Datasheet ADA4666-2 (Analog Devices) - 26

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ADA4666-2 Data Sheet C2 6.8nF C4 6.8nF R1 R2 +VSY 2.55kΩ 2.55kΩ V R3 R4 +V IN SY 6.19kΩ 6.19kΩ 1/2 C1 VOUT1 V 5.6nF ADA4666-2 1/2 OUT2 C3 –VSY 1nF ADA4666-2 –VSY
1 -08
SECTION A SECTION B
1382 1 Figure 81. Four-Pole Low-Pass Filter
Table 8. Q Requirements and Pole Positions CAPACITIVE LOAD DRIVE Section Poles Q
The ADA4666-2 can safely drive capacitive loads of up to 50 pF A −0.9239 ± j0.3827 0.5412 in any configuration. As with most amplifiers, driving larger B −0.3827 ± j0.9239 1.3065 capacitive loads than specified may cause excessive overshoot The Sallen-Key topology is widely used due to its simple design and ringing, or even oscillation. Heavy capacitive load reduces with few circuit elements. This topology provides the user the phase margin and causes the amplifier frequency response to flexibility of implementing either a low-pass or a high-pass filter peak. Peaking corresponds to overshooting or ringing in the by simply interchanging the resistors and capacitors. The time domain. Therefore, it is recommended that external ADA4666-2 is configured in unity gain with a corner frequency compensation be used if the ADA4666-2 must drive a load at 10 kHz. An active filter requires an op amp with a unity-gain exceeding 50 pF. This compensation is particularly important in bandwidth that is at least 100 times greater than the product of the unity-gain configuration, which is the worst case for the corner frequency, f stability. C, and the quality factor, Q. The resistors and capacitors are also important in determining the perfor- A quick and easy way to stabilize the op amp for capacitive load mance over manufacturing tolerances, time, and temperature. drive is by adding a series resistor, RISO, between the amplifier At least 1% or better tolerance resistors and 5% or better output terminal and the load capacitance, as shown in Figure 83. tolerance capacitors are recommended. RISO isolates the amplifier output and feedback network from Figure 82 shows the frequency response of the low-pass Sallen- the capacitive load. However, with this compensation scheme, Key filter, where: the output impedance as seen by the load increases, and this reduces gain accuracy. VOUT1 is the output of the first stage.
+VSY
VOUT2 is the output of the second stage.
RISO VOUT
V
1/2
OUT1 shows a 40 dB/decade roll-off and VOUT2 shows an
VIN ADA4666-2
80 dB/decade roll-off. The transition band becomes sharper as
CL –VSY
083 the order of the filter increases. 1382- 1
20
Figure 83. Stability Compensation with Isolating Resistor, RISO
0
Figure 84 shows the effect of the compensation scheme on the frequency response of the amplifier in unity-gain configuration
–20
driving 250 pF of load.
VOUT1 B) –40 d N ( VOUT2 AI –60 G –80 –100 VSY = ±9V VIN = 50mV p-p –120100 1k 10k 100k 1M
082
FREQUENCY (Hz)
1382- 1 Figure 82. Low-Pass Filter: Gain vs. Frequency Rev. 0 | Page 26 of 32 Document Outline Features Applications General Description Pin Connection Diagrams Revision History Specifications Electrical Characteristics—18 V Operation Electrical Characteristics—10 V Operation Electrical Characteristics—3.0 V Operation Absolute Maximum Ratings Thermal Resistance ESD Caution Pin Configurations and Function Descriptions Typical Performance Characteristics Applications Information Input Stage Gain Stage Output Stage Maximum Power Dissipation Rail-to-Rail Input and Output Comparator Operation EMI Rejection Ratio Current Shunt Monitor Active Filters Capacitive Load Drive Noise Considerations with High Impedance Sources Outline Dimensions Ordering Guide