Datasheet

ADA4861-3
Rev. A | Page 13 of 16
APPLICATIONS
GAIN CONFIGURATIONS 20 MHz ACTIVE LOW-PASS FILTER
Unlike conventional voltage feedback amplifiers, the feedback
resistor has a direct impact on the closed-loop bandwidth and
stability of the current feedback op amp circuit. Reducing the
resistance below the recommended value can make the amplifier
response peak and even become unstable. Increasing the size
of the feedback resistor reduces the closed-loop bandwidth.
The ADA4861-3 triple amplifier lends itself to higher order
active filters.
Figure 48 shows a 28 MHz, 6-pole, Sallen-Key
low-pass filter.
V
IN
U1
OP AMP
OUT
+
–
R1
562Ω
R2
562Ω
C2
10pF
C1
10pF
R12
301Ω
R11
210kΩ
U2
OP AMP
OUT
+
–
R3
562Ω
R4
562Ω
C4
10pF
C3
10pF
R10
301Ω
R9
210Ω
05708-007
U3
OP AMP
OUT
+
–
R5
562Ω
R6
562Ω
C6
10pF
C5
10pF
R8
301Ω
R7
210Ω
V
OUT
Tabl e 5 provides a convenient reference for quickly determining
the feedback and gain set resistor values and bandwidth for
common gain configurations.
Table 5. Recommended Values and Frequency Performance
1
Large Signal
0.1 dB Flatness
Gain R
F
(Ω) R
G
(Ω) −3 dB SS BW (MHz)
+1 499 N/A 730 90
−1 301 301 350 60
[phone]
+5 200 49.9 180 30
+10 200 22.1 80 15
1
Conditions: V
S
= ±5 V, T
A
= 25°C, R
L
= 150 Ω.
Figure 46 and Figure 47 show the typical noninverting and
inverting configurations and recommended bypass capacitor
values.
0
5708-005
0.1µF
10µF
–V
S
V
IN
R
G
V
OUT
10µF
0.1µF
+
V
S
ADA4861-3
+
–
R
F
Figure 48. 28 MHz, 6-Pole Low-Pass Filter
The filter has a gain of approximately 23 dB and flat frequency
response out to 22 MHz. This type of filter is commonly used at
the output of a video DAC as a reconstruction filter. The frequency
response of the filter is shown in
Figure 49.
Figure 46. Noninverting Gain
30
20
–70
–60
–50
–40
–30
–20
–10
0
10
1 10 100 200
05708-047
MAGNITUDE (dB)
FREQUENCY (MHz)
0
5708-006
0.1µF
10µF
–V
S
V
IN
V
OUT
10µF
0.1µF
+V
S
ADA4861-3
+
–
R
F
R
G
Figure 47. Inverting Gain
Figure 49. 20 MHz Low-Pass Filter Frequency Response