THS4500 Datasheet, PDF (26/37 Pages) Texas Instruments – WIDEBAND, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4500
THS4501
SLOS350D − APRIL 2002 − REVISED JANUARY 2004
of an analog-to-digital converter to drive V_OCM. Output
current drive capabilities differ from part to part, so a
voltage buffer may be necessary in some applications.
VOCM
IIN
VS+
R = 50 kΩ
IIN =
2 VOCM − VS+ − VS−
R
R = 50 kΩ
VS−
Equivalent Input Circuit for VOCM
Figure 105
By design, the input signal applied to the V_OCM pin
propagates to the outputs as a common-mode signal. As
shown in the equivalent circuit diagram, the V_OCM input
has a high impedance associated with it, dictated by the
two 50-kΩ resistors. While the high impedance allows for
relaxed drive requirements, it also allows the pin and any
associated printed-circuit board traces to act as an
antenna. For this reason, a decoupling capacitor is
recommended on this node for the sole purpose of filtering
any high frequency noise that could couple into the signal
path through the V_OCM circuitry. A 0.1-µF or 1-µF
capacitance is a reasonable value for eliminating a great
deal of broadband interference, but additional, tuned
decoupling capacitors should be considered if a specific
source of electromagnetic or radio frequency interference
is present elsewhere in the system. Information on the ac
performance (bandwidth, slew rate) of the V_OCM circuitry
is included in the specification table and graph section.
Since the V_OCM pin provides the ability to set an output
common-mode voltage, the ability for increased power
dissipation exists. While this does not pose a performance
problem for the amplifier, it can cause additional power
dissipation of which the system designer should be aware.
The circuit shown in Figure 106 demonstrates an
example of this phenomenon. For a device operating on
a single 5-V supply with an input signal referenced around
ground and an output common-mode voltage of 2.5 V, a
dc potential exists between the outputs and the inputs of
the device. The amplifier sources current into the
feedback network in order to provide the circuit with the
proper operating point. While there are no serious effects
on the circuit performance, the extra power dissipation
may need to be included in the system’s power budget.
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I1 =
VOCM
Rf1+ Rg1 + RS || RT
DC Current Path to Ground
RS
Rg1
Rf1
VS
RT
5V
+−
VOCM = 2.5 V
−+
2.5-V DC
RL
2.5-V DC
Rg2
Rf2
DC Current Path to Ground
I2
=
VOCM
Rf2 + Rg2
Depiction of DC Power Dissipation Caused By
Output Level-Shifting in a DC-Coupled Circuit
Figure 106
SAVING POWER WITH POWER-DOWN
FUNCTIONALITY
The THS4500 family of fully differential amplifiers contains
devices that come with and without the power-down
option. Even-numbered devices have power-down
capability, which is described in detail here.
The power-down pin of the amplifiers defaults to the
positive supply voltage in the absence of an applied
voltage (i.e. an internal pullup resistor is present), putting
the amplifier in the power-on mode of operation. To turn off
the amplifier in an effort to conserve power, the
power-down pin can be driven towards the negative rail.
The threshold voltages for power-on and power-down are
relative to the supply rails and given in the specification
tables. Above the enable threshold voltage, the device is
on. Below the disable threshold voltage, the device is off.
Behavior in between these threshold voltages is not
specified.
Note that this power-down functionality is just that; the
amplifier consumes less power in power-down mode. The
power-down mode is not intended to provide a
high-impedance output. In other words, the power-down
functionality is not intended to allow use as a 3-state bus
driver. When in power-down mode, the impedance looking
back into the output of the amplifier is dominated by the
feedback and gain setting resistors.
The time delays associated with turning the device on and
off are specified as the time it takes for the amplifier to
reach 50% of the nominal quiescent current. The time
delays are on the order of microseconds because the
amplifier moves in and out of the linear mode of operation
in these transitions.
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