THS6012 Datasheet, PDF (22/35 Pages) Texas Instruments – 500-mA DUAL DIFFERENTIAL LINE DRIVER

THS6012
500-mA DUAL DIFFERENTIAL LINE DRIVER
SLOS226C– SEPTEMBER 1998 – REVISED FEBRUARY 2000
APPLICATION INFORMATION
recommended feedback and gain resistor values
As with all current feedback amplifiers, the bandwidth of the THS6012 is an inversely proportional function of
the value of the feedback resistor. This can be seen from Figures 17 – 20. The recommended resistors with a
±15 V power supply for the optimum frequency response with a 25-Ω load system are 680-Ω for a gain = 1 and
620-Ω for a gain = 2 or –1. Additionally, using a ±5 V power supply, it is recommended that a 1-kΩ feedback
resistor be used for a gain of 1 and a 820-Ω feedback resistor be used for a gain of 2 or –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 11, Figure 23, and Figure 24. 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. For 100-Ω loads, it is recommended that the
feedback resistor be changed to 820 Ω for a gain of 1 and 560 Ω for a gain of 2 or –1. 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.
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.
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
+
VO
RS
ǒ ǒ ǓǓ ǒ ǒ ǓǓ ^IIB+
+ ) VOO VIO 1
RF
RG
" ₎ IIB RS
)1
RF
RG
" IIB– RF
Figure 40. Output Offset Voltage Model
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