THS3001 Datasheet, PDF (23/32 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS

THS3001, THS3002
420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS
APPLICATION INFORMATION
SLOS217A – JULY 1998 – REVISED JUNE 1999
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 THS300x 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 53. 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.
1 kΩ
Input
1 kΩ
_
THS300x
+
20 Ω
Output
CLOAD
Figure 53. Driving a Capacitive Load
PCB design considerations
Proper PCB design techniques in two areas are important to assure proper operation of the THS300x. These
areas are high-speed layout techniques and thermal-management techniques. Because the THS300x is a
high-speed part, the following guidelines are recommended.
D Ground plane – It is essential that a ground plane be used on the board to provide all components with a
low inductive ground connection. Although a ground connection directly to a terminal of the THS300x is not
necessarily required, it is recommended that the thermal pad of the package be tied to ground. This serves
two functions: it provides a low inductive ground to the device substrate to minimize internal crosstalk, and
it provides the path for heat removal.
D Input stray capacitance – To minimize potential problems with amplifier oscillation, the capacitance at the
inverting input of the amplifiers must be kept to a minimum. To do this, PCB trace runs to the inverting input
must be as short as possible, the ground plane must be removed under any etch runs connected to the
inverting input, and external components should be placed as close as possible to the inverting input. This
is especially true in the noninverting configuration. An example of this can be seen in Figure 54, which shows
what happens when a 1-pF capacitor is added to the inverting input terminal. The bandwidth increases at
the expense of peaking. This is because some of the error current is flowing through the stray capacitor
instead of the inverting node of the amplifier. Although, while the device is in the inverting mode, stray
capacitance at the inverting input has a minimal effect. This is because the inverting node is at a virtual
ground and the voltage does not fluctuate nearly as much as in the noninverting configuration. This can be
seen in Figure 55, where a 10-pF capacitor adds only 0.35 dB of peaking. In general, as the gain of the
system increases, the output peaking due to this capacitor decreases. While this can initially look like a
faster and better system, overshoot and ringing are more likely to occur under fast transient conditions. So
proper analysis of adding a capacitor to the inverting input node should be performed for stable operation.
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