
resistors of around to 51Ω to de-Q the inputs.
The EL4450 is a complete four-quadrant multiplier with
90MHz bandwidth. It has three sets of inputs; a differential
multiplying X-input, a differential multiplying Y-input, and
another differential input which is used to complete a
feedback loop with the output. Here is a typical connection:
Signal Amplitudes
Signal input common-mode voltage must be between (V-)
+2.5V and (V+) -2.5V to ensure linearity. Additionally, the
differential voltage on any input stage must be limited to ±6V
to prevent damage. The differential signal range is ±2V in the
EL4450. The input range is substantially constant with
temperature.
The gain of the feedback divider is H, and
H = R /(R + R ). The transfer function of the part is:
G
G
F
V
= A × (1/2 × ((V
+–V -) × (V +–V -)) +
INX INX INY INY
OUT
O
(V
–V )).
REF FB
V
V
is connected to V
through a feedback network, so
FB
OUT
= H*V
. A is the open-loop gain of the amplifier, and
FB
OUT
O
The Ground Pin
is about 600. The large value of A drives:
O
The ground pin draws only 6µA maximum DC current, and
may be biased anywhere between (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 over
frequency, and if connected to a potential other than ground,
it must be bypassed.
(1/2 × ((V
+–V -) × (V
INX INX
+–V -)) + (V
INY INY
–
REF
V
))→0.
FB
Rearranging and substituting for V
:
REF
V
= (1/2 × ((V
+–V -) × (V +–V -))
INY INY
OUT
+V
INX
INX
= (XY/2 + V
OUT
)/H, or V
)/H
REF
REF
Thus the output is equal to one-half the product of X and Y
inputs and offset by V , all gained up by the feedback
Power Supplies
REF
divider ratio. The EL4450 is stable for a direct connection
between V and FB, and the feedback divider may be
The EL4450 works well on supplies from ±3V to ±15V. The
supplies may be of different voltages as long as the
requirements of the GND pin are observed (see the Ground
Pin section for a discussion). The supplies should be
bypassed close to the device with short leads. 4.7µF
tantalum capacitors are very good, and no smaller bypasses
need be placed in parallel. Capacitors as low as 0.01µF can
be used if small load currents flow.
OUT
used for higher output gain, although with the traditional loss
of bandwidth.
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 impedance of the feedback resistors and stray
capacitance should be at least 150MHz; typical strays of 3pF
thus require a feedback impedance of 360Ω or less.
Single-polarity supplies, such as +12V with +5V can be
used, where the ground pin is connected to +5V and V- to
ground. The inputs and outputs will have to have their levels
shifted above ground to accommodate the lack of negative
supply.
Alternatively, a small capacitor across R can be used to
F
create more of a frequency-compensated divider. The value
of the capacitor should scale with the parasitic capacitance
at the FB input. It is also practical to place small capacitors
across both the feedback resistors (whose values maintain
the desired gain) to swamp out parasitics. For instance, two
10pF capacitors across equal divider resistors for a
maximum gain of 1 will dominate parasitic effects and allow
a higher divider resistance.
The power dissipation of the EL4450 increases with power
supply voltage, and this must be compatible with the
package chosen. This is a close estimate for the dissipation
of a circuit:
P
=2*I ,max*V + (V –V )*V /R
S S S O O PAR
D
where
I ,max is the maximum supply current
The REF pin can be used as the output’s ground reference,
or for DC offsetting of the output, or it can be used to sum in
another signal.
S
V is the ± supply voltage (assumed equal)
S
Input Connections
V
is the output voltage
O
The input transistors can be driven from resistive and
capacitive sources, but are capable of oscillation when
presented with an inductive input. It takes about 80nH of
series inductance to make the inputs actually oscillate,
equivalent to four inches of unshielded wiring or about 6 of
unterminated input transmission line. The oscillation has a
characteristic frequency of 500MHz. Placing one’s finger (via
R
is the parallel of all resistors loading the output
PAR
For instance, the EL4450 draws a maximum of 18mA. With
light loading, R →∞ and the dissipation with ±5V supplies
PAR
is 180mW. The maximum supply voltage that the device can
run on for a given P and the other parameters is:
D
2
V ,max = (P + V /R
)/(2I + V /R )
S
D
O
PAR PAR
S
O
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