Datasheet

REV. C
AD9631/AD9632
–15–
Active Filters
The wide bandwidth and low distortion of the AD9631 and
AD9632 are ideal for the realization of higher bandwidth active
filters. These characteristics, while being more common in
many current feedback op amps, are offered in the AD9631 and
AD9632 in a voltage feedback configuration. Many active filter
configurations are not realizable with current feedback amplifiers.
A multiple feedback active filter requires a voltage feedback
amplifier and is more demanding of op amp performance than
other active filter configurations, such as the Sallen-Key. In
general, the amplifier should have a bandwidth that is at least
10 times the bandwidth of the filter if problems due to phase shift
of the amplifier are to be avoided.
Figure 9 is an example of a 20 MHz low-pass multiple feedback
active filter using an AD9632.
–5V
10F
0.1F
0.1F
+5V
10F
AD9632
100
C1
50pF
R3
78.7
R4
154
C2
100pF
R1
154
V
IN
V
OUT
Figure 9. Active Filter Circuit
Choose
F
O
= Cutoff Frequency = 20 MHz
a = Damping Ratio = 1/Q = 2
H = Absolute Value of Circuit Gain =
–R4
R1
= 1
Then
kFC
C
CH
R
HK
R
KH
RHR
O
=
=
+
=
=
+
=
21
2
41 1
1
2
3
21
41




()
()
()
2
A/D Converter Driver
As A/D converters move toward higher speeds with higher resolu-
tions, there becomes a need for high performance drivers that
will not degrade the analog signal to the converter. It is desirable
from a system’s standpoint that the A/D be the element in the
signal chain that ultimately limits overall distortion. This places
new demands on the amplifiers that are used to drive fast, high
resolution A/Ds.
With high bandwidth, low distortion, and fast settling time, the
AD9631 and AD9632 make high performance A/D drivers for
advanced converters. Figure 10 is an example of an AD9631 used
as an input driver for an AD872, a 12-bit, 10 MSPS A/D converter.
–5V ANALOG
10F
0.1F
0.1F
+5V ANALOG
10F
AD9631
140
130
AV
DD
AGND
0.1F
4
5
+5V ANALOG
V
INA
1
2
27
28
V
INB
REF GND
0.1F
REF IN
26
REF OUT
1F
AV
SS
AV
SS
DV
DD
DGND
10
0.1F
7
6
+5V DIGITAL
CLK
MSB
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
BIT8
BIT9
BIT10
BIT11
BIT12
AGND
DV
DD
DGND
0.1F
22
23
+5V DIGITAL
21
20
OTR
19
18
17
16
15
14
13
12
11
10
9
8
49.9
DIGITAL OUTPUT
0.1F 0.1F
–5V ANALOG
325
AD872
CLOCK INPUT
ANALOG
IN
Figure 10. AD9631 Used as Driver for an AD872, a 12-Bit, 10 MSPS A/D Converter