ADL5391
Rev. 0 | Page 10 of 16
GENERAL DESCRIPTION
BASIC THEORY
The multiplication of two analog variables is a fundamental
signal processing function that has been around for decades.
By convention, the desired transfer function is given by
W = αXY/U + Z (1)
where:
X and Y are the multiplicands.
U is the multiplier scaling factor.
α is the multiplier gain.
W is the product output.
Z is a summing input.
All the variables and the scaling factor have the dimension of volts.
In the past, analog multipliers, such as the
AD835, were
implemented almost exclusively with a Gilbert Cell topology
or a close derivative. The inherently asymmetric signal paths
for X and Y inevitably create amplitude and delay imbalances
between X and Y. In the ADL5391, the novel multiplier core
provides absolute symmetry between X and Y, minimizing
scaling and phasing differences inherent in the Gilbert Cell.
The simplified block diagram of the ADL5391 shows a main
multiplier cell that receives inputs X and Y and a second
multiplier cell in the feedback path around an integrating
buffer. The inputs to this feedback multiplier are the difference
of the output signal and the summing input, W − Z, and the
internal scaling reference, U. At dc, the integrating buffer
ensures that the output of both multipliers is exactly 0, therefore
(W − Z)xU = XY, or W = XY/U + Z (2)
By using a feedback multiplier that is identical to the main
multiplier, the scaling is traced back solely to U, which is
an accurate reference generated on-chip. As is apparent in
Equation 2, noise, drift, or distortion that is common to both
multipliers is rejected to first-order because the feedback
multiplier essentially compensates the impairments generated
in the main multiplier.
The scaling factor, U, is fixed by design to 1.12 V. However, the
multiplier gain, α, can be adjusted by driving the GADJ pin with
a voltage ranging from 0 V to 2 V. If left floating, then α = 1 or
0 dB, and the overall scaling is simply U = 1 V. For VGADJ = 0 V,
the gain is lowered by approximately 4 dB; for VGADJ = 2 V,
the gain is raised by approximately 6 dB.
Figure 5 shows the
relationship between α(V/V) and VGADJ.
The small-signal bandwidth from the inputs X, Y, and Z to
the output W is a single-pole response. The pole is inversely
proportional to α. For α = 1 (GADJ floating), the bandwidth is
about 2 GHz; for α > 1, the bandwidth is reduced; and for α < 1,
the bandwidth is increased.
All input ports, X, Y, and Z, are differential and internally
biased to midsupply, V
POS
/2. The differential input impedance is
500 Ω up to 100 MHz, rolling off to 50 at 2 GHz. All inputs
can be driven in single-ended fashion and can be ac-coupled. In
dc-coupled operation, the inputs can be biased to a common
mode that is lower than V
POS
/2. The bias current flowing out of
the input pins to accommodate the lower common mode is
subtracted from the 50 mA total available from the internal
reference V
POS
/2 at the VREF pin. Each input pin presents an
equivalent 250 dc resistance to V
POS
/2. If all six input pins sit
1 V below V
POS
/2, a total of 6 × 1 V/250 Ω = 24 mA must flow
internally from VREF to the input pins.
Calibration
The dc offset of the ADL5391 is approximately 20 mV but
changes over temperature and has variation from part to part
(see
Figure 4). It is generally not of concern unless the ADL5391
is operated down to dc (close to the point X = 0 V or Y = 0 V),
where 0 V is expected on the output (W = 0 V). For example,
when the ADL5391 is used as a VGA and a large amount of
attenuation is needed, the maximum attenuation is determined
by the input dc offset.
Applying the proper voltage on the Z input removes the W
offset. Calibration can be accomplished by making the appropriate
cross plots and adjusting the Z input to remove the offset.
Additionally, gain scaling can be adjusted by applying a dc
voltage to the GADJ pin, as shown in
Figure 5.
BASIC CONNECTIONS
Multiplier Connections
The best ADL5391 performance is achieved when the X, Y, and
Z inputs and W output are driven differentially; however, they
can be driven single-ended. Single-ended-to-differential
transformations (or differential-to-single-ended transformations)
can be done using a balun or active components, such as the
AD8313, the AD8132 (both with operation down to dc), or the
AD8352 (for higher drive capability). If using the ADL5391
single-ended without ac coupling capacitors, the reference
voltage of 2.5 V needs to be taken into account. Voltages above
2.5 V are positive voltages and voltages below 2.5 V are negative
voltages. Care needs to be taken not to load the ADL5391 too
heavily, the maximum reference current available is 50 mA.
