AD835
PRODUCT DESCRIPTION
The AD835 is a four-quadrant, voltage output, analog multi-
plier fabricated on an advanced, dielectrically isolated, comple-
mentary bipolar process. In its basic mode, it provides the linear
product of its X and Y voltage inputs. In this mode, the –3 dB
output voltage bandwidth is 250 MHz (a small signal rise time
of 1 ns). Full-scale (–1 V to +1 V) rise/fall times are 2.5 ns (with
the standard R
L
of 150
Ω)
and the settling time to 0.1% under
the same conditions is typically 20 ns.
As in earlier multipliers from Analog Devices, a unique sum-
ming feature is provided at the Z-input. As well as providing in-
dependent ground references for inputs and output, and
enhanced versatility, this feature allows the AD835 to operate
with voltage
gain.
Its X-, Y- and Z-input voltages are all nomi-
nally
±
1 V FS, with overrange of at least 20%. The inputs are
fully differential and at high impedance (100 kΩ 2 pF) and pro-
vide a 70 dB CMRR (f
≤
1 MHz).
The low impedance output is capable of driving loads as small
as 25
Ω.
The peak output can be as large as
±
2.2 V minimum
for R
L
= 150
Ω,
or
±
2.0 V minimum into R
L
= 50
Ω.
The
AD835 has much lower noise than the AD534 or AD734, mak-
ing it attractive in low level signal-processing applications, for
example, as a wideband gain-control element or modulator.
Basic Theory
Simplified representations of this sort, where all signals are pre-
sumed to be expressed in
volts,
are used throughout this data
sheet, to avoid the needless use of less-intuitive subscripted vari-
ables (such as V
X1
). We can view all variables as being
normal-
ized to 1 V.
For example, the input X can either be stated as
being in the range –1 V to +1 V, or simply –1 to +1. The latter
representation will be found to facilitate the development of new
functions using the AD835. The explicit inclusion of the de-
nominator, U, is also less helpful, as in the case of the AD835, if
it is not an electrical input variable.
Scaling Adjustment
The basic value of U in Equation 1 is nominally 1.05 V. Figure
18, which shows the basic multiplier connections, also
shows how the effective value of U can be adjusted to have any
lower voltage (usually 1 V) through the use of a resistive-divider
between W (Pin 5) and Z (Pin 4). Using the general resistor val-
ues shown, we can rewrite Equation 1 as
W
=
XY
+
kW
+
(1 –
k)Z
'
U
(3)
(where Z' is distinguished from the signal Z at Pin 4). It follows
that
XY
W
=
+
Z
'
(4)
(1 –
k)U
In this way, we can modify the effective value of U to
U
'
=
(1 –
k)U
The multiplier is based on a classic form, having a translinear
core, supported by three (X, Y, Z) linearized voltage-to-current
converters, and the load driving output amplifier. The scaling
voltage (the denominator U, in the equations below) is provided
by a bandgap reference of novel design, optimized for ultralow
noise. Figure 17 shows the functional block diagram.
In general terms, the AD835 provides the function
(5)
without altering the scaling of the Z' input. (This is to be ex-
pected, since the only “ground reference” for the output is
through the Z' input.)
Thus, to set U' to 1 V, remembering that the basic value of U is
1.05 V, we need to choose R1 to have a nominal value of 20
times R2. The values shown here allow U to be adjusted
through the nominal range 0.95 V to 1.05 V, that is, R2 pro-
vides a 5% gain adjustment.
+5V
+5V
FB
4.7µF TANTALUM
W
=
(X 1 –
X
2)(Y 1 –
Y
2)
+
Z
U
(1)
where the variables W, U, X, Y and Z are all voltages. Con-
nected as a simple multiplier, with X = X1 – X2, Y = Y1 – Y2
and Z = 0, and with a scale factor adjustment (see below) which
sets U = 1 V, the output can be expressed as
W = XY
(2)
0.01µF CERAMIC
X
8
7
X2
Y2
2
6
VP
VN
3
5
W
R1 = (1–k) R
2kΩ
Z
4
W
X1
X1
Y1
X1
X2
X = X1 –X2
AD835
AD835
1
XY
∑
XY + Z
+1
W OUTPUT
Y
4.7µF TANTALUM
R2 = kR
200Ω
Y1
Y2
Y = Y1 –Y2
0.01µF CERAMIC
FB
Z
1
Z INPUT
–5V
Figure 17. Functional Block Diagram
Figure 18. Multiplier Connections
Note that in many applications, the exact gain of the multiplier
may not be very important; in which case, this network may be
omitted entirely, or R2 fixed at 100
Ω.
–6–
REV. A
AD [ ANALOG DEVICES ]
