THS6062 Datasheet, PDF (23/31 Pages) Texas Instruments – LOW-NOISE ADSL DUAL DIFFERENTIAL RECEIVER

THS6062
LOW-NOISE ADSL DUAL DIFFERENTIAL RECEIVER
SLOS228B – JANUARY 1999 – REVISED JUNE 1999
APPLICATION INFORMATION
optimizing frequency response (continued)
The next thing that helps to maintain a smooth frequency response is to keep the feedback resistor (R_f) and
the gain resistor (R_g) values fairly low. These two resistors are effectively in parallel when looking at the ac
small-signal response. This is why in Figure 29, a feedback resistor of 3.9 kΩ with a gain resistor of 1 kΩ only
shows a small peaking in the frequency response. The parallel resistance is only 800 Ω. This value, in
conjunction with a very small stray capacitance test PCB, forms a zero on the edge of the amplifier’s natural
frequency response. To eliminate this peaking, all that needs to be done is to reduce the feedback and gain
resistances. One other way to compensate for this stray capacitance is to add a small capacitor in parallel with
the feedback resistor. This helps to neutralize the effects of the stray capacitance. To keep this zero out of the
operating range, the stray capacitance and resistor value’s time constant must be kept low. But, as can be seen
in Figures 23 – 28, a value too low starts to reduce the bandwidth of the amplifier. Table 1 shows some
recommended feedback resistors to be used with the THS6062.
Table 1. Recommended Feedback Resistors
GAIN
2
–1
5
Rf for VCC = ±15 V, ± 5 V, 5 V
300 Ω
360 Ω
3.3 kΩ (low stray-c PCB only)
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 THS6062 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 44. 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.
360 Ω
Input
360 Ω
_
THS6062
+
20 Ω
Output
CLOAD
Figure 44. Driving a Capacitive Load
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
23