AD8031B Analog Devices, AD8031B Datasheet - Page 14

• Manufacturer Part Number: AD8031B
• Description: 2.7 V/ 800 uA/ 80 MHz Rail-to-Rail I/O Amplifiers
• Manufacturer: Analog Devices

AD8031/AD8032
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 resistors
connected between +5 V and ground. This bias point is also
decoupled to ground with a 0.1 F 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.
Figure 42. A 2 MHz Biquad Bandpass Filter Using AD8031/
AD8032
V
IN
0.1 F
3k
R1
1k
2k
1k
R2
+5V
50pF
0.1 F
AD8031
C1
V
OUT
2k
R3
+5V
1k
2k
AD8032
R4
R6
0.1 F
1/2
2k
R5
50pF
C2
AD8032
1/2
–14–
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.
Figure 43. Frequency Response of 2 MHz Bandpass Filter
Figure 44. Low Distortion Line Driver for Single Supply
Ground Referenced Signals
V
IN
–10
–20
–30
–40
–50
0
10k
49.9
2.49k
3
2
100k
7
4
0.1 F
10 F
AD8031
2.49k
+5V
6
FREQUENCY – Hz
2N3904
1M
200
49.9
10M
49.9
100M
V
REV. B
OUT