Datasheet

R
PU
+
V
P
– V
CC
ǒ
V
IN
– V
P
Ǔ
1
RG
)
ǒ
V
OUT
– V
P
Ǔ
1
RF
DRIVING A CAPACITIVE LOAD
THS415x
Output
Output
20 Ω
20 Ω
390 Ω
390 Ω
390 Ω
390 Ω
ACTIVE ANTIALIAS FILTERING
V
IN
-
V
IN
+
+
-
+
-
V
OCM
V
OCM
V
IN
-
V
IN
+
V
CC
-
THS1050
THS415x
C3
C3
R4
R
(t)
R2
R4
+
C1
+
V
CC
C1
R2
R3
R3
C2
R1
R1
Vs
V
IC
THS4150
THS4151
www.ti.com ........................................................................................................................................................... SLOS321G – MAY 2000 – REVISED MARCH 2009
The following equation is used to calculate R
PU
:
Driving capacitive loads with high-performance amplifiers is not a problem as long as certain precautions are
taken. The first is to realize that the THS415x has been internally compensated to maximize its bandwidth and
slew rate performance. When the amplifier is compensated in this manner, capacitive loading directly on the
output will decrease the device's phase margin leading to high-frequency ringing or oscillations. Therefore, for
capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with the output of
the amplifier, as shown in Figure 34 . A minimum value of 20 Ω should work well for most applications. For
example, in 50- Ω transmission systems, setting the series resistor value to 20 Ω both isolates any capacitance
loading and provides the proper line impedance matching at the source end.
Figure 34. Driving a Capacitive Load
For signal conditioning in ADC applications, it is important to limit the input frequency to the ADC. Low-pass
filters can prevent the aliasing of the high frequency noise with the frequency of operation. Figure 35 presents a
method by which the noise may be filtered in the THS415x.
Figure 35. Antialias Filtering
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