Datasheet

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Driving a Capacitive Load
+
_
THS402x
C
LOAD
1kW
Input
Output
50 W
20 W
S0278-01
Offset Nulling
_
+
THS402x
V
CC–
V
CC+
0.1 Fm
0.1 Fm
10kW
S0279-01
THS4021
THS4022
SLOS265C – SEPTEMBER 1999 – REVISED JULY 2007
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 THS402x 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 75- Ω transmission systems, setting the series resistor value to 75 Ω both isolates any capacitance
loading and provides the proper line impedance matching at the source end.
Figure 33. Driving a Capacitive Load
The THS402x has very low input offset voltage for a high-speed amplifier. However, if additional correction is
required, an offset nulling function has been provided on the THS4021. The input offset can be adjusted by
placing a potentiometer between terminals 1 and 8 of the device and tying the wiper to the negative supply. This
is shown in Figure 34 .
Figure 34. Offset Nulling Schematic
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Product Folder Link(s): THS4021 THS4022