THS4140 Datasheet, PDF (13/24 Pages) Texas Instruments – HIGH SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4140, THS4141
HIGHĆSPEED FULLY DIFFERENTIAL I/O AMPLIFIERS
SLOS320E − MAY 2000 − REVISED JANUARY 2004
APPLICATION INFORMATION
data converters (continued)
Some single supply applications may require the input voltage to exceed the common-mode input voltage
range. In such cases, the following circuit configuration is suggested to bring the common-mode input voltage
within the specifications of the amplifier.
0.1 µF
VCC
Rf VCC
RPU
Rg
5V
VIN
+
VP _VOCM
−
Rg
RPU
VCC
−+
Rf
VDD
VOUT
AVDD DVDD
AIN
VOUT ^{AIN AVSS}
Vref
Figure 24. Circuit With Improved Common-Mode Input Voltage
The following equation is used to calculate R_PU:
RPU
+
VP – VCC
ǒ_VIN
–
VP^Ǔ
1
RG
)
ǒ_VOUT
–
VP^Ǔ
1
RF
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
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