THS4140 Datasheet, PDF (13/33 Pages) Texas Instruments – HIGH SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

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THS4140
THS4141
SLOS320F – MAY 2000 – REVISED JANUARY 2006
APPLICATION INFORMATION (continued)
The following equation is used to calculate R_PU:
ǒ Ǔ ǒ Ǔ ^{RPU +}
VP – VCC
VIN – VP
1
RG
)
VOUT – VP
1
RF
(1)
DRIVING A CAPACITIVE LOAD
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 THS414x 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 25. 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 50 Ω both isolates any capacitance
loading and provides the proper line impedance matching at the source end.
390 Ω
390 Ω
20 Ω
Output
390 Ω
THS414x
20 Ω
390 Ω
Output
Figure 25. Driving a Capacitive Load
ACTIVE ANTIALIAS FILTERING
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 26 presents a
method by which the noise may be filtered in the THS414x.
R2
C1
R1
R3
V_IN-
Vs
C2
V_IN+
R1
R3
V_IC
R2
V_CC
+
-
+
THS414x
+-
V_OCM
V_CC-
+
C1
R4
R_(t)
R4
C3
V_IN+
THS1050
V_IN-
V_OCM
C3
Figure 26. Antialias Filtering
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