Datasheet

Driving large capacitive loads increases the chance of
oscillations occurring in most amplifier circuits. This is
especially true for circuits with high loop gains, such as
voltage followers. The buffer’s output resistance and the
load capacitor combine to add a pole and excess phase
to the loop response. If the frequency of this pole is low
enough to interfere with the loop response and degrade
phase margin sufficiently, oscillations can occur.
A second problem when driving capacitive loads
results from the amplifier’s output impedance, which
looks inductive at high frequencies. This inductance
forms an L-C resonant circuit with the capacitive load,
which causes peaking in the frequency response and
degrades the amplifier’s gain margin.
Figure 6 shows the devices’ frequency response under
different capacitive loads. To drive loads with greater
than 20pF of capacitance or to settle out some of
the peaking, the output requires an isolation resistor
like the one shown in Figure 7. Figure 8 is a graph of
the Optimal Isolation Resistor vs. Load Capacitance.
Figure 9 shows the frequency response of the
MAX4214/MAX4215/MAX4217/MAX4219/MAX4222
when driving capacitive loads with a 27Ω isolation
resistor.
Coaxial cables and other transmission lines are easily
driven when properly terminated at both ends with their
characteristic impedance. Driving back-terminated
transmission lines essentially eliminates the lines’
capacitance.
MAX4214/MAX4215/MAX4217/MAX4219/MAX4222
High-Speed, Single-Supply, Gain of 2,
Closed-Loop, Rail-to-Rail Buffers with Enable
______________________________________________________________________________________ 11
MAX4214
MAX4215
MAX4217
MAX4219
MAX4222
500Ω500Ω
OUT
IN-
IN+
Figure 5. Input Protection Circuit
6
-4
100k 10M 100M1M 1G
-2
FREQUENCY (Hz)
NORMALIZED GAIN (dB)
0
2
4
5
-3
-1
1
3
C
L
= 10pF
C
L
= 5pF
C
L
= 15pF
Figure 6. Small-Signal Gain vs. Frequency with Load
Capacitance and No Isolation Resistor
500Ω
500Ω
R
ISO
C
L
V
OUT
V
IN
R
TIN
50Ω
MAX42_ _
Figure 7. Driving a Capacitive Load Through an Isolation
Resistor
14
16
12
10
6
4
2
8
0
C
LOAD
(pF)
0 50 100 150 200 250
R
ISO
(Ω)
Figure 8. Isolation Resistance vs. Capacitive Load