THS6022_16 Datasheet, PDF (32/44 Pages) Texas Instruments – 250-mA DUAL DIFFERENTIAL LINE DRIVER

THS6022
SLOS225D – SEPTEMBER 1998 – REVISED JULY 2007
www.ti.com
The THS6022 has been specifically designed for ultralow distortion by careful circuit implementation and by
taking advantage of the superb characteristics of the complementary bipolar process. Driver single-ended
distortion measurements are shown in Figure 37 through Figure 40. It is commonly known that in the differential
driver configuration, the second-order harmonics tend to cancel out. Thus, the dominant total harmonic distortion
(THD) is primarily due to the third-order harmonics. Additionally, distortion should be reduced as the feedback
resistance drops. This is because the bandwidth of the amplifier increases, which allows the amplifier to react
faster to any nonlinearities in the closed-loop system.
Another significant point is the fact that distortion decreases as the impedance load increases. This is because
the output resistance of the amplifier becomes less significant as compared to the output load resistance. This is
illustrated in Figure 40.
One problem that has been receiving a lot of attention in the ADSL area is power dissipation. One way to
substantially reduce power dissipation is to lower the power supply voltages. This is because the RMS voltage of
an ADSL remote terminal signal is 1.35-V RMS. But to meet ADSL requirements, the drivers must have a
voltage RMS-to-peak crest factor of 5.6 in order to keep the bit-error probability rate below 10^–7. Hence, the
power supply voltages must be high enough to accomplish the peak output voltage of 1.35 V × 5.6 = 7.6
V(PEAK). If ±15-V power supplies are used for the THS6022 drivers in the circuit shown in Figure 61, the power
dissipation of the THS6022 is approximately 600 mW. This is assuming that part of the quiescent current is
diverted back to the load, which typically happens in a class-AB amplifier. But if the power supplies are dropped
down to ±12 V, then the power dissipation drops to approximately 460 mW. This is a 23% reduction of power,
which ultimately lowers the temperature of the drivers and increases efficiency.
Another way to reduce power dissipation in the drivers is to increase the transformer ratio. The drawback in
doing this is that it increases the loading on the drivers and reduces the signals being received from the central
office. If this can be overcome, then a power reduction in the drivers results. By going to a 1:2 transformer ratio,
the power supply voltages can drop to ±6 V. The driver output voltage has now been reduced to 675 mV RMS.
But the loading on the output of the drivers drops to 25 Ω. The power dissipated is now approximately 360 mW,
a reduction of 22% over the previous example. But, the received signal is now 1/2 of the previous example. This
must be dealt with by requiring low-noise receivers. There are always trade offs when it comes to dealing with
power, so proper analysis of the system should always be considered.
General Configurations
A common error for the first-time CFB user is to create a unity-gain buffer amplifier by shorting the output
directly to the inverting input. A CFB amplifier in this configuration oscillates and is not recommended. The
THS6022, like all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing
capacitors directly from the output to the inverting input is not recommended. This is because, at high
frequencies, a capacitor has a very low impedance. This results in an unstable amplifier and should not be
considered when using a current-feedback amplifier. Because of this, integrators and simple low-pass filters,
which are easily implemented on a VFB amplifier, must be designed slightly differently. If filtering is required,
simply place an RC-filter at the noninverting terminal of the operational-amplifier (see Figure 62).
R_G
R_F
V_I
R1
–
V_O
+
C1
f_-3dB
=
1
2pR1C1
V_O
V_I
æ
= ççè1+
R_F
R_G
öæ _{1 ö}
÷÷ø _{çè 1+ sR1C1÷ø}
S0281-01
Figure 61. Single-Pole Low-Pass Filter
32
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