ADA4661-2ARMZ-R7 datasheet(26/32 Pages) AD

26 / 32 page

ADA4661-2

Data Sheet

Rev. 0 | Page 26 of 32

Figure 81. Four-Pole Low-Pass Filter

Table 8. Q Requirements and Pole Positions

Section

Poles

−0.9239 ± j0.3827

0.5412

−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

ADA4661-2 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 ADA4661-2 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 ADA4661-2 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

5.6nF

ADA4661-2

SECTION B

SECTION A

2.55k

Ω

2.55k

Ω

6.8nF

1/2

–VSY

+VSY

VOUT2

1nF

ADA4661-2

6.19k

Ω

6.19k

Ω

6.8nF

–120

–100

–80

–60

–40

–20

100

10k

100k

VSY = ±9V

VOUT1

VOUT2

VIN = 50mV p-p

FREQUENCY (Hz)

1/2

–VSY

VIN

+VSY

VOUT

ADA4661-2

RISO

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