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ADA4666-2 Datasheet(PDF) 26 Page - Analog Devices |
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ADA4666-2 Datasheet(HTML) 26 Page - Analog Devices |
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26 / 32 page  ADA4666-2 Data Sheet Rev. 0 | Page 26 of 32 Figure 81. Four-Pole Low-Pass Filter Table 8. Q Requirements and Pole Positions Section Poles Q A −0.9239 ± j0.3827 0.5412 B −0.3827 ± j0.9239 1.3065 The Sallen-Key topology is widely used due to its simple design with few circuit elements. This topology provides the user the flexibility of implementing either a low-pass or a high-pass filter by simply interchanging the resistors and capacitors. The ADA4666-2 is configured in unity gain with a corner frequency at 10 kHz. An active filter requires an op amp with a unity-gain bandwidth that is at least 100 times greater than the product of the corner frequency, fC, and the quality factor, Q. The resistors and capacitors are also important in determining the perfor- mance over manufacturing tolerances, time, and temperature. At least 1% or better tolerance resistors and 5% or better tolerance capacitors are recommended. Figure 82 shows the frequency response of the low-pass Sallen- Key filter, where: VOUT1 is the output of the first stage. VOUT2 is the output of the second stage. VOUT1 shows a 40 dB/decade roll-off and VOUT2 shows an 80 dB/decade roll-off. The transition band becomes sharper as the order of the filter increases. Figure 82. Low-Pass Filter: Gain vs. Frequency CAPACITIVE LOAD DRIVE The ADA4666-2 can safely drive capacitive loads of up to 50 pF in any configuration. As with most amplifiers, driving larger capacitive loads than specified may cause excessive overshoot and ringing, or even oscillation. Heavy capacitive load reduces phase margin and causes the amplifier frequency response to peak. Peaking corresponds to overshooting or ringing in the time domain. Therefore, it is recommended that external compensation be used if the ADA4666-2 must drive a load exceeding 50 pF. This compensation is particularly important in the unity-gain configuration, which is the worst case for stability. A quick and easy way to stabilize the op amp for capacitive load drive is by adding a series resistor, RISO, between the amplifier output terminal and the load capacitance, as shown in Figure 83. RISO isolates the amplifier output and feedback network from the capacitive load. However, with this compensation scheme, the output impedance as seen by the load increases, and this reduces gain accuracy. Figure 83. Stability Compensation with Isolating Resistor, RISO Figure 84 shows the effect of the compensation scheme on the frequency response of the amplifier in unity-gain configuration driving 250 pF of load. 1/2 –VSY VIN +VSY VOUT1 C1 5.6nF ADA4666-2 SECTION B SECTION A R2 2.55k Ω R1 2.55k Ω C2 6.8nF 1/2 –VSY +VSY VOUT2 C3 1nF ADA4666-2 R4 6.19k Ω R3 6.19k Ω C4 6.8nF –120 –100 –80 –60 –40 –20 0 20 100 1k 10k 100k 1M VSY = ±9V VOUT1 VOUT2 VIN = 50mV p-p FREQUENCY (Hz) 1/2 –VSY VIN +VSY VOUT CL ADA4666-2 RISO |
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