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MC1494 Scheda tecnica(PDF) 7 Page - ON Semiconductor |
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MC1494 Scheda tecnica(HTML) 7 Page - ON Semiconductor |
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7 / 16 page  MC1494 7 MOTOROLA ANALOG IC DEVICE DATA Figure 18. Typical Multiplier Connection +15 V +15 V 15 5 RL 50 k 22 k 10 pF 2 3 3 14 1 R1 16 k P4 6 4 7 10 11 30 k RX 12 13 9 8 7 6 42 P1 20 k P2 P3 20 k 50 k R* 510 10 pF 510 10 pF R* 62 k RY MC1494 + + – – + – + – + MC1456 VO 0.1 µF +15 V –15 V VO = –VX VY 10 –10 V ≤ VX ≤ +10 V –10 V ≤ VY ≤ +10 V *R is not necessary if inputs are DC coupled. 0.1 µF 0.1 µF 0.1 µF VX VY It should be pointed out that there is nothing magic about setting the scale factor to 1/10. This is merely a convenient factor to use if the VX and VY input voltages are expected to be large, say ±10 V. Obviously with VX = VY = 10 V and a scale factor of unity, the device could not hope to provide a 100 V output, so the scale factor is set to 1/10 and provides an output scaled down by a factor of ten. For many applications it may be desirable to set K = 1/2 or K = 1 or even K = 100. This can be accomplished by adjusting RX, RY and RL appropriately. The selection of RL is arbitrary and can be chosen after resistors RX and RY are found. Note in Figure 18 that RY is 62 k Ω while RX is 30 kΩ. The reason for this is that the “Y” side of the multiplier exhibits a second order nonlinearity whereas the “X” side exhibits a simple nonlinearity. By making the RY resistor approximately twice the value of the RX resistor, the linearity on both the “X” and “Y” sides are made equal. The selection of the RX and RY resistor values is dependent upon the expected amplitude of VX and VY inputs. To maintain a specified linearity, resistors RX and RY should be selected according to the following equations: RX ≥ 3 VX (max) in kΩ when VX is in Volts, RY ≥ 6 VY (max) in kΩ when VY is in Volts. For example, if the maximum input on the “X” side is ±1.0 V, resistor RX can be selected to be 3.0 kΩ. If the maximum input on the “Y” side is also ±1.0 V, then resistor RY can be selected to be 6.0 k Ω (6.2 kΩ nominal value). If a scale factor of K = 10 is desired, the load resistor is found to be 47 k Ω. In this example, the multiplier provides a gain of 20 dB. Operational Amplifier Selection The operational amplifier connection in Figure 18 is a simple but extremely accurate current–to–voltage converter. The output current of the multiplier flows through the feedback resistor RL to provide a low impedance output voltage from the op amp. Since the offset current and bias currents of the op amp will cause errors in the output voltage, particularly with temperature, one with very low bias and offset currents is recommended. The MC1456 or MC1741 are excellent choices for this application. Since the MC1494 is capable of operation at much higher frequencies than the op amp, the frequency characteristics of the circuit in Figure 18 will be primarily dependent upon the operational amplifier. Stability The current–to–voltage converter mode is a most demanding application for an operational amplifier. Loop gain is at its maximum and the feedback resistor in conjunction with stray or input capacitance at the multiplier output adds additional phase shift. It may therefore be necessary to add (particularly in the case of internally compensated op amps) a small feedback capacitor to reduce loop gain at the higher frequencies. A value of 10 pF in parallel with RL should be adequate to insure stability over production and temperature variations, etc. An externally compensated op amp might be employed using slightly heavier compensation than that recommended for unity–gain operation. Offset Adjustment The noninverting input of the op amp provides a convenient point to adjust the output offset voltage. By connecting this point to the wiper arm of a potentiometer (P3), the output offset voltage can be adjusted to zero (see Offset and Scale Factor Adjustment Procedure). The input offset adjustment potentiometers, P1 and P2 will be necessary for most applications where it is desirable to take advantage of the multiplier’s excellent linearity characteristics. Depending upon the particular application, some of the potentiometers can be omitted (see Figures 19, 21, 24, 26 and 27). |
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