AD8698

Rev. 0 | Page 16 of 20

LOW NOISE APPLICATIONS

In some applications, it is critical to minimize the noise, and

although the AD8698 has a low noise of typically 8 nV/√Hz at

1 kHz, paralleling the two amplifiers within the same package

reduces the total noise referred to the input to approximately

5.5 nV/√Hz. This simple technique is depicted in Figure 55.

V–

V+

V+

R4

10k

Ω

V–

R2

10k

Ω

R3

1k

Ω

R1

1k

Ω

R5

100

Ω

R3

100

Ω

Figure 55. Paralleling Amplifiers

DRIVING ADCs

The AD8698 can drive extremely heavy capacitive loads

without any compensation. Sometimes capacitors are placed at

the output of the amplifier to absorb transient currents while

the op amp is interfaced with the ADC. Most op amps need a

small resistor with the output to isolate the load capacitance.

This results in a loss of bandwidth and slows the amplifier

down substantially. However, the AD8698 maintains a unity

gain bandwidth of 1 MHz with loads of up to 1 nF, as shown in

Figure 52.

USING THE AD8698 IN ACTIVE FILTER DESIGNS

The AD8698 is recommended for unity gain filter designs with

a corner frequency of up to 100 kHz, one tenth of the op amp’s

unity gain bandwidth.

If a higher gain is desired, the corner frequency should be

chosen accordingly. For example, if the amplifier is configured

with a gain of 10, the corner frequency of the filter should not

be more than 10 kHz.

An example of an active filter is the Sallen Key. This topology

gives the user the flexibility of implementing a low-pass or a

high-pass filter by simply interchanging the resistors and the

capacitors.

In the high-pass filter of Figure 56, the damping factor Q is set

to 1/√2 for a maximally flat response (Butterworth).

The gain is unity and the bandwidth is 10 kHz with the values

shown.

V–

V+

R2

22k

Ω

R1

11k

Ω

C1

1nF

C2

1nF

Figure 56. Two Pole High-Pass Filter

V–

V+

R2

11k

Ω

R1

11k

Ω

C1

2nF

C2

1nF

VIN

Figure 57. Two Pole Low-Pass Filter

The circuit of Figure 57 has a bandwidth of 10 kHz and a

maximally flat response. In this case, the damping factor is

controlled by the ratio of the capacitors and the gain is unity.