Datasheet

FOUR-QUADRANT MULTIPLIER DIFFERENCE AMPLIFIER
V =
OUT
´ V V +
G
´
IN
R
F
R
G
R
F
R
G
-
R
F
R
1
´ V
IN
(1)
V =
OUT
´ V V
IN
´
G
R
F
R
G
(2)
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
R
2
R
3
20W
V
IN
V
G
R
S
Source
Impedance
R
1
VCA824
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
R
S
R
S
20W
V
IN+
V
IN-
VCA824
85
80
75
70
65
60
55
50
45
40
Frequency(Hz)
Common-ModeRejectionRatio(dB)
10k 100k 1M 10M 100M
InputReferred
1.5
1.0
-1.5
Time( s)m
Amplitude(V)
0 1 102
0.5
-1.0
-0.5
0
9876543
V
OUT
V
IN
V
G
f =1MHz
f =0.1MHz
IN
VG
VCA824
SBOS394C – NOVEMBER 2007 – REVISED DECEMBER 2008 ....................................................................................................................................... www.ti.com
A four-quadrant multiplier can easily be implemented Because both inputs of the VCA824 are
using the VCA824. By placing a resistor between FB high-impedance, a difference amplifier can be
and V
IN
, the transfer function depends upon both V
IN
implemented without any major problem. Figure 78
and V
G
, as shown in Equation 1 . shows this implementation. This circuit provides
excellent common-mode rejection ratio (CMRR) as
long as the input is within the CMRR range of – 2.1V
to +1.6V. Note that this circuit does not make use of
the gain control pin, V
G
. Also, it is recommended to
choose R
S
such that the pole formed by R
S
and the
Setting R
1
to equal R
G
, the term that depends only on
parasitic input capacitance does not limit the
V
IN
drops out of the equation, leaving only the term
bandwidth of the circuit. Figure 79 shows the
that depends on both V
G
and V
IN
. V
OUT
then follows
common-mode rejection ratio for this circuit
Equation 2 .
implemented in a gain of +10V/V for V
G
= +1V. Note
that because the gain control voltage is fixed and is
normally set to +1V, the feedback element can be
reduced in order to increase the bandwidth. When
reducing the feedback element, make sure that the
VCA824 is not limited by common-mode input
voltage, the current flowing through R
G
, or any other
limitation described in this data sheet.
Figure 76. Four-Quadrant Multiplier Circuit
Figure 78. Difference Amplifier
Figure 77 illustrates the behavior of this circuit.
Keeping the input amplitude of a 1MHz signal
constant and varying the V
G
voltage (100kHz, 2V
PP
)
gives the modulated output voltage shown in
Figure 77 .
Figure 79. Common-Mode Rejection Ratio
Figure 77. Modulated Output Signal of the
4-Quadrant Multiplexer Circuit
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