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AD534SH Fiches technique(PDF) 5 Page - Analog Devices |
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AD534SH Fiches technique(HTML) 5 Page - Analog Devices |
5 / 12 page AD534 REV. B –5– FUNCTIONAL DESCRIPTION Figure 2 is a functional block diagram of the AD534. Inputs are converted to differential currents by three identical voltage-to- current converters, each trimmed for zero offset. The product of the X and Y currents is generated by a multiplier cell using Gilbert’s translinear technique. An on-chip “Buried Zener” provides a highly stable reference, which is laser trimmed to provide an overall scale factor of 10 V. The difference between XY/SF and Z is then applied to the high gain output amplifier. This permits various closed loop configurations and dramati- cally reduces nonlinearities due to the input amplifiers, a domi- nant source of distortion in earlier designs. The effectiveness of the new scheme can be judged from the fact that under typical conditions as a multiplier the nonlinearity on the Y input, with X at full scale ( ±10 V), is ±0.005% of FS; even at its worst point, which occurs when X = ±6.4 V, it is typically only ±0.05% of FS Nonlinearity for signals applied to the X input, on the other hand, is determined almost entirely by the multi- plier element and is parabolic in form. This error is a major factor in determining the overall accuracy of the unit and hence is closely related to the device grade. V-1 X1 X2 V-1 Y1 Y2 V-1 Z1 Z2 SF AD534 +VS –VS A OUT TRANSFER FUNCTION VO = A – (Z1 – Z2) (X1 – X2) (Y1 – Y2) SF HIGH GAIN OUTPUT AMPLIFIER + – + – + – STABLE REFERENCE AND BIAS TRANSLINEAR MULTIPLIER ELEMENT 0.75 ATTEN Figure 2. Functional Block Diagram The generalized transfer function for the AD534 is given by: VOUT = A ( X1 − X2 )(Y1 − Y2 ) SF − (Z 1 − Z2 ) where A = open loop gain of output amplifier, typically 70 dB at dc X, Y, Z = input voltages (full scale = ±SF, peak = ±1.25 SF) SF = scale factor, pretrimmed to 10.00 V but adjustable by the user down to 3 V. In most cases the open loop gain can be regarded as infinite, and SF will be 10 V. The operation performed by the AD534, can then be described in terms of equation: ( X1 − X2 )(Y1 − Y2 ) = 10 V ( Z1 − Z2 ) The user may adjust SF for values between 10.00 V and 3 V by connecting an external resistor in series with a potentiometer between SF and –VS. The approximate value of the total resis- tance for a given value of SF is given by the relationship: RSF = 5.4K SF 10 − SF Due to device tolerances, allowance should be made to vary RSF; by ±25% using the potentiometer. Considerable reduction in bias currents, noise and drift can be achieved by decreasing SF. This has the overall effect of increasing signal gain without the customary increase in noise. Note that the peak input signal is always limited to 1.25 SF (i.e., ±5 V for SF = 4 V) so the overall transfer function will show a maximum gain of 1.25. The per- formance with small input signals, however, is improved by using a lower SF since the dynamic range of the inputs is now fully utilized. Bandwidth is unaffected by the use of this option. Supply voltages of ±15 V are generally assumed. However, satisfactory operation is possible down to ±8 V (see Figure 16). Since all inputs maintain a constant peak input capability of ±1.25 SF some feedback attenuation will be necessary to achieve output voltage swings in excess of ±12 V when using higher supply voltages. OPERATION AS A MULTIPLIER Figure 3 shows the basic connection for multiplication. Note that the circuit will meet all specifications without trimming. X1 X2 Y1 Y2 Z1 Z2 AD534 = + Z2 (X1 – X2) (Y1 – Y2) 10V OUTPUT , 12V PK X INPUT 10V FS 12V PK Y INPUT 10V FS 12V PK +15V OUT –VS +VS –15V OPTIONAL SUMMING INPUT, Z, 10V PK SF Figure 3. Basic Multiplier Connection In some cases the user may wish to reduce ac feedthrough to a minimum (as in a suppressed carrier modulator) by applying an external trim voltage ( ±30 mV range required) to the X or Y input (see Figure 1). Figure 19 shows the typical ac feedthrough with this adjustment mode. Note that the Y input is a factor of 10 lower than the X input and should be used in applications where null suppression is critical. The high impedance Z2 terminal of the AD534 may be used to sum an additional signal into the output. In this mode the out- put amplifier behaves as a voltage follower with a 1 MHz small signal bandwidth and a 20 V/ µs slew rate. This terminal should always be referenced to the ground point of the driven system, particularly if this is remote. Likewise, the differential inputs should be referenced to their respective ground potentials to realize the full accuracy of the AD534. |
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Description similaire - AD534SH |
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