THS4521 Datasheet, PDF (34/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
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8.4 Device Functional Modes
This wideband FDA requires external resistors for correct signal-path operation. When configured for the desired
input impedance and gain setting with these external resistors, the amplifier can be either on with the PD pin
asserted to a voltage greater than V_S– + 1.7 V, or turned off by asserting PD low. Disabling the amplifier shuts off
the quiescent current and stops correct amplifier operation. The signal path is still present for the source signal
through the external resistors.
The V_OCM control pin sets the output average voltage. Left open, V_OCM defaults to an internal midsupply value.
Driving this high-impedance input with a voltage reference within its valid range sets a target for the internal V_CM
error amplifier.
8.4.1 Operation from Single-Ended Sources to Differential Outputs
One of the most useful features supported by the FDA device is an easy conversion from a single-ended input to
a differential output centered on a user-controlled, common-mode level. While the output side is relatively
straightforward, the device input pins move in a common-mode sense with the input signal. This common-mode
voltage at the input pins moving with the input signal acts to increase the apparent input impedance to be greater
than the R_G value. This input-active-impedance issue applies to both ac- and dc-coupled designs, and requires
somewhat more complex solutions for the resistors to account for this active impedance, as shown in the
following subsections.
8.4.1.1 AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output Conversion
When the signal path can be ac-coupled, the dc biasing for the THS452x family becomes a relatively simple task.
In all designs, start by defining the output common-mode voltage. The ac-coupling issue can be separated for the
input and output sides of an FDA design. The input can be ac-coupled and the output dc-coupled, or the output
can be ac-coupled and the input dc-coupled, or they can both be ac-coupled.
One situation where the output might be dc-coupled (for an ac-coupled input), is when driving directly into an
ADC where the V_OCM control voltage uses the ADC common-mode reference to directly bias the FDA output
common-mode to the required ADC input common-mode. In any case, the design starts by setting the desired
V_OCM.
When an ac-coupled path follows the output pins, the best linearity is achieved by operating V_OCM at midsupply.
The V_OCM voltage must be within the linear range for the common-mode loop, as specified in the headroom
specifications (approximately 0.91 V greater than the negative supply and 1.1 V less than the positive supply). If
the output path is also ac-coupled, simply letting the V_OCM control pin float is usually preferred in order to get a
midsupply default V_OCM bias with minimal elements. To limit noise, place a 0.1-µF decoupling capacitor on the
V_OCM pin to ground.
After V_OCM is defined, check the target output voltage swing to ensure that the V_OCM plus the positive or negative
output swing on each side do not clip into the supplies. If the desired output differential swing is defined as V_OPP,
divide by 4 to obtain the ±V_P swing around V_OCM at each of the two output pins (each pin operates 180° out of
phase with the other). Check that V_OCM ±V_P does not exceed the absolute supply rails for this rail-to-rail output
(RRO) device.
Going to the device input pins side, because both the source and balancing resistor on the non-signal input side
are dc-blocked (see Figure 74), no common-mode current flows from the output common-mode voltage, thus
setting the input common-mode equal to the output common-mode voltage.
This input headroom also sets a limit for higher V_OCM voltages. Because the input V_ICM is the output V_OCM for ac-
coupled sources, the 1.2-V minimum headroom for the input pins to the positive supply overrides the 1.1-V
headroom limit for the output V_OCM. Also, the input signal moves this input V_ICM around the dc bias point, as
described in the section Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA.
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