AD8021
Rev. F | Page 23 of 28
G = –2
G = +2
GAIN (dB)
FREQUENCY (Hz)
100k 1M 10M 100M 1G
12
9
6
3
0
–3
–6
–9
–12
–15
–18
01888-066
Figure 66. AC Response of Two Identically Compensated High Speed Op
Amps Configured for a Gain of +2 and a Gain of −2
100k 1M 10M 100M 1G
FREQUENCY (Hz)
12
9
6
3
0
–3
–6
–9
–12
–15
–18
GAIN (dB)
01888-067
G = ±2
Figure 67. AC Response of Two Dissimilarly Compensated AD8021 Op Amps
(
Figure 66) Configured for a Gain of +2 and a Gain of −2,
(Note the Close Gain Match)
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.
C
C
C2
AD8021
3
2
R
F
6
V
OUT
R
G
+V
S
R2R1
V
IN
5
–V
S
C1
01888-068
Figure 68. Schematic of a Second-Order, Low-Pass Active Filter
Table 9. Typical Component Values for Second-Order, Low-
Pass Active Filter of
Figure 68
Gain R1
(Ω)
R2
(Ω)
R
F
(Ω)
R
G
(Ω)
C1
(nF)
C2
(nF)
C
C
(pF)
2 71.5 215 499 499 10 10 7
5 44.2 365 365 90.9 10 10 2
1k 10k 100k 1M 10M
FREQUENCY (Hz)
50
40
30
20
10
0
–10
–20
–30
–40
–50
GAIN (dB)
G = 2
G = 5
01888-069
Figure 69. Frequency Response of the Filter Circuit of
Figure 68
for Two Different Gains
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
C
, which reduces the peaking while
maintaining gain flatness at low frequencies. The second
technique is to add a resistor, R
SNUB
, in series between the output
pin of the AD8021 and the capacitive load, C
B
L
. shows
the response of the AD8021 when both C
Figure 70
C
and R
SNUB
B are used to
reduce peaking. For a given C
L
, Figure 71 can be used to
determine the value of R
SNUB
that maintains 2 dB of peaking in
the frequency response. Note, however, that using R
B
SNUB
attenuates
the low frequency output by a factor of R
LOAD
/(R
SNUB
B + R
LOAD
).
