THS4521 datasheet(37/66 Pages) TI2

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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 THS4541 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 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 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 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 confirm 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 7-1), 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 Section 8.4.1.3 section.

8.4.1.2 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 can have some advantages to

move the input Vicm down if the source is ground referenced. When the source is dc coupled into the THS4541

(see Figure 7-3), 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

grounded for dc-coupled, bipolar-input applications. This configuration gives a balanced differential output if the

source is swinging 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 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 in range. If the average source is at ground, the negative rail input stage

for the THS4541 is in range for applications using a single positive supply and a positive output Vocm setting

because this dc current lifts the average FDA input summing junctions up off of ground to a positive voltage (the

average of the V+ and V– input pin voltages on the FDA).

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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