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AD734BN Datasheet(PDF) 6 Page - Analog Devices |
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AD734BN Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page AD734 –6– REV. C Table I. Component Values for Setting Up Nonstandard Denominator Values Denominator R1 (Fixed) R1 (Variable) R2 5 V 34.8 k Ω 20 k Ω 120 k Ω 3 V 64.9 k Ω 20 k Ω 220 k Ω 2 V 86.6 k Ω 50 k Ω 300 k Ω 1 V 174 k Ω 100 k Ω 620 k Ω The denominator can also be current controlled, by grounding Pin 3 (U0) and withdrawing a current of Iu from Pin 4 (U1). The nominal scaling relationship is U = 28 × Iu, where u is expressed in volts and Iu is expressed in milliamps. Note, however, that while the linearity of this relationship is very good, it is subject to a scale tolerance of ±20%. Note that the common mode range on Pins 3 through 5 actually extends from 4 V to 36 V below VP, so it is not necessary to restrict the connection of U0 to ground if it should be desirable to use some other voltage. The output ER may also be buffered, re-scaled and used as a general-purpose reference voltage. It is generated with respect to the negative supply line Pin 8 (VN), but this is acceptable when driving one of the signal interfaces. An example is shown in Figure 12, where a fixed numerator of 10 V is generated for a divider application. There, Y2 is tied to VN but Y1 is 10 V above this; therefore the common-mode voltage at this interface is still 5 V above VN, which satisfies the internal biasing requirements (see Specifications table). OPERATION AS A MULTIPLIER All of the connection schemes used in this section are essentially identical to those used for the AD534, with which the AD734 is pin-compatible. The only precaution to be noted in this regard is that in the AD534, Pins 3, 5, 9, and 13 are not internally connected and Pin 4 has a slightly different purpose. In many cases, an AD734 can be directly substituted for an AD534 with immediate benefits in static accuracy, distortion, feedthrough, and speed. Where Pin 4 was used in an AD534 application to achieve a reduced denominator voltage, this function can now be much more precisely implemented with the AD734 using alter- native connections (see Direct Denominator Control, page 5). Operation from supplies down to ±8 V is possible. The supply current is essentially independent of voltage. As is true of all high speed circuits, careful power-supply decoupling is impor- tant in maintaining stability under all conditions of use. The decoupling capacitors should always be connected to the load ground, since the load current circulates in these capacitors at high frequencies. Note the use of the special symbol (a triangle with the letter ‘L’ inside it) to denote the load ground. Standard Multiplier Connections Figure 5 shows the basic connections for multiplication. The X and Y inputs are shown as optionally having their negative nodes grounded, but they are fully differential, and in many applications the grounded inputs may be reversed (to facilitate interfacing with signals of a particular polarity, while achieving some desired output polarity) or both may be driven. The AD734 has an input resistance of 50 k Ω ± 20% at the X, Y, and Z interfaces, which allows ac-coupling to be achieved with moderately good control of the high-pass (HP) corner frequency; a capacitor of 0.1 µF provides a HP corner frequency 1 2 3 4 5 6 7 10 8 9 11 13 12 14 W ER VN VP DD Z1 Z2 X1 X2 U1 U2 U0 Y1 Y2 AD734 NC NC LOAD GROUND 0.1 F 0.1 F X – INPUT 10V FS Y – INPUT 10V FS +15V –15V OPTIONAL SUMMING INPUT ±10V FS W = (X1 – X2) 10V (Y1 – Y2) + Z2 L L Z2 Figure 5. Basic Multiplier Circuit of 32 Hz. When a tighter control of this frequency is needed, or when the HP corner is above about 100 kHz, an external resis- tor should be added across the pair of input nodes. At least one of the two inputs of any pair must be provided with a dc path (usually to ground). The careful selection of ground returns is important in realizing the full accuracy of the AD734. The Z2 pin will normally be connected to the load ground, which may be remote, in some cases. It may also be used as an optional summing input (see Equations (3) and (4), above) having a nominal FS input of ±10 V and the full 10 MHz bandwidth. In applications where high absolute accuracy is essential, the scaling error caused by the finite resistance of the signal source(s) may be troublesome; for example, a 50 Ω source resistance at just one input will introduce a gain error of –0.1%; if both the X- and Y-inputs are driven from 50 Ω sources, the scaling error in the product will be –0.2%. Provided the source resistance(s) are known, this gain error can be completely compensated by including the appropriate resistance (50 Ω or 100 Ω, respectively, in the above cases) between the output W (Pin 12) and the Z1 feedback input (Pin 11). If Rx is the total source resistance associated with the X1 and X2 inputs, and Ry is the total source resistance associated with the Y1 and Y2 inputs, and neither Rx nor Ry exceeds 1 k Ω, a resistance of Rx+Ry in series with pin Z1 will provide the required gain restoration. Pins 9 (ER) and 13 (DD) should be left unconnected in this application. The U-inputs (Pins 3, 4 and 5) are shown connected to ground; they may alternatively be connected to VN, if desired. In applications where Pin 2 (X2) happens to be driven with a high-amplitude, high-frequency signal, the capacitive coupling to the denominator control circuitry via an ungrounded Pin 3 can cause high-frequency distortion. However, the AD734 can be operated without modification in an AD534 socket, and these three pins left unconnected, with the above caution noted. 1 2 3 4 5 6 7 10 8 9 11 13 12 14 W ER VN VP DD Z1 Z2 X1 X2 U1 U2 U0 Y1 Y2 AD734 NC NC L 0.1 F 0.1 F X – INPUT 10V FS Y – INPUT 10V FS +15V –15V I W = (X 1 – X 2 ) 10V (Y 1 – Y 2 ) 1 R S + 1 50k R S L 10mA MAX FS 10V MAXIMUM LOAD VOLTAGE L I W Figure 6. Conversion of Output to a Current |
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