THS4541-Q1 Datasheet, PDF (36/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
www.ti.com
9.4 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 (Vs–) + 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 Vocm control pin sets the output average voltage. Left open, Vocm 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 Vcm
error amplifier.
9.4.1 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 Rg 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.
9.4.1.1 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 THS4541-Q1 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 Vocm
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 Vocm. When an AC-
coupled path follows the output pins, the best linearity is achieved by operating Vocm at midsupply. The Vocm
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 Vocm control pin float is usually preferred in order to get a midsupply
default Vocm bias with minimal elements. To limit noise, place a 0.1-µF decoupling capacitor on the Vocm pin to
ground.
After Vocm is defined, check the target output voltage swing to ensure that the Vocm plus the positive or
negative output swing on each side does not clip into the supplies. If the desired output differential swing is
defined as Vopp, divide by 4 to obtain the ±Vp swing around Vocm at each of the two output pins (each pin
operates 180° out of phase with the other). Check that Vocm ±Vp 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 61), 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 Vocm voltages. Because the input Vicm is the output Vocm 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 Vocm. Also, the input signal moves this input Vicm around the DC bias point, as
described in the Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA section.
9.4.1.2 DC-Coupled Input Signal Path Considerations for Single-Ended to Differential Conversion
The output considerations remain the same as for the AC-coupled design. Again, the input can be DC coupled
while the output is AC coupled. A DC-coupled input with an AC-coupled output might have some advantages to
move the input Vicm down if the source is ground referenced. When the source is DC coupled into the THS4541-
Q1 (see Figure 63), both sides of the input circuit must be DC coupled to retain differential balance. Normally, the
nonsignal input side has an Rg element biased to whatever the source midrange is expected to be. Providing this
midscale reference gives a balanced differential swing around Vocm at the outputs. Often, Rg2 is simply
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