THS4541-Q1 Datasheet, PDF (26/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
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8.3 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 midsupply voltage, in order 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 V_PP 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 they are
within the supply rails for this rail-to-rail (RR) output device.
Vo_min
Vocm
Vopp
4
(2)
Vo_max
Vocm
Vopp
4
(3)
For instance, driving the ADC3223 with its 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 V_PP 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 V_PP is also well beyond the maximum required 2-V_PP 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 their 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-Q1
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.
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