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AD835ARREEL7 Datasheet(PDF) 10 Page  Analog Devices 

AD835ARREEL7 Datasheet(HTML) 10 Page  Analog Devices 
10 / 16 page AD835 Rev. D  Page 10 of 16 THEORY OF OPERATION The AD835 is a fourquadrant, voltage output analog multiplier, fabricated on an advanced dielectrically isolated complementary 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 (with small signal rise time of 1 ns). Fullscale (−1 V to +1 V) rise to fall times are 2.5 ns (with a standard RL 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 summing feature is provided at the Z input. As well as providing independent ground references for the input and the output and enhanced versatility, this feature allows the AD835 to operate with voltage gain. Its X, Y, and Zinput voltages are all nominally ±1 V FS, with an overrange of at least 20%. The inputs are fully differential at high impedance (100 kΩ2 pF) and provide 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 RL = 150 Ω, or ±2.0 V minimum into RL = 50 Ω. The AD835 has much lower noise than the AD534 or AD734, making 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, and Z) linearized voltagetocurrent converters, and the load driving output amplifier. The scaling voltage (the denominator U in the equations) is provided by a band gap reference of novel design, optimized for ultralow noise. Figure 19 shows the functional block diagram. In general terms, the AD835 provides the function ()( ) Z U Y Y X X W + − − = 2 1 2 1 (1) where the variables W, U, X, Y, and Z are all voltages. Connected as 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, the output can be expressed as W = XY (2) X1 X2 X = X1 – X2 Z INPUT Y = Y1 – Y2 AD835 W OUTPUT Y1 Y2 XY XY + Z X1 + + Figure 19. Functional Block Diagram Simplified representations of this sort, where all signals are presumed expressed in V, are used throughout this data sheet to avoid the needless use of less intuitive subscripted variables (such as, VX1). All variables as being normalized to 1 V. For example, the input X can either be stated as being in the −1 V to +1 V range or simply –1 to +1. The latter representation is found to facilitate the development of new functions using the AD835. The explicit inclusion of the denominator, 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 20, 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 values shown, Equation 1can be rewritten as () ' 1 kW Z k U XY W − + + = (3) where Z' is distinguished from the signal Z at Pin 4. It follows that () ' 1 Z U k XY W + − = (4) In this way, the effective value of U can be modified to U’ = (1 − k)U (5) without altering the scaling of the Z' input, which is expected because the only ground reference for the output is through the Z' input. Therefore, to set U' to 1 V, remembering that the basic value of U is 1.05 V, R1 must have a nominal value of 20 × R2. The values shown allow U to be adjusted through the nominal range of 0.95 V to 1.05 V. That is, R2 provides a 5% gain adjustment. 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. + + R1 = (1–k) R 2k Ω R2 = kR 200 Ω Z1 FB +5V –5V FB AD835 4.7 μF TANTALUM 0.01 μF CERAMIC 0.01 μF CERAMIC 4.7 μF TANTALUM 1 2 3 4 8 X W Y X2 VP W Y1 X1 Y2 VN Z 7 6 5 Figure 20. Multiplier Connections 
