AD835

REV. A

–6–

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

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

U

+ kW + (1 – k)Z '

(3)

(where Z' is distinguished from the signal Z at Pin 4). It follows

that

(4)

In this way, we can modify the effective value of U to

U '

= (1 – k)U

(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.

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.

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

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 k

Ω 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

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

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

W

=

( X 1– X 2)(Y 1– Y 2)

U

+ Z

(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)

Figure 17. Functional Block Diagram

Y1

Y2

Z INPUT

Y = Y1 –Y2

X = X1 –X2

XY + Z

X1

X2

W OUTPUT

XY

AD835

∑

+1

W

= XY

(1 – k)U

+ Z'

R1 = (1–k) R

2k

Ω

W

R2 = kR

200

Ω

+5V

+5V

X1

X2

VP

W

Z

VN

Y2

Y1

X1

AD835

FB

+5V

X

Y

–5V

FB

3

4

2

1

5

7

0.01

µF CERAMIC

4.7

µF TANTALUM

8

0.01

µF CERAMIC

4.7

µF TANTALUM

6