THS6182_08 Datasheet, PDF (17/35 Pages) Texas Instruments – LOW-POWER DISSIPATION ADSL LINE DRIVER

THS6182
www.ti.com
SLLS544H – SEPTEMBER 2002 – REVISED JUNE 2007
APPLICATION INFORMATION (continued)
RECOMMENDED FEEDBACK AND GAIN RESISTOR VALUES
As with all current feedback amplifiers, the bandwidth of the THS6182 is an inversely proportional function of the
value of the feedback resistor. The recommended resistors with a ±12-V power supply for the optimum
frequency response with a 25-Ω load system is 1 kΩ for a gain of 5. 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.
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.
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.
Finally, in a differential configuration as shown in Figure 1, it is important to note that there is a differential gain
and a common-mode gain which are different from each other. Differentially, the gain is at 1 + R_F/R_G. While
common-mode gain = 1 due to R_G being connected directly between each amplifier and not to ground. This can
lead to potential problems as the stability of the amplifier is determined by R_F. Thus, R_F must be large enough to
ensure the common-mode stability, even though a large differential gain may be required.
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:
Figure 70. Output Offset Voltage Model
NOISE CALCULATIONS
Noise can cause errors on very small signals. This is especially true for the amplifying small signals. The noise
model for current feedback amplifiers (CFB) is the same as voltage feedback amplifiers (VFB). The only
difference between the two is that the CFB amplifiers generally specify different current noise parameters for
each input while VFB amplifiers usually only specify one noise current parameter. The noise model is shown in
Figure 71. This model includes all of the noise sources as follows:
• e_n = Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN- = Inverting current noise (pA/√Hz)
• e_RX = Thermal voltage noise associated with each resistor (e_RX = 4 kTR_x)
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