AD8021 Analog Devices, AD8021 Datasheet - Page 23

• Manufacturer Part Number: AD8021
• Description: Low Noise, High Speed Amplifier for 16-Bit Systems
• Manufacturer: Analog Devices

Specifications of AD8021

• -3db Bandwidth: 490MHz
• Slew Rate: 150V/µs
• Vos: 400µV
• Ib: 7.5µA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 2.1nV/rtHz
• Vcc-vee: 4.5V to 24V
• Isy Per Amplifier: 7.7mA
• Packages: SOIC,SOP

USING THE AD8021 IN ACTIVE FILTERS
The low noise and high gain bandwidth of the AD8021 make it
an excellent choice in active filter circuits. Most active filter
literature provides resistor and capacitor values for various
filters but neglects the effect of the op amp’s finite bandwidth on
filter performance; ideal filter response with infinite loop gain is
implied. Unfortunately, real filters do not behave in this manner.
Instead, they exhibit finite limits of attenuation, depending on
the gain bandwidth of the active device. Good low-pass filter
performance requires an op amp with high gain bandwidth for
attenuation at high frequencies, and low noise and high dc gain
for low frequency, pass-band performance.
Figure 68 shows the schematic of a 2-pole, low-pass active filter
and lists typical component values for filters having a Bessel-
type response with a gain of 2 and a gain of 5. Figure 69 is a
network analyzer plot of this filter’s performance.
Figure 67. AC Response of Two Dissimilarly Compensated AD8021 Op Amps
Figure 66. AC Response of Two Identically Compensated High Speed Op
–12
–15
–18
–12
–15
–18
12
–3
–6
–9
12
–3
–9
–6
100k
9
6
3
0
9
6
3
0
100k
(Figure 66) Configured for a Gain of +2 and a Gain of −2,
Amps Configured for a Gain of +2 and a Gain of −2
1M
1M
(Note the Close Gain Match)
FREQUENCY (Hz)
FREQUENCY (Hz)
10M
10M
G = –2
G = +2
G = ±2
100M
100M
1G
1G
Rev. F | Page 23 of 28
Table 9. Typical Component Values for Second-Order, Low-
Pass Active Filter of Figure 68
Gain
2
5
DRIVING CAPACITIVE LOADS
When the AD8021 drives a capacitive load, the high frequency
response can show excessive peaking before it rolls off. Two
techniques can be used to improve stability at high frequency
and reduce peaking. The first technique is to increase the
compensation capacitor, C
maintaining gain flatness at low frequencies. The second
technique is to add a resistor, R
pin of the AD8021 and the capacitive load, C
the response of the AD8021 when both C
reduce peaking. For a given C
determine the value of R
the frequency response. Note, however, that using R
the low frequency output by a factor of R
Figure 68. Schematic of a Second-Order, Low-Pass Active Filter
–10
–20
–30
–40
–50
Figure 69. Frequency Response of the Filter Circuit of Figure 68
50
40
30
20
10
R1
(Ω)
71.5
44.2
0
1k
V
IN
R2
(Ω)
215
365
R1
10k
C2
G = 2
for Two Different Gains
R2
R
(Ω)
499
365
SNUB
F
C
FREQUENCY (Hz)
, which reduces the peaking while
R
B
C1
G
that maintains 2 dB of peaking in
L
, Figure 71 can be used to
SNUB
100k
R
(Ω)
499
90.9
3
2
G
–V
+V
, in series between the output
B
S
G = 5
C
S
C
R
AD8021
LOAD
F
C1
(nF)
10
10
5
6
C
/(R
and R
1M
L
SNUB
.
Figure 70
V
OUT
SNUB
B
SNUB
C2
(nF)
10
10
+ R
AD8021
B
LOAD
are used to
attenuates
10M
).
shows
C
(pF)
7
2
C