AD8031/AD8032

–14–

REV. B

APPLICATIONS

A 2 MHz Single Supply Biquad Bandpass Filter

Figure 42 shows a circuit for a single supply biquad bandpass

filter with a center frequency of 2 MHz. A 2.5 V bias level is

easily created by connecting the noninverting inputs of all three

op amps to a resistor divider consisting of two 1 k

Ω resistors

connected between +5 V and ground. This bias point is also

decoupled to ground with a 0.1

µF capacitor. The frequency

response of the filter is shown in Figure 43.

In order to maintain an accurate center frequency, it is essential

that the op amp has sufficient loop gain at 2 MHz. This requires

the choice of an op amp with a significantly higher unity gain

crossover frequency. The unity gain crossover frequency of the

AD8031/AD8032 is 40 MHz. Multiplying the open-loop gain by

the feedback factors of the individual op amp circuits yields the

loop gain for each gain stage. From the feedback networks of the

individual op amp circuits, we can see that each op amp has a

loop gain of at least 21 dB. This level is high enough to ensure

that the center frequency of the filter is not affected by the op

amp’s bandwidth. If, for example, an op amp with a gain band-

width product of 10 MHz was chosen in this application, the

resulting center frequency would shift by 20% to 1.6 MHz.

+5V

0.1 F

0.1 F

R2

2k

1k

1k

AD8031

C1

50pF

R1

3k

R3

2k

+5V

0.1 F

R4

2k

1/2

AD8032

R5

2k

1/2

AD8032

C2

50pF

R6

1k

Figure 42. A 2 MHz Biquad Bandpass Filter Using AD8031/

AD8032

1M

FREQUENCY – Hz

10k

100M

100k

10M

–50

0

–10

–30

–40

–20

Figure 43. Frequency Response of 2 MHz Bandpass Filter

High Performance Single Supply Line Driver

Even though the AD8031/AD8032 swing close to both rails,

the AD8031 has optimum distortion performance when the

signal has a common-mode level half way between the supplies

and when there is about 500 mV of headroom to each rail. If

low distortion is required in single supply applications for sig-

nals that swing close to ground, an emitter follower circuit can

be used at the op amp output.

+5V

0.1 F

7

3

2

2N3904

200

6

2.49k

49.9

4

10 F

AD8031

2.49k

49.9

49.9

Figure 44. Low Distortion Line Driver for Single Supply

Ground Referenced Signals