Datasheet

Data Sheet ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4
Rev. E | Page 17 of 24
08054-025
–0.3
–0.2
–0.1
0
0.1
0.2
0.1
1
10 100
NORMALIZED CLOSED-LOOP GAIN (dB)
FREQUENC
Y (MHz)
C
F
= 3.3pF
C
F
= 0pF
C
F
= 1pF
V
S
= 5V
G = +2
R
F
= 604Ω
R
L
= 150Ω
V
OUT
= 2V p-p
Figure 54. 0.1 dB Gain Flatness vs. C
F
, V
S
= 5 V,
ADA4891-1/ADA4891-2
DRIVING CAPACITIVE LOADS
A highly capacitive load reacts with the output impedance of
the amplifiers, causing a loss of phase margin and subsequent
peaking or even oscillation. The ADA4891-1/ADA4891-2 are
used to demonstrate this effect (see Figure 55 and Figure 56).
–10
–8
–6
–4
–2
0
2
4
6
8
0.1 1 10 100
MAGNITUDE (dB)
FREQUENCY (MHz)
V
S
= 5V
V
OUT
= 200mV p-p
G = +1
R
L
= 1kΩ
C
L
= 6.8pF
08054-032
Figure 55. Closed-Loop Frequency Response, C
L
= 6.8 pF,
ADA4891-1/ADA4891-2
OUTPUT VOLTAGE (mV)
50ns/DIV50mV/DIV
V
S
= 5V
G = +1
R
L
= 1kΩ
C
L
= 6.8pF
0
100
–100
08054-034
Figure 56. 200 mV Step Response, C
L
= 6.8 pF,
ADA4891-1/ADA4891-2
These four methods minimize the output capacitive loading effect.
• Reducing the output resistive load. This pushes the pole
further away and, therefore, improves the phase margin.
• Increasing the phase margin with higher noise gains. As
the closed-loop gain is increased, the larger phase margin
allows for large capacitive loads with less peaking.
• Adding a parallel capacitor (C
F
) with R
F
, from −IN to the
output. This adds a zero in the closed-loop frequency
response, which tends to cancel out the pole formed by the
capacitive load and the output impedance of the amplifier.
See the Effect of R
F
on 0.1 dB Gain Flatness section for
more information.
• Placing a small value resistor (R
S
) in series with the output
to isolate the load capacitor from the output stage of the
amplifier.
Figure 57 shows the effect of using a snub resistor (R
S
) on reducing
the peaking in the worst-case frequency response (gain of +1).
Using R
S
= 100 Ω reduces the peaking by 3 dB, with the trade-off
that the closed-loop gain is reduced by 0.9 dB due to attenuation
at the output. R
S
can be adjusted from 0 Ω to 100 Ω to maintain
an acceptable level of peaking and closed-loop gain, as shown in
Figure 57.
MAGNITUDE (dB)
–10
–8
–6
–4
–2
0
2
4
6
8
0.1
1
10
100
FREQUENCY
(MHz)
V
S
= 5V
V
OUT
= 200mV p-p
G = +1
R
L
= 1kΩ
C
L
= 6.8pF
R
S
= 0Ω
R
S
= 100Ω
50
Ω
R
L
R
S
C
L
OUT
V
IN
200mV
STEP
08054-033
Figure 57. Closed-Loop Frequency Response with Snub Resistor, C
L
= 6.8 pF
Figure 58 shows that the transient response is also much improved
by the snub resistor (R
S
= 100 Ω) compared to that of Figure 56.
V
S
= 5V
G = +1
R
L
= 1kΩ
C
L
= 6.8pF
R
S
= 100Ω
08054-035
50ns/DIV50mV/DIV
OUTPUT VOLTAGE (mV)
0
100
–100
Figure 58. 200 mV Step Response, C
L
= 6.8 pF, R
S
= 100 Ω