Datasheet AD835 (Analog Devices) - 10

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AD835 Data Sheet THEORY OF OPERATION
The AD835 is a four-quadrant, voltage output analog multiplier, avoid the needless use of less intuitive subscripted variables fabricated on an advanced dielectrical y isolated complementary (such as, VX1). All variables as being normalized to 1 V. bipolar process. In its basic mode, it provides the linear product For example, the input X can either be stated as being in the −1 V of its X and Y voltage inputs. In this mode, the −3 dB output to +1 V range or simply –1 to +1. The latter representation is found voltage bandwidth is 250 MHz (with small signal rise time of 1 ns). to facilitate the development of new functions using the AD835. Ful -scale (−1 V to +1 V) rise to fall times are 2.5 ns (with a The explicit inclusion of the denominator, U, is also less helpful, as standard RL of 150 Ω), and the settling time to 0.1% under the in the case of the AD835, if it is not an electrical input variable. same conditions is typically 20 ns.
SCALING ADJUSTMENT
As in earlier multipliers from Analog Devices a unique summing feature is provided at the Z input. As well as providing The basic value of U in Equation 1 is nominal y 1.05 V. Figure 20, independent ground references for the input and the output and which shows the basic multiplier connections, also shows how enhanced versatility, this feature allows the AD835 to operate the effective value of U can be adjusted to have any lower with voltage gain. Its X-, Y-, and Z-input voltages are al voltage (usually 1 V) through the use of a resistive divider nominal y ±1 V FS, with an overrange of at least 20%. The between W (Pin 5) and Z (Pin 4). Using the general resistor inputs are ful y differential at high impedance (100 kΩ||2 pF) values shown, Equation 1can be rewritten as and provide a 70 dB CMRR (f ≤ 1 MHz). XY W = + kW + (1− k)Z' (3) The low impedance output is capable of driving loads as small U as 25 Ω. The peak output can be as large as ±2.2 V minimum where Z' is distinguished from the signal Z at Pin 4. It follows that for RL = 150 Ω, or ±2.0 V minimum into RL = 50 Ω. The AD835 XY has much lower noise than the AD534 or AD734, making it W = ( + Z (4) 1 − k) ' attractive in low level, signal processing applications, for U example, as a wideband gain control element or modulator. In this way, the effective value of U can be modified to
BASIC THEORY
U’ = (1 − k)U (5) The multiplier is based on a classic form, having a translinear core, without altering the scaling of the Z' input, which is expected because supported by three (X, Y, and Z) linearized voltage-to-current the only ground reference for the output is through the Z' input. converters, and the load driving output amplifier. The scaling Therefore, to set U' to 1 V, remembering that the basic value of voltage (the denominator U in the equations) is provided by a U is 1.05 V, R1 must have a nominal value of 20 × R2. The values band gap reference of novel design, optimized for ultralow noise. shown al ow U to be adjusted through the nominal range of Figure 19 shows the functional block diagram. 0.95 V to 1.05 V. That is, R2 provides a 5% gain adjustment. In general terms, the AD835 provides the function In many applications, the exact gain of the multiplier may not (X1− X2)(Y1−Y ) be very important; in which case, this network may be omitted W = 2 + Z (1) U entirely, or R2 fixed at 100 Ω. where the variables W, U, X, Y, and Z are al voltages. Connected as
FB 4.7µF TANTALUM + +5V
a simple multiplier, with X = X1 − X2, Y = Y1 − Y2, and Z = 0 and with a scale factor adjustment (see Figure 19) that sets U = 1 V,
0.01µF CERAMIC
the output can be expressed as
X W 8 7 6 5
W = XY (2)
X1 X2 VP W R1 = (1–k) R AD835 2kΩ X1 X = X1 – X2 AD835 Y1 Y2 VN Z 1 2 3 4 X2 Y XY XY + Z + X1 W OUTPUT + FB 4.7µF TANTALUM R2 = kR + +5V 200Ω Y1 0.01µF CERAMIC Y2 Y = Y1 – Y2 Z’
025 020 00883-
Z INPUT
00883- Figure 19. Functional Block Diagram Figure 20. Multiplier Connections Simplified representations of this sort, where all signals are presumed expressed in V, are used throughout this data sheet to Rev. E | Page 10 of 14 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION BASIC THEORY SCALING ADJUSTMENT APPLICATIONS INFORMATION MULTIPLIER CONNECTIONS WIDEBAND VOLTAGE-CONTROLLED AMPLIFIER AMPLITUDE MODULATOR SQUARING AND FREQUENCY DOUBLING OUTLINE DIMENSIONS ORDERING GUIDE