
AD8057/AD8058
–11–
REV. A
FREQUENCY – Hz
100
10
0.110
100M
100
I
N
H
1k
10k
100k
1
1M
10M
Figure 37. Current Noise vs. Frequency
FREQUENCY – MHz
100
10
0.1
0.1
1000
1
Z
O
V
10
100
1
Figure 38. Output Impedance vs. Frequency
APPLICATIONS
Driving Capacitive Loads
When driving a capacitive load, most op amps will exhibit over-
shoot in their pulse response.
Figure 39 shows the relationship between the capacitive load that
results in 30% overshoot and closed loop gain of an AD8058. It can
be seen that, under the Gain = +2 condition, the device is stable
with capacitive loads of up to 69 pF.
In general, to minimize peaking or to ensure device stability for
larger values of capacitive loads, a small series resistor, R
S
, can
be added between the op amp output and the load capacitor, C
L
,
as shown in Figure 40.
For the setup shown in Figure 40, the relationship between R
S
and C
L
was empirically derived and is shown in Table I.
CLOSED-LOOP GAIN
500
400
0
1
5
2
C
L
3
4
300
200
100
R
S
= 2.4
V
R
S
= 0
V
Figure 39. Capacitive Load Drive vs. Closed-Loop Gain
Table I. Recommended Value for Resistors R
S
, R
F
, R
G
vs.
Capacitive Load, C
L
, Which Results in 30% Overshoot
Gain
1
2
3
4
5
10
R
F
100
100
100
100
100
100
R
G
C
L
w/R
S
= 0
V
C
L
w/R
S
= 2.4
V
11
13
51
69
104
153
186
270
245
500
870
1580
100
50
33.2
25
11
–2.5V
R
G
50k
V
V
IN
= 200mV p-p
AD8058
0.1
m
F
10
m
F
0.1
m
F
10
m
F
+2.5V
R
F
C
L
R
S
V
OUT
FET PROBE
Figure 40. Capacitive Load Drive Circuit
100mV
50ns/DIV
200mV
–100mV
–200mV
+ OVERSHOOT
29.0%
100mV
Figure 41. Typical Pulse Response with C
L
= 65 pF,
Gain = +2, and V
S
=
±
2.5 V