THS4121IDGN Datasheet, PDF (12/30 Pages) Texas Instruments – HIGH-SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4120
THS4121
SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004
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APPLICATION INFORMATION (continued)
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 THS412x 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 decreases 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 21. 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.
R_f
R_(g)
20 Ω
Output
THS412x
20 Ω
R_(g)
R_f
Output
Figure 21. 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 22 presents a
method by which the noise may be filtered in the THS412x. Proper ground referencing should be considered.
R2
C1
V_DD
R1
R3
V_IN-
Vs
C2
V_IN+
R1
R3
V_IC
R2
-
+
THS412x
+-
V_OCM
V_SS
C1
R4
R_(t)
R4
V_DD
C3
AV_DD DV_DD
V_IN+
V_IN-
C3
V_OCM
Figure 22. Antialias Filtering
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