THS4131 Datasheet, PDF (15/37 Pages) National Semiconductor (TI) – HIGH-SPEED, LOW-NOISE, FULLY-DIFFERENTIAL I/O AMPLIFIERS

THS4130
THS4131
www.ti.com
SLOS318H – MAY 2000 – REVISED MAY 2011
Some single-supply applications may require the input voltage to exceed the common-mode input voltage range.
In such cases, the circuit configuration of Figure 32 is suggested to bring the common-mode input voltage within
the specifications of the amplifier.
0.1 µF
V_CC
R_{f VCC}
R_PU
R_g
5V
V_IN
+
V_{P VOCM}
−
R_{g RPU}
V_CC
−+
R_f
V_DD
V_OUT
AV_DD
A_IN1
DV_DD
THS1206
V_OUT ^{AIN2 AVSS}
V_ref
Figure 32. Circuit With Improved Common-Mode Input Voltage
Equation 2 is used to calculate R_PU:
ǒ Ǔ ǒ Ǔ ^{RPU +}
VP – VCC
VIN – VP
1
RG
)
VOUT – VP
1
RF
(2)
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 THS413x 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 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 33. 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
THS413x
20 Ω
390 Ω
Output
Figure 33. Driving a Capacitive Load
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