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AD835AR 데이터시트(PDF) 11 Page - Analog Devices |
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AD835AR 데이터시트(HTML) 11 Page - Analog Devices |
11 / 16 page AD835 Rev. D | Page 11 of 16 APPLICATIONS INFORMATION The AD835 is 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 inputs and Y inputs connected in parallel). In this mode, it is useful at input frequencies to well over 250 MHz because that is the bandwidth limitation of the output amplifier only. MULTIPLIER CONNECTIONS Figure 20 shows the basic connections for multiplication. The inputs are often single sided, in which case the X2 and Y2 inputs are normally grounded. Note that by assigning Pin 7 (X2) and Pin 2 (Y2), respectively, to these (inverting) inputs, an extra measure of isolation between inputs and output is provided. The X and Y inputs may 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 recommendations shown in Figure 20 should be followed closely. In Figure 21, Figure 23, and Figure 24, these power supply decoupling components are omitted for clarity but should be used wherever optimal performance with high speed inputs is required. However, if the full, high frequency capabilities of the AD835 are not being exploited, these components can be omitted. WIDEBAND VOLTAGE-CONTROLLED AMPLIFIER Figure 21 shows the AD835 configured to provide a gain of nominally 0 dB 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 addition 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 a slight revision. R1 97.6 Ω C1 33pF R2 301 Ω AD835 1 2 3 4 8 VOLTAGE OUTPUT VG (GAIN CONTROL) VIN (SIGNAL) X2 VP W Y1 X1 Y2 +5V –5V VN Z 7 6 5 Figure 21. Voltage-Controlled 50 MHz Amplifier Using the AD835 The ac response of this amplifier for gains of 0 dB (VG = 0.25 V), 6 dB (VG = 0.5 V), and 12 dB (VG = 1 V) is shown in Figure 22. 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 VG. 10k 100k 1M FREQUENCY (Hz) –9 –6 –3 0 3 6 9 12 15 18 21 10M 100M 12dB (VG = 1V) 6dB (VG = 0.5V) 0dB (VG = 0.25V) Figure 22. AC Response of VCA AMPLITUDE MODULATOR Figure 23 shows a simple modulator. The carrier is applied 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%. Carrier and modulation frequencies can be up to 300 MHz, somewhat 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. +5V –5V AD835 1 2 3 4 8 X2 MODULATION SOURCE MODULATED CARRIER OUTPUT CARRIER SOURCE VP W Y1 X1 Y2 VN Z 7 6 5 Figure 23. Simple Amplitude Modulator Using the AD835 |
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