THS6007 Datasheet, PDF (25/36 Pages) Texas Instruments – DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS

THS6007
DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS
APPLICATION INFORMATION
SLOS334– DECEMBER 2000
recommended feedback and gain resistor values
As with all current-feedback amplifiers, the bandwidth of the THS6007 drivers is an inversely proportional
function of the value of the feedback resistor. This can be seen from Figures 18 and 19. For the driver, the
recommended resistors for the optimum frequency response for a 25-Ω load system are 680 Ω for a gain = 1
and 620 Ω for a gain = 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 the drivers, which tend to drive low-impedance loads. This can be seen
in Figure 12, Figure 24, and Figure 25. 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 independent 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.
The receivers of the THS6007 are voltage feedback amplifiers (VFB). Therefore the amplifiers follow the
classical amplifier use of a gain-bandwidth-product. As gain increases, the bandwidth (–3 dB) decreases
accordingly. There are no limitations on using capacitors within the feedback loop of VFB amplifier circuits.
Figures 56 through 67 show the effects of feedback resistance and gain versus frequency. Using these graphs
as a reference point is highly recommended.
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 81. Output Offset Voltage Model
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