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AD835AN Datasheet(PDF) 6 Page - Analog Devices |
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AD835AN Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page 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 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 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 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 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 L = 150 Ω, or ± 2.0 V minimum into RL = 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 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 Z1 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 |
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