HA-2556 查看數據表（PDF） - Intersil
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HA-2556 Datasheet PDF : 15 Pages
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HA-2556
Application Information
Operation at Reduced Supply Voltages
The HA-2556 will operate over a range of supply voltages,
±5V to ±15V. Use of supply voltages below ±12V will reduce
input and output voltage ranges. See “Typical Performance
Curves” for more information.
Offset Adjustment
X and Y channel offset voltages may be nulled by using a
20K potentiometer between the VYIO or VXIO adjust pin A
and B and connecting the wiper to V-. Reducing the channel
offset voltage will reduce AC feedthrough and improve the
multiplication error. Output offset voltage can also be nulled
by connecting VZ- to the wiper of a potentiometer which is
tied between V+ and V-.
Capacitive Drive Capability
When driving capacitive loads >20pF a 50Ω resistor should
be connected between VOUT and VZ+, using VZ+ as the
output (see Figure 1). This will prevent the multiplier from
going unstable and reduce gain peaking at high frequencies.
The 50Ω resistor will dampen the resonance formed with the
capacitive load and the inductance of the output at pin 8.
Gain accuracy will be maintained because the resistor is
inside the feedback loop.
Theory of Operation
The HA-2556 creates an output voltage that is the product
of the X and Y input voltages divided by a constant scale
factor of 5V. The resulting output has the correct polarity in
each of the four quadrants defined by the combinations of
positive and negative X and Y inputs. The Z stage provides
the means for negative feedback (in the multiplier
configuration) and an input for summation into the output.
This results in the following equation, where X, Y and Z are
high impedance differential inputs.
NC
NC
NC
VY+
-15V
1
16
REF
2
15
3
14
4
5
+-
6
7
8
+- 13
12
11
+
Σ
-
+-
10
9
NC
NC
NC
VX+
+15 V
VZ -
VZ +
50Ω
1kΩ
VOUT
20pF
FIGURE 1. DRIVING CAPACITIVE LOAD
VOUT = Z = X------5x-----Y--
To accomplish this the differential input voltages are first
converted into differential currents by the X and Y input
transconductance stages. The currents are then scaled by a
constant reference and combined in the multiplier core. The
multiplier core is a basic Gilbert Cell that produces a
differential output current proportional to the product of X and
Y input signal currents. This current becomes the output for
the HA-2557.
The HA-2556 takes the output current of the core and feeds it
to a transimpedance amplifier, that converts the current to a
voltage. In the multiplier configuration, negative feedback is
provided with the Z transconductance amplifier by connecting
VOUT to the Z input. The Z stage converts VOUT to a current
which is subtracted from the multiplier core before being
applied to the high gain transimpedance amp. The Z stage, by
virtue of it’s similarity to the X and Y stages, also cancels
second order errors introduced by the dependence of VBE on
collector current in the X and Y stages.
The purpose of the reference circuit is to provide a stable
current, used in setting the scale factor to 5V. This is
achieved with a bandgap reference circuit to produce a
temperature stable voltage of 1.2V which is forced across a
NiCr resistor. Slight adjustments to scale factor may be
possible by overriding the internal reference with the VREF
pin. The scale factor is used to maintain the output of the
multiplier within the normal operating range of ±5V when
full scale inputs are applied.
The Balance Concept
The open loop transfer equation for the HA-2556 is:
VOUT = A (---V----X----+--------V----X-------)--5--x--V---(--V-----Y---+-----–----V----Y-------) - (VZ+ -VZ-)
where;
A
= Output Amplifier Open Loop Gain
VX, VY, VZ = Differential Input Voltages
5V
= Fixed Scaled Factor
An understanding of the transfer function can be gained by
assuming that the open loop gain, A, of the output amplifier
is infinite. With this assumption, any value of VOUT can be
generated with an infinitesimally small value for the terms
within the brackets. Therefore we can write the equation:
0 = (---V----X----+--------V----X-------)--5--x-V----(--V----Y----+--------V----Y-------) - (VZ+ -VZ-)
which simplifies to:
(VX+ -VX-) x (VY+ -VY-) = 5V (VZ+ -VZ-)
This form of the transfer equation provides a useful tool to
analyze multiplier application circuits and will be called the
Balance Concept.
4
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