THS6022 Datasheet, PDF (24/38 Pages) Texas Instruments – 250-mA DUAL DIFFERENTIAL LINE DRIVER

THS6022
250-mA DUAL DIFFERENTIAL LINE DRIVER
SLOS225C – SEPTEMBER 1998 – REVISED JANUARY 2000
APPLICATION INFORMATION
recommended feedback and gain resistor values
As with all current feedback amplifiers, the bandwidth of the THS6022 is an inversely proportional function of
the value of the feedback resistor. This can be seen from Figures 19 to 32. The recommended resistors for the
optimum frequency response are shown in Table 1. These should be used as a starting point and once optimum
values are found, 1% tolerance resistors should be used to maintain frequency response characteristics.
Because there is a finite amount of output resistance of the operational amplifier, load resistance can play a
major part in frequency response. This is especially true with these drivers, which tend to drive low-impedance
loads. This can be seen in Figure 10 and Figures 25 – 28. As the load resistance increases, the output resistance
of the amplifier becomes less dominant at high frequencies. To compensate for this, the feedback resistor
should change. Although, for most applications, a feedback resistor value of 1 kΩ is recommended, which is
a good compromise between bandwidth and phase margin that yields a very stable amplifier.
Table 1. Recommended Feedback (R_F) Values for Optium Frequency Response
GAIN
1
2
–1
VCC = ± 15 V
RL = 50 Ω
RL = 100 Ω
787 Ω
750 Ω
590 Ω
590 Ω
560 Ω
—
RL = 25 Ω
1 kΩ
820 Ω
—
VCC = ± 15 V
RL = 50 Ω
910 Ω
715 Ω
680 Ω
RL = 100 Ω
820 Ω
680 Ω
—
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 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. This is illustrated in Figure 40.
offset voltage
The output offset voltage, (V_OO) is the sum of the input offset voltage (V_IO) and both input bias currents (I_IB) times
the corresponding gains. The following schematic and formula can be used to calculate the output offset
voltage:
RF
IIB–
RG
+
–
VI
+
RS
VO
ǒ ǒ ǓǓ ǒ ǒ ǓǓ + ) " ₎ ) " VOO VIO 1
RF
RG
IIB RS 1
RF
RG
IIB– RF
IIB+
Figure 50. Output Offset Voltage Model
24
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265