THS4551 Datasheet, PDF (30/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
www.ti.com
With the output headrooms confirmed, the input junctions must also stay within the operating range. The input
range limitations only appear when approaching the positive supply where a maximum 1.3-V headroom is
required over the full temperature range because the input range extends to the negative supply voltage over the
full temperature range.
The input pins operate at voltages set by the external circuit design, the required output V_OCM, and the input
signal characteristics. For differential-to-differential designs where there is no signal-related movement in the
input V_ICM voltages, ac-coupled differential input designs have a V_ICM equal to the output V_OCM. The maximum
input operating point is always within range if the output common-mode voltage is in range for ac-coupled input
designs because the input V_ICM requires approximately a 1.3-V headroom to the positive supply, but the
common-mode loop requires a 1.5-V headroom. This value is well within the 0-V minimum headroom on the
input V_ICM because the lower limit on the output V_OCM is approximately 0.55 V to the negative supply over the full
temperature range.
DC-coupled differential input designs must check the voltage divider from the source common-mode input
voltage to the THS4551 V_OCM setting. This result must be equal to an input V_ICM within the specified range. If the
source V_CM can vary over some voltage range, validate this result over that range before proceeding.
For single-ended input to differential output designs, the V_ICM is nominally at a voltage set by the external
configuration with a small swing around the nominal value because of the common-mode loop. An ac-coupled,
single-ended input to differential output design places an average input V_ICM equal to the output V_OCM for the
FDA with an ac-coupled swing around the V_OCM voltage following the input voltage. A dc-coupled, single-ended
input to differential design gets a nominal input V_ICM set by the source signal common-mode level and the V_OCM
output voltage with a small signal-related swing around the nominal V_ICM voltage.
One approach to deriving the V_ICM voltage range for any single-ended input to differential output design is to
observe the voltage swing on the non-signal input side of the FDA outputs and simply take the voltage division
on the input pin to ground or to the dc reference used on that side. An example analysis is shown in Figure 70
using a Thevenized version of the gain of 2 values listed in Table 1 for a 50-Ω matched impedance, ac-coupled
design.
In this section, a single 3.3-V supply is used with the V_OCM defaulted to midsupply or 1.65 V as a common-mode
output voltage. This value is also the common-mode voltage on the input pins for the ac-coupled input to the
FDA. Targeting a 4-V_PP differential output swing means each output pin swings ±1 V around this 1.65-V
common-mode voltage. This output swing is in range because the full swing is 0.65 V to 2.65 V relative to
ground, which is well within the 0.2-V output headroom requirements on a single 3.3-V supply.
THS4551 Wideband,
Fully Differential Amplifier _RF1
1.02 k
VS+
VS-
Thevinized
Source
RS1
25.6
+
3.3 V
±
+
± ^{0 V VIN = ±1.022 V}
1 µF
1 µF
RG1
499
1 µF
VS+
VOCM
±
+
FDA
±
+
PD
VS- ^VS+
VOUT = 4-VPP
Differential
RL
1k
RG2
523
RF2
1.02 k
Copyright © 2016, Texas Instruments Incorporated
Figure 70. Input Swing Analysis Circuit with AC-Coupled, Single-Ended to Differential Signal Path
The output on the lower side of this design ranges from 0.65 V to 2.65 V. This 2-V_PP swing (on just one side, the
other output is an inverted version and gives the 4-V_PP differential maximum) is divided back by the R_F2 and R_G2
divider to the input pins to form a common-mode input swing on top of the 1.65-V input common-mode voltage.
This divider is 0.339 × 2 V_PP = 0.678 V_PP or ±0.34 V around the 1.65-V input common-mode voltage. The 1.31-V
to 1.99-V common-mode input swing for this design is in range for the 0 V to 2.2 V available input range (the
maximum headroom is 3.3 V – 1.1 V, which is equal to 2.2 V). Shifting the V_OCM down slightly (if allowed by the
design requirements) is a good way to improve the positive-swinging input headroom for this low-voltage design.
30
Submit Documentation Feedback
Product Folder Links: THS4551
Copyright © 2016, Texas Instruments Incorporated