THS6032_14 Datasheet, PDF (18/35 Pages) Texas Instruments – LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER

THS6032
SLOS233F – APRIL 1999 – REVISED AUGUST 2009....................................................................................................................................................... www.ti.com
RECOMMENDED FEEDBACK AND GAIN RESISTOR VALUES
As with all current feedback amplifiers, the bandwidth of the THS6032 is an inversely-proportional function of the
value of the feedback resistor. This can be seen from Figure 1 to Figure 6. The recommended resistors for the
optimum frequency response with a 25-Ω load system can be seen in Table 1. These should be used as a
starting point. When optimum values are found, 1%- tolerance resistors should be used to maintain frequency
response characteristics. For most applications, a feedback-resistor value of 1.3 kΩ is recommended; this value
provides a good compromise between bandwidth and phase margin that yields a very stable amplifier.
Consistent with current-feedback amplifiers, increasing the gain is best accomplished by changing the gain
resistor, not the feedback resistor. This is because the bandwidth of the amplifier is dominated by the feedback
resistor value and the internal dominant-pole capacitor. The ability to control the amplifier gain independently of
the bandwidth constitutes a major advantage of current-feedback amplifiers over conventional voltage-feedback
amplifiers. Therefore, once a frequency response is found suitable to a particular application, adjust the value of
the gain resistor to increase or decrease the overall amplifier gain.
Finally, it is important to realize the effects of the feedback resistance on distortion. Increasing the resistance
decreases the loop gain and increases the distortion. It is also important to know that decreasing load impedance
increases total harmonic distortion (THD). Typically, the third-order harmonic distortion increases more than the
second-order harmonic distortion.
Table 1. Recommended Feedback Resistor Values for
25-Ω Loads
GAIN
1
2, –1
5
7.8
10
R_f
1.3 kΩ
1.1 kΩ
820 kΩ
680 kΩ
510 kΩ
SHUTDOWN CONTROL
There are two shutdown pins that control the shutdown for each amplifier of the THS6032. When the shutdown
pin signals are low, the THS6032 is active. But, when a shutdown pin is high (≥2 V), the corresponding amplifier
is turned off. The shutdown logic is not latched, and should always have a signal applied to it. To help ensure a
fixed logic state, an internal 50-kΩ resistor to DGND is utilized. An external resistor, such as a 3.3 kΩ, to DGND
may be added to help improve noise immunity in harsh environments. If no external resistor is used and SHDN_X
pins are left unconnected, the THS6032 defaults to a power-on state. A simplified circuit is shown in Figure 43.
+V_CC(H)
To Internal
Bias Circuitry
Control
SHDN_X
50 kΩ
DGND
DGND
−V_CC(H)
Figure 43. Simplified THS6032 Shutdown Control Circuit
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