Datasheet

AD829 Data Sheet
Rev. I | Page 14 of 20
Because the closed-loop bandwidth is a function of R
F
and
C
COMP
(see Figure 39), it is independent of the amplifier closed-
loop gain, as shown in Figure 41. To preserve stability, the time
constant of R
F
and C
COMP
needs to provide a bandwidth of
<65 MHz. For example, with C
COMP
= 15 pF and R
F
= 1 kΩ, the
small signal bandwidth of the AD829 is 10 MHz. Figure 40
shows that the slew rate is in excess of 60 V/µs. As shown in
Figure 41, the closed-loop bandwidth is constant for gains of
−1 to −4; this is a property of the current feedback amplifiers.
Figure 39. Inverting Amplifier Connection Using Current Feedback
Compensation
Figure 40. Large Signal Pulse Response of Inverting Amplifier Using Current
Feedback Compensation, C
COMP
= 15 pF, C1 = 15 pF R
F
= 1 kΩ, R1 = 1 kΩ
Figure 41. Closed-Loop Gain vs. Frequency for the Circuit of Figure 38
Figure 42 is an oscilloscope photo of the pulse response of a unity-
gain inverter that has been configured to provide a small signal
bandwidth of 53 MHz and a subsequent slew rate of 180 V/µs;
R
F
= 3 kΩ and C
COMP
= 1 pF. Figure 43 shows the excellent pulse
response as a unity-gain inverter, this using component values
of R
F
= 1 kΩ and C
COMP
= 4 pF.
Figure 42. Large Signal Pulse Response of the Inverting Amplifier Using
Current Feedback Compensation, C
COMP
= 1 pF, R
F
= 3 kΩ, R1 = 3 kΩ
Figure 43. Small Signal Pulse Response of Inverting Amplified Using Current
Feedback Compensation, C
COMP
= 4 pF, R
F
= 1 kΩ, R1 = 1 kΩ
00880-039
2
3
7
6
4
–
+
AD829
+V
S
–V
S
0.1µF
5
0.1µF
C1*
50Ω
50Ω
COAX
CABLE
R1
C
COMP
R
F
V
OUT
R
L
1kΩ
V
IN
IN4148
*RECOMMENDED VALUE
OF C
COMP
FOR C1
<7pF 0pF
≥7pF 15pF
C
COMP
SHOULD NEVER EXCEED
15pF FOR THIS CONNECTION
00880-040
5V 200ns
0%
10
90
100%
00880-041
FREQUENCY (Hz)
CLOSED-LOOP GAIN (dB)
100k 1M 10M
–15
–12
–9
–6
–3
0
3
6
9
12
15
100M
GAIN = –4
–3dB @ 8.2MHz
GAIN = –2
–3dB @ 9.6MHz
GAIN = –1
–3dB @ 10.2MHz
V
IN
= –30dBm
V
S
= ±15V
R
L
= 1kΩ
R
F
= 1kΩ
C
COMP
= 15pF
C1 = 15pF
00880-042
5V 200ns
0%
10
90
100%
00880-043
20mV
10ns
0%
10
90
100%