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AD734BQ Datasheet(PDF) 5 Page - Analog Devices |
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AD734BQ Datasheet(HTML) 5 Page - Analog Devices |
5 / 12 page AD734 –5– REV. C 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 denominator, and their associated control pins U0, U1 and U2 will normally be grounded. The reference voltage is also made available, via the 100 k Ω resistor Rr, at Pin 9 (ER); the purpose of Qr is explained below. When the control pin DD (denominator disable) is connected to VP, the internal source of Iu is shut off, and the collector cur- rent 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 pins U1 and U2). Note that this trimming only sets up the correct internal ratio; the absolute value of Ru (nominally 28 k Ω) has a tolerance of ±20%. Also, the alpha of Qu, (typically 0.995) which might be seen as a source of scaling error, is canceled by the alpha of other transistors in the complete circuit. In the simplest scheme (Figure 3), 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 will be about 1.3 V. This error will not be important in some closed-loop applications, such as automatic gain control (AGC), but clearly is not acceptable where the denominator value must be well- defined. When it is required to set up an accurate, fixed value of U, the on-chip reference may be used. The transistor Qr is provided to cancel the VBE of Qu, and is biased by an external resistor, R2, as shown in Figure 4. R1 is chosen to set the de- sired value of U and consists of a fixed and adjustable resistor. Ru 28k Qu Iu U0 U1 U2 VP DD AD734 ~60 A +VS –VS NC NC ER VN Qr Rr 100k VG 3 4 5 9 8 13 14 Figure 3. Low-Accuracy Denominator Control R1 Ru 28k Qu Iu U0 U1 U2 VP DD AD734 +VS –VS NOM 8V NC ER VN Qr Rr 100k 3 4 5 9 8 13 14 R2 Figure 4. Connections for a Fixed Denominator Table I shows useful values of the external components for set- ting up nonstandard denominator values. the AD734 can be operated using the standard (AD534) divider connections (Figure 8), when the negative feedback path is established via the Y2 input. Substituting W for Y2 in Equation (2), we get W = U Z 2 − Z1 () X 1 − X 2 () + Y 1. (5) In this case, note that the variable X is now the denominator, and the above restriction (X/U ≤ 1.25) on the magnitude of the X input does not apply. However, X must be positive in order 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 ±W () =+U () ±Z () +X () +±Y (). (6) In some cases, feedback may be connected to two of the avail- able inputs. This is true for the square-rooting connections (Fig-ure 9), where W is connected to both X1 and Y2. Setting X1 = W and Y2 = W in Equation (2), and anticipating the possibility of again providing a summing input, so setting X2 = S and Y1 = S, we find, in shorthand form ±W () =+U () +Z () +±S (). (7) This is seen more generally to be the geometric-mean function, since both U and Z can be variable; operation is restricted to one quadrant. Feedback may also be taken to the U-interface. Full details of the operation in these modes is provided in the appropriate section of this data sheet. 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 (1) to simply alter the multiplier scaling, thus improve accuracy and achieve reduced noise levels when operating with small input signals; (2) to implement an accurate two-quadrant divider, with a 1000:1 gain range and an asymptotic gain-bandwidth product of 200 MHz; (3) to achieve certain other special functions, such as AGC or rms. Figure 2 shows the internal circuitry associated with denomina- tor 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 have a very low temperature coefficient. This voltage is nominally 8 V with a tolerance of ±10%. Ru 28k Rd NOM 22.5k Qu Qd NOM 8V Rr 100k TC Qr NEGATIVE SUPPLY NOMINALLY 356 A for U = 10V Iu U0 U1 U2 VP DD ER VN AD734 LINK TO DISABLE 3 4 5 8 9 14 13 Figure 2. Denominator Control Circuitry |
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