AD8031/AD8032

Data Sheet

Rev. G | Page 16 of 20

APPLICATIONS

A 2 MHz SINGLE-SUPPLY, BIQUAD BAND-PASS

FILTER

Figure 47 shows a circuit for a single-supply, biquad band-pass

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 kΩ 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 48.

To maintain an accurate center frequency, it is essential that the

op amp have 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, it can be seen 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

bandwidth product of 10 MHz was chosen in this application,

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

5V

0.1µF

0.1µF

1k

Ω

1k

Ω

AD8031

5V

VOUT

C2

50pF

C1

50pF

R6

1k

Ω

R4

2k

Ω

1/2

AD8032

1/2

AD8032

R5

2k

Ω

R1

3k

Ω

VIN

R2

2k

Ω

R3

2k

Ω

0.1µF

Figure 47. A 2 MHz, Biquad Band-Pass Filter Using AD8031/AD8032

FREQUENCY (Hz)

–50

0

–10

–30

–40

–20

10k

100k

1M

10M

100M

Figure 48. Frequency Response of 2 MHz Band-Pass 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 signals

that swing close to ground, an emitter-follower circuit can be

used at the op amp output.

10µF

5V

7

3

2

2N3904

200

Ω

6

2.49k

Ω

49.9

Ω

4 AD8031

2.49k

Ω

49.9

Ω

49.9

Ω

0.1µF

VIN

VOUT

Figure 49. Low Distortion Line Driver for Single-Supply, Ground Referenced Signals

Figure 49 shows the AD8031 configured as a single-supply, gain-

of-2 line driver. With the output driving a back-terminated

50 Ω line, the overall gain from VIN to VOUT is unity. In addition

to minimizing reflections, the 50 Ω back termination resistor

protects the transistor from damage if the cable is short circuited.

The emitter follower, which is inside the feedback loop, ensures

that the output voltage from the AD8031 stays about 700 mV

above ground. Using this circuit, low distortion is attainable

even when the output signal swings to within 50 mV of ground.

The circuit was tested at 500 kHz and 2 MHz.