THS4531A_16 Datasheet, PDF (41/59 Pages) Texas Instruments – Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

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THS4531A
SLOS823C – DECEMBER 2012 – REVISED JANUARY 2016
9.2.6 Single-Ended Input to Differential Output Amplifier
The THS4531A can also be used to amplify and convert single-ended input signals to differential output signals.
A basic block diagram of the circuit is shown in Figure 95 (V_OCM and PD inputs not shown). The gain of the
circuit is again set by R_F divided by R_G.
R_G
R_F
V_IN+
V_OUT±
Single-Ended
Input
V_S+
Differential
Output
+
THS4531A
±
V_S±
R_G
R_F
V_OUT+
Figure 95. Single-Ended Input to Differential Output Amplifier
9.2.6.1 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 THS4531A 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 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 and
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 nonsignal input side
are DC-blocked (see Figure 96), 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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