THS4532_16 Datasheet, PDF (44/58 Pages) Texas Instruments – Ultra Low Power, Rail-to-Rail Output, Fully-Differential Amplifier

THS4532
SLOS829A – FEBRUARY 2013 – REVISED JULY 2015
www.ti.com
Systems Examples (continued)
9.3.3.1 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 THS5432 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 98), 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.
50-Input Match,
Gain of 2 V/V from Rt,
Single-Ended Source to
Differential Output
Wideband,
Fully-Differential Amplifier
Rf1
1.02 kΩ
50-Ω
Source
C1
Rg1
100 nF _{499 Ω}
Rt
52.3 Ω
Vocm
Rg2
523 Ω
Vcc
–
+
FDA _–
+
PD
Vcc
Rload
500 Ω
Output
Measurement
Point
C2
100 nF
Rf2
1.02 kΩ
Figure 98. AC-Coupled, Single-Ended Source to a Differential Gain of 2 V/V Test Circuit
44
Submit Documentation Feedback
Product Folder Links: THS4532
Copyright © 2013–2015, Texas Instruments Incorporated