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9.4.2.1 AC-Coupled Signal Path Considerations for Single-Ended Input to Differential Output

Conversions

When the signal path can be AC-coupled, the DC biasing for the THS4561 becomes a relatively simple task. In

all designs, start by defining the output common-mode voltage. The AC-coupling requirement 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 both can be AC-coupled. One situation where the output

can 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 voltage to the

required ADC input common-mode voltage. 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, which

can be easily delivered by floating the VOCM pin. The VOCM voltage must be within the linear range for the

common-mode loop, as specified in the headroom specifications. If the output path is also AC-coupled, simply

letting the VOCM control pin float is usually preferred in order to obtain a midsupply default VOCM bias with

minimal elements. To limit noise, place a 0.1-µF decoupling capacitor on the VOCM control pin to ground.

After VOCM is defined, check the target output voltage swing to make certain that the VOCM plus the positive and

negative output swing on each side does not clip into the supplies. If the desired peak-to-peak output differential

swing is defined as VOPP, divide by 4 to obtain the ±VP (peak voltage) 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 the rail-to-rail output (RRO) device. Common-mode current does not flow from the

common-mode output voltage set by the VOCM pin towards the device input pins side, because both the source

and balancing resistor on the non-signal input side are DC blocked. The AC-coupled input path sets the input pin

common-mode voltage equal to the output common-mode voltage. If the VOCM voltage is within the headroom

requirement, then the input pins are also in range for the AC-coupled input configuration. This headroom

requirement functions similarly for when the VOCM voltage approaches the negative supply.

The input pin voltages move in a common-mode manner with the input signal. Confirm that the VOCM voltage

plus the input VPP common-mode swing also stays in the VICM specification range for the input pins.

9.4.2.2 DC-Coupled Input Signal Path Considerations for Single-Ended to Differential Conversions

The output considerations remain the same as for the AC-coupled design. Again, the input can be DC-coupled

when the output is ac coupled. A DC-coupled input with an AC-coupled output can have some advantages to

move the input VICM down by adjusting the VOCM down if the source is ground referenced. When the source is

DC-coupled into the THS4561 (see Figure 10-3), both sides of the input circuit must be DC-coupled to retain

differential balance. Normally, the non-signal input side has an RG element biased to whatever the source

midrange is expected to be, provided that this midscale reference gives a balanced differential swing around

VOCM at the outputs. Often, RG2 is simply grounded for DC-coupled, bipolar-input applications. This configuration

provides a balanced differential output if the source swings around ground. If the source swings from ground to

some positive voltage, grounding RG2 gives a unipolar output differential swing from both outputs at VOCM (when

the input is at ground) to one polarity of the swing. Biasing RG2 to an expected midpoint for the input signal

creates a differential output swing around VOCM.

One significant consideration for a DC-coupled input is that VOCM sets up a common-mode bias current from

the output back through RF and RG to the source on both sides of the feedback. Without input balancing

networks, the source must sink or source this dc current. After the input signal range and biasing on the other RG

element is set, check that the voltage divider from VOCM to VIN through RF and RG (and possibly RS) establishes

an input VICM at the device input pins that is within the specification range. If the average source is at ground, the

negative rail input stage for the THS4561 is in range for applications using a single positive supply and a positive

output VOCM setting because this dc common-mode current lifts the average FDA input summing junctions

above ground to a positive voltage (the average of the V+ and V– input pin voltages on the FDA). TINA-TI™

simulations of the intended circuit offer a good check for input and output pin voltage swings.

THS4561

SBOS874D – AUGUST 2017 – REVISED FEBRUARY 2021

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