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AD603AR Fiches technique(PDF) 10 Page - Analog Devices |
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AD603AR Fiches technique(HTML) 10 Page - Analog Devices |
10 / 14 page REV. C –10– AD603 output signal. The automatic gain control voltage, VAGC, is the time-integral of this error current. In order for VAGC (and thus the gain) to remain insensitive to short-term amplitude fluctuations in the output signal, the rectified current in Q1 must, on average, exactly balance the current in Q2. If the output of A2 is too small to do this, VAGC will increase, causing the gain to increase, until Q1 conducts sufficiently. Consider the case where R8 is zero and the output voltage VOUT is a square wave at, say, 455 kHz, which is well above the corner frequency of the control loop. During the time VOUT is negative with respect to the base voltage of Q1, Q1 conducts; when VOUT is positive, it is cut off. Since the average collector current of Q1 is forced to be 300 µA, and the square wave has a duty-cycle of 1:1, Q1’s collector current when conducting must be 600 µA. With R8 omitted, the peak amplitude of VOUT is forced to be just the VBE of Q1 at 600 µA, typically about 700 mV, or 2 VBE peak-to-peak. This voltage, hence the amplitude at which the output stabilizes, has a strong negative temperature coefficient (TC), typically –1.7 mV/ °C. Although this may not be troublesome in some applications, the correct value of R8 will render the output stable with temperature. To understand this, first note that the current in Q2 is made to be proportional to absolute temperature (PTAT). For the moment, continue to assume that the signal is a square wave. When Q1 is conducting, VOUT is now the sum of VBE and a voltage that is PTAT and which can be chosen to have an equal but opposite TC to that of the VBE. This is actually nothing more than an application of the “bandgap voltage reference” principle. When R8 is chosen such that the sum of the voltage across it and the VBE of Q1 is close to the bandgap voltage of about 1.2 V, VOUT will be stable over a wide range of temperatures, provided, of course, that Q1 and Q2 share the same thermal environment. Since the average emitter current is 600 µA during each half- cycle of the square wave a resistor of 833 Ω would add a PTAT voltage of 500 mV at 300 K, increasing by 1.66 mV/ °C. In prac- tice, the optimum value will depend on the type of transistor used and, to a lesser extent, on the waveform for which the temperature stability is to be optimized; for the inexpensive 2N3904/2N306 pair and sine wave signals, the recommended value is 806 Ω. This resistor also serves to lower the peak current in Q1 when more typical signals (usually, sinusoidal) are involved, and the 1.8 kHz LP filter it forms with CAV helps to minimize distortion due to ripple in VAGC. Note that the output amplitude under sine wave conditions will be higher than for a square wave, since the average value of the current for an ideal rectifier would be 0.637 times as large, causing the output amplitude to be 1.88 (=1.2/0.637) V, or 1.33 V rms. In practice, the somewhat nonideal rectifier results in the sine wave output being regulated to about 1.4 V rms, or 3.6 V p-p. The bandwidth of the circuit exceeds 40 MHz. At 10.7 MHz, the AGC threshold is 100 µV (–67 dBm) and its maximum gain is 83 dB (20 log 1.4 V/100 µV). The circuit holds its output at 1.4 V rms for inputs as low as –67 dBm to +15 dBm (82 dB), where the input signal exceeds the AD603’s maximum input rating. For a 30 dBm input at 10.7 MHz, the second harmonic is 34 dB down from the fundamental and the third harmonic is 35 dB down. CAUTION Careful component selection, circuit layout, power-supply decoupling, and shielding are needed to minimize the AD603’s susceptibility to interference from radio and TV stations, etc. In bench evaluation, we recommend placing all of the components in a shielded box and using feedthrough decoupling networks for the supply voltage. Circuit layout and construction are also critical, since stray capacitances and lead inductances can form resonant circuits and are a potential source of circuit peaking, oscillation, or both. |
Numéro de pièce similaire - AD603AR |
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Description similaire - AD603AR |
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