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AD835 Fiches technique(PDF) 7 Page - Analog Devices |
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AD835 Fiches technique(HTML) 7 Page - Analog Devices |
7 / 8 page AD835 REV. A –7– APPLICATIONS The AD835 is both easy to use and versatile. The capability for adding another signal to the output at the Z input is frequently valuable. Three applications of this feature are presented here: a wideband voltage controlled amplifier, an amplitude modulator and a frequency doubler. Of course, the AD835 may also be used as a square law detector (with its X- and Y-inputs con- nected in parallel) in which mode it is useful at input frequen- cies to well over 250 MHz, since that is the bandwidth limitation only of the output amplifier. Multiplier Connections Figure 18 shows the basic connections for multiplication. The inputs will often be single sided, in which case the X2 and Y2 inputs will normally be grounded. Note that by assigning Pins 7 and 2 to these (inverting) inputs, respectively, an extra measure of isolation between inputs and output is provided. The X and Y inputs may, of course, be reversed to achieve some desired overall sign with inputs of a particular polarity, or they may be driven fully differentially. Power supply decoupling and careful board layout are always important in applying wideband circuits. The decoupling rec- ommendations shown in Figure 18 should be followed closely. In remaining figures in this data sheet, these power supply decoupling components have been omitted for clarity, but should be used wherever optimal performance with high speed inputs is required. However, they may be omitted if the full high frequency capabilities of AD835 are not being exploited. A Wideband Voltage Controlled Amplifier Figure 19 shows the AD835 configured to provide a gain of nominally 0 to 12 dB. (In fact, the control range extends from well under –12 dB to about +14 dB.) R1 and R2 set the gain to be nominally ×4. The attendant bandwidth reduction that comes with this increased gain can be partially offset by the ad- dition of the peaking capacitor C1. Although this circuit shows the use of dual supplies, the AD835 can operate from a single 9 V supply with slight revision. Figure 19. Voltage Controlled 50 MHz Amplifier Using the AD835 The ac response of this amplifier for gains of 0 dB (V G = 0.25 V), 6 dB (V G = 0.5 V) and 12 dB (VG = 1 V) is shown in Figure 20. In this application, the resistor values have been slightly adjusted to reflect the nominal value of U = 1.05 V. The overall sign of the gain may be controlled by the sign of V G. V IN (SIGNAL) R2 301 Ω VOLTAGE OUTPUT X1 X2 VP W Z VN Y2 Y1 X1 AD835 +5V –5V R1 97.6 Ω 3 4 2 1 5 6 7 8 C1 33pF V G (GAIN CONTROL) Figure 20. AC Response of VCA An Amplitude Modulator Figure 21 shows a simple modulator. The carrier is applied both to the Y-input and the Z-input, while the modulating signal is applied to the X-input. For zero modulation, there is no product term, so the carrier input is simply replicated at unity gain by the voltage follower action from the Z-input. At X = 1 V, the RF output is doubled, while for X = –1 V, it is fully suppressed. That is, an X-input of approximately ±1 V (actually ±U, or about 1.05 V) corresponds to a modulation index of 100%. Car- rier and modulation frequencies can be up to 300 MHz, some- what beyond the nominal –3 dB bandwidth. Of course, a suppressed carrier modulator can be implemented by omitting the feedforward to the Z-input, grounding that pin instead. Figure 21. Simple Amplitude Modulator Using the AD835 Squaring and Frequency Doubling Amplitude domain squaring of an input signal, E, is achieved simply by connecting the X- and Y-inputs in parallel to pro- duce an output of E 2/U. The input may have either polarity, but the output in this case will always be positive. The output polar- ity may be reversed by interchanging either the X or Y inputs. When the input is a sine wave E sin ωt, a signal squarer behaves as a frequency doubler, since (6) While useful, Equation 6 shows a dc term at the output which will vary strongly with the amplitude of the input, E. MODULATED CARRIER OUTPUT MODULATION INPUT CARRIER OUTPUT X1 X2 VP W Z VN Y2 Y1 X1 AD835 +5V –5V 3 4 2 1 5 6 7 8 E sin ωt ()2 U = E 2 2U (1 – cos 2 ωt) 100k 100M 10M 1M 10k 12dB (VG = 1V) 0dB (VG = 0.25V) 6dB (VG = 0.5V) START 10 000.000Hz STOP 100 000 000.000Hz |
Numéro de pièce similaire - AD835 |
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Description similaire - AD835 |
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