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7.3 I/O Headroom Considerations

The starting point for most designs is usually to assign an output common-mode voltage. For ac-coupled signal

paths, this voltage is often the default mid-supply voltage to retain the most available output swing around the

centered Vocm. For dc-coupled designs, set this voltage with consideration for the required minimum headroom

to the supplies shown in the specifications for the Vocm control. From the target output Vocm, the next step is to

verify that the desired output differential VPP stays within the supplies. For any desired differential Vopp, check

that the absolute maximum output pin swings with Equation 2 and Equation 3, and confirm the differential Vopp

are within the supply rails for this rail-to-rail (RR) output device.

min

Vopp

Vocm

(2)

max

Vopp

Vocm

(3)

For instance, driving the ADC3223 with a 0.95 Vcm control using a single 3.3 V supply, the maximum output

swing is set by the negative-going signal from 0.95 Vcm to +0.2 V above ground. This 0.75 V, single-sided swing

becomes an available 4 × 0.75 V = 3 VPP differential around the nominal 0.95 Vcm output common mode. On

the high side, the maximum output is 0.95 + 0.75 = 1.7 V. This result is well within the allowed maximum of 3.3

V – 0.2 V = 3.1 V. This 3 VPP is also well beyond the maximum required 2-VPP full-scale differential input for this

ADC. However, having this extra swing range is useful if an interstage filter to the ADC adds insertion loss.

With the output headrooms confirmed, the input junctions must also stay within the operating range. The input

range extends to the negative supply voltage (over the full temperature range); therefore, input range limitations

usually appear only approaching the positive supply, where a maximum 1.3 V headroom is required over the full

temperature range.

The input pins operate at voltages set by the external circuit design, the required output Vocm, and the input

signal characteristics. For differential-to-differential designs where the input Vicm voltage does not move with the

input signal, there are two configurations to consider:

• AC-coupled, differential-input designs have a Vicm equal to the output Vocm. The input Vicm requires

approximately a 1.3 V headroom to the positive supply; therefore, the maximum Vocm to that value reduces

from the Vocm positive headroom requirement of 1.2 V to the 1.3 V required on the input pins. The lower limit

on the output Vocm is approximately 0.95 V to the negative supply over the full temperature range, and well

within the 0-V minimum headroom on the input Vicm.

• DC-coupled, differential-input designs, check the voltage divider from the source Vcm to the THS4541 Vocm

setting to confirm the resulting voltage divider solves to an input Vicm within the allowed range. If the source

Vcm can vary over some voltage range, this result must be validated over that range.

For single-ended input to differential output designs, there is a dc Vicm voltage set by the external configuration

with a small-signal related swing around that. The two conditions to consider are:

• AC-coupled, single-ended input to differential designs place an average input Vicm equal to the output Vocm

voltage with an ac-coupled swing around that Vocm following the input voltage.

• DC-coupled, single-ended input to differential designs get a nominal input Vicm set by the source-signal

common mode and the output Vocm setting with a small, signal-related swing around the dc Vicm level set by

the voltage divider.

One method of deriving the voltage range for Vicm for any single-ended input to differential output design is

to determine the voltage swing on the nonsignal-input side of the FDA outputs and simply take the respective

divider back to the input pin to ground or the dc reference used on that side. An example analysis is shown in

Figure 7-8, where the circuit of Figure 7-1 is simplified to show just a Thevenized source impedance.

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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