AD835_15 Datasheet, PDF (11/16 Pages) Analog Devices – 250 MHz, Voltage Output, 4-Quadrant Multiplier

APPLICATIONS INFORMATION
The AD835 is easy to use and versatile. The capability for adding
another signal to the output at the Z input is frequently valuable.
Three applications of this feature are presented here: a wideband
voltage-controlled amplifier, an amplitude modulator, and a
frequency doubler. Of course, the AD835 may also be used as a
square law detector (with its X inputs and Y inputs connected in
parallel). In this mode, it is useful at input frequencies to well
over 250 MHz because that is the bandwidth limitation of the
output amplifier only.
MULTIPLIER CONNECTIONS
Figure 20 shows the basic connections for multiplication. The
inputs are often single sided, in which case the X2 and Y2 inputs
are normally grounded. Note that by assigning Pin 7 (X2) and
Pin 2 (Y2), respectively, to these (inverting) inputs, an extra
measure of isolation between inputs and output is provided.
The X and Y inputs may be reversed to achieve some desired
overall sign with inputs of a particular polarity, or they may be
driven fully differentially.
Power supply decoupling and careful board layout are always
important in applying wideband circuits. The decoupling
recommendations shown in Figure 20 should be followed
closely. In Figure 21, Figure 23, and Figure 24, these power
supply decoupling components are omitted for clarity but should
be used wherever optimal performance with high speed inputs
is required. However, if the full, high frequency capabilities of the
AD835 are not being exploited, these components can be
omitted.
WIDEBAND VOLTAGE-CONTROLLED AMPLIFIER
Figure 21 shows the AD835 configured to provide a gain of
nominally 0 dB to 12 dB. (In fact, the control range extends from
well under –12 dB to about +14 dB.) R1 and R2 set the gain to
be nominally ×4. The attendant bandwidth reduction that comes
with this increased gain can be partially offset by the addition of
the peaking capacitor C1. Although this circuit shows the use of
dual supplies, the AD835 can operate from a single 9 V supply
with a slight revision.
+5V
V_G
(GAIN CONTROL)
8
7
6
5
X1
X2
VP
W
AD835
Y1
Y2
VN
Z
1
2
3
4
R1
97.6Ω
VOLTAGE
OUTPUT
C1
33pF
V_IN
(SIGNAL)
R2
301Ω
–5V
Figure 21. Voltage-Controlled 50 MHz Amplifier Using the AD835
AD835
The ac response of this amplifier for gains of 0 dB (VG = 0.25 V),
6 dB (VG = 0.5 V), and 12 dB (VG = 1 V) is shown in Figure 22.
In this application, the resistor values have been slightly adjusted to
reflect the nominal value of U = 1.05 V. The overall sign of the
gain may be controlled by the sign of VG.
21
18
15
12dB
12
(V_G = 1V)
9
6dB
(V_G = 0.5V)
6
3
0dB
(V_G = 0.25V)
0
–3
–6
–9
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 22. AC Response of VCA
100M
AMPLITUDE MODULATOR
Figure 23 shows a simple modulator. The carrier is applied to the
Y input and the Z input, while the modulating signal is applied to
the X input. For zero modulation, there is no product term so the
carrier input is simply replicated at unity gain by the voltage
follower action from the Z input. At X = 1 V, the RF output is
doubled, while for X = –1 V, it is fully suppressed. That is, an
X input of approximately ±1 V (actually ±U or about 1.05 V)
corresponds to a modulation index of 100%. Carrier and
modulation frequencies can be up to 300 MHz, somewhat
beyond the nominal −3 dB bandwidth.
Of course, a suppressed carrier modulator can be implemented
by omitting the feedforward to the Z input, grounding that
pin instead.
+5V
MODULATION
SOURCE
8
7
6
5
MODULATED
X1
X2
VP
W ^CARRIER
AD835
OUTPUT
Y1
Y2
VN
Z
1
2
3
4
–5V
CARRIER
SOURCE
Figure 23. Simple Amplitude Modulator Using the AD835
Rev. D | Page 11 of 16