THS6022 Datasheet, PDF (27/38 Pages) Texas Instruments – 250-mA DUAL DIFFERENTIAL LINE DRIVER

THS6022
250-mA DUAL DIFFERENTIAL LINE DRIVER
SLOS225C – SEPTEMBER 1998 – REVISED JANUARY 2000
slew rate
APPLICATION INFORMATION
The slew rate performance of a current feedback amplifier, like the THS6022, is affected by many different
factors. Some of these factors are external to the device, such as amplifier configuration and PCB parasitics,
and others are internal to the device, such as available currents and node capacitance. Understanding some
of these factors should help the PCB designer arrive at a more optimum circuit with fewer problems.
Whether the THS6022 is used in an inverting amplifier configuration or a noninverting configuration can impact
the output slew rate. Slew rate performance in the inverting configuration is generally faster than the
noninverting configuration. This is because in the inverting configuration the input terminals of the amplifier are
at a virtual ground and do not significantly change voltage as the input changes. Consequently, the time to
charge any capacitance on these input nodes is less than for the noninverting configuration, where the input
nodes actually do change in voltage an amount equal to the size of the input step. In addition, any PCB parasitic
capacitance on the input nodes degrades the slew rate further simply because there is more capacitance to
charge. If the supply voltage (V_CC ) to the amplifier is reduced, slew rate decreases because there is less current
available within the amplifier to charge the capacitance on the input nodes as well as other internal nodes. Also,
as the load resistance decreases, the slew rate typically decreases due to the increasing internal currents, which
slow down the transitions (see Figures 13 and 14)
Internally, the THS6022 has other factors that impact the slew rate. The amplifier’s behavior during the slew rate
transition varies slightly depending upon the rise time of the input. This is because of the way the input stage
handles faster and faster input edges. Slew rates (as measured at the amplifier output) of less than about
1300 V/µs are processed by the input stage in a very linear fashion. Consequently, the output waveform
smoothly transitions between initial and final voltage levels. This is shown in Figure 53. For slew rates greater
than 1300 V/µs, additional slew-enhancing transistors present in the input stage begin to turn on to support
these faster signals. The result is an amplifier with extremely fast slew rate capabilities. Figure 54 shows
waveforms for these faster slew rates. The additional aberrations present in the output waveform with these
faster slewing input signals are due to the brief saturation of the internal current mirrors. This phenomenon,
which typically lasts less than 20 ns, is considered normal operation and is not detrimental to the device in any
way. If for any reason this type of response is not desired, then increasing the feedback resistor or slowing down
the input signal slew rate reduces the effect.
SLEW RATE — SATURATION
16
SLEW RATE — LINEAR
16
12
12
8
8
4
4
0
0
–4
SR = 3500 V/µs
VCC = ±15 V
–8
Gain = 5
RL = 1 kΩ
–12
RF = 50 Ω
tr/tf = 900 ns
–16
0 10 20 30 40 50 60 70 80 90 100
t – Time – ns
Figure 53
–4
SR ≅ 1300 V/µs
VCC = ±15 V
–8
Gain = 5
RF = 1 kΩ
–12
RL = 50 Ω
tr/tf = 10 ns
–16
0 10 20 30 40 50 60 70 80 90 100
t – Time – ns
Figure 54
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