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ADA4000-1 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADA4000-1 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page ADA4000-1/ADA4000-2/ADA4000-4 Rev. 0 | Page 10 of 16 APPLICATIONS OUTPUT PHASE REVERSAL AND INPUT NOISE Phase reversal is a change of polarity in the transfer function of the amplifier. This can occur when the voltage applied at the input of the amplifier exceeds the maximum common-mode voltage. Phase reversal happens when the part is configured in the gain of 1. Most JFET amplifiers invert the phase of the input signal if the input exceeds the common-mode input. Phase reversal is a temporary behavior of the ADA4000-x family. Each part returns to normal operation by bringing back the common- mode voltage. The cause of this effect is saturation of the input stage, which leads to the forward-biasing of a drain-gate diode. In noninverting applications, a simple fix for this is to insert a series resistor between the input signal and the noninverting terminal of the amplifier. The value of the resistor depends on the application, because adding a resistor adds to the total input noise of the amplifier. The total noise density of the circuit is () S S n n nTOTAL kTR R i e e 4 2 2 + + = where: en is the input voltage noise density of the part. in is the input current noise density of the part. RS is the source resistance at the noninverting terminal. k is Boltzmann’s constant (1.38 × 10−23 J/K). T is the ambient temperature in Kelvin (T = 273 + °C). In general, it is good practice to limit the input current to less than 5 mA to avoid driving a great deal of current into the amplifier inputs. CAPACITIVE LOAD DRIVE The ADA4000-1/ADA4000-2/ADA4000-4 are stable at all gains in both inverting and noninverting configurations. The parts are capable of driving up to 1000 pF of capacitive loads without oscillations in unity gain configurations. However, as with most amplifiers, driving larger capacitive loads in a unity gain configuration can cause excessive overshoot and ringing. A simple solution to this problem is to use a snubber network (see Figure 30). ADA4000-1 V+ V– +15V –15V RS CS CL 500pF RL 10kΩ 0 SNUBBER NETWORK 400mV p-p 0 V1 3 2 1 U1 Figure 30. Snubber Network Configuration The advantage of this compensation method is that the swing at the output is not reduced because RS is out of the feedback network, and the gain accuracy does not change. Depending on the capacitive loading of the circuit, the values of RS and CS change, and the optimum value can be determined empirically. In Figure 31, the oscilloscope image shows the output of the ADA4000-x family in response to a 400 mV pulse. The circuit is configured in the unity gain configuration with 500 pF in parallel with 10 kΩ of load capacitive. TIME (1µs/DIV) INPUT SIGNAL OUTPUT SIGNAL Figure 31. Capacitive Load Drive Without Snubber Network When the snubber circuit is used, the overshoot is reduced from 30% to 6% with the same load capacitance. Ringing is virtually eliminated, as shown in Figure 32. In this circuit, RS is 41 Ω and CS is 10 nF. TIME (1µs/DIV) INPUT SIGNAL OUTPUT SIGNAL Figure 32. Capacitive Load with Snubber Network |
Podobny numer części - ADA4000-1_07 |
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Podobny opis - ADA4000-1_07 |
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