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AD734BN 데이터시트(PDF) 11 Page - Analog Devices |
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AD734BN 데이터시트(HTML) 11 Page - Analog Devices |
11 / 20 page AD734 Rev. E | Page 11 of 20 Most of the functions of the AD734 (including division, unlike the AD534 in this respect) are realized with Z1 connected to W. Therefore, substituting W in place of Z1 in Equation 2 results in an output. 2 2 1 2 1 ) )( ( Z U Y Y X X W + − − = (3) The free input, Z2, can be used to sum another signal to the output; in the absence of a product signal, W simply follows the voltage at Z2 with the full 10 MHz bandwidth. When not needed for summation, Z2 should be connected to the ground associated with the load circuit. The allowable polarities can be shown in the following shorthand form: Z U Y X W ± + + ± ± = ± ) ( ) )( ( ) ( (4) In the recommended direct divider mode, the Y input is set to a fixed voltage (typically 10 V) and U is varied directly; it can have any value from 10 mV to 10 V. The magnitude of the ratio X/U cannot exceed 1.25; for example, the peak X input for U = 1 V is ±1.25 V. Above this level, clipping occurs at the positive and negative extremities of the X input. Alternatively, the AD734 can be operated using the standard (AD534) divider connections (see Figure 27), when the negative feedback path is established via the Y2 input. Substituting W for Y2 in Equation 2, ( ) () 1 2 1 1 2 Y X X Z Z U W + − − = (5) In this case, note that the variable X is now the denominator, and the previous restriction (X/U ≤ 1.25) on the magnitude of the X input does not apply. However, X must be positive for the feedback polarity to be correct. Y1 can be used for summing purposes or connected to the load ground if not needed. The shorthand form in this case is ) ( ) ( ) ( ) ( ) ( Y X Z U W ± + + ± + = ± (6) In some cases, feedback can be connected to two of the available inputs. This is true for the square-rooting connections (see Figure 28), where W is connected to both X1 and Y2. Set X1 = W and Y2 = W in Equation 2, and anticipating the possibility of again providing a summing input, set X2 = S and Y1 = S, so that, in shorthand form, ) ( ) )( ( ) ( S Z U W ± + + + = ± (7) This is seen more generally to be the geometric-mean function, because both U and Z can be variable; operation is restricted to one quadrant. Feedback can also be taken to the U interface. Full details of the operation in these modes is provided in the Wideband RMS-to-DC Converter Using U Interface section. DIRECT DENOMINATOR CONTROL A valuable new feature of the AD734 is the provision to replace the internal denominator voltage, U, with any value from 10 mV to 10 V. This can be used • To simply alter the multiplier scaling, thus improve accu- racy and achieve reduced noise levels when operating with small input signals. • To implement an accurate two-quadrant divider, with a 1000:1 gain range and an asymptotic gain-bandwidth product of 200 MHz. • To achieve certain other special functions, such as AGC or rms. Figure 21 shows the internal circuitry associated with denominator control. Note, first, that the denominator is actually proportional to a current, Iu, having a nominal value of 356 μA for U = 10 V, whereas the primary reference is a voltage generated by a buried-Zener circuit and laser-trimmed to ha very low temperature coefficient. This voltage is nominally 8 with a tolerance of ±10% , ve a V . TC 4 3 5 14 9 13 8 U0 Iu Qu Qr Ru 28kΩ Rr 100kΩ Rd NOM 22.5kΩ Qd VP VN LINK TO DISABLE DD ER U1 U2 AD734 NOMINALLY 356µA for U = 10V NOM 8V NEGATIVE SUPPLY + Figure 21. Denominator Control Circuitry After temperature-correction (block TC), the reference voltage is applied to Transistor Qd and trimmed Resistor Rd, which generate the required reference current. Transistor Qu and Resistor Ru are not involved in setting up the internal denomina- tor, and their associated control pins, U0, U1, and U2, are normally grounded. The reference voltage is also made available, via the 100 kΩ resistor, Rr, at Pin 9 (ER). When the control pin, DD (denominator disable), is connected to VP, the internal source of Iu is shut off, and the collector current of Qu must provide the denominator current. The resistor Ru is laser-trimmed such that the multiplier denominator is exactly equal to the voltage across it (that is, across Pin U1 and Pin U2). Note that this trimming only sets up the correct internal ratio; the absolute value of Ru (nominally 28 kΩ) has a tolerance of ±20%. Also, the alpha of Qu (typically 0.995), which may be seen as a source of scaling error, is canceled by the alpha of other transistors in the complete circuit. In the simplest scheme (see Figure 22), an externally provided control voltage, VG, is applied directly to U0 and U2 and the resulting voltage across Ru is therefore reduced by one VBE. For example, when VG = 2 V, the actual value of U is about 1.3 V. |
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