Datasheet ADA4000-1, ADA4000-2, ADA4000-4 (Analog Devices) - 10

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ADA4000-1/ADA4000-2/ADA4000-4 Data Sheet APPLICATIONS INFORMATION OUTPUT PHASE REVERSAL AND INPUT NOISE
The advantage of this compensation method is that the swing at Phase reversal is a change of polarity in the transfer function of the output is not reduced because RS is out of the feedback network, the amplifier. This can occur when the voltage applied at the and the gain accuracy does not change. Depending on the input of the amplifier exceeds the maximum common-mode capacitive loading of the circuit, the values of RS and CS change, voltage. Phase reversal happens when the device is configured and the optimum value can be determined empirically. In in the gain of 1. Figure 31, the oscilloscope image shows the output of the ADA4000-1/ADA4000-2/ADA4000-4 family in response to Most JFET amplifiers invert the phase of the input signal if the a 400 mV pulse. The circuit is configured in the unity gain input exceeds the common-mode input. Phase reversal is a configuration with 500 pF in parallel with 10 kΩ of load temporary behavior of the ADA4000-1/ADA4000-2/ADA4000-4 capacitive. family. Each device 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-
INPUT SIGNAL
gate diode. In noninverting applications, a simple fix for this is
)
to insert a series resistor between the input signal and the non-
V /DI
inverting terminal of the amplifier. The value of the resistor depends on the application, because adding a resistor adds to the
200mV ( E
total input noise of the amplifier. The total noise density of the
AG T
circuit is
L O OUTPUT SIGNAL V
e  e 2  i R 2  4kTR nTOTAL n  n S S where: 2 03 1- en is the input voltage noise density of the device. 79
TIME (1µs/DIV)
05 in is the input current noise density of the device. Figure 31. Capacitive Load Drive Without Snubber Network RS is the source resistance at the noninverting terminal. When the snubber circuit is used, the overshoot is reduced from k is Boltzmann’s constant (1.38 × 10−23 J/K). 30% to 6% with the same load capacitance. Ringing is virtually T is the ambient temperature in Kelvin (T = 273 + °C). eliminated, as shown in Figure 32. In this circuit, RS is 41 Ω and In general, it is good practice to limit the input current to less CS is 10 nF. than 5 mA to avoid driving a great deal of current into the amplifier inputs.
CAPACITIVE LOAD DRIVE INPUT SIGNAL )
The ADA4000-1/ADA4000-2/ADA4000-4 are stable at all gains
IV /D
in both inverting and noninverting configurations. The devices
V
are capable of driving up to 1000 pF of capacitive loads without
00m
oscillations in unity gain configurations.
E (2 G A T
However, as with most amplifiers, driving larger capacitive loads
L VO OUTPUT SIGNAL
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). 33 0
+15V
1- 79
TIME (1µs/DIV)
05
3 U1 SNUBBER NETWORK
Figure 32. Capacitive Load with Snubber Network
V1 V+ 400mV p-p 1 ADA4000-1 2 V– RS C 0 L RL 500pF C 10kΩ S –15V
31 -0 791
0
05 Figure 30. Snubber Network Configuration Rev. B | Page 10 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE POWER SEQUENCING ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION OUTPUT PHASE REVERSAL AND INPUT NOISE CAPACITIVE LOAD DRIVE SETTLING TIME OUTLINE DIMENSIONS ORDERING GUIDE