THS4541-Q1 Datasheet, PDF (49/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

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THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
11 Power-Supply Recommendations
The THS4541-Q1 is principally intended to operate with a nominal single-supply voltage of +3 V to +5 V. Supply-
voltage tolerances are supported with the specified operating range of 2.7 V (10% low on a 3-V nominal supply)
and 5.4 V (8% high on a 5-V nominal supply). Supply decoupling is required, as described in the Terminology
and Application Assumptions section. Split (or bipolar) supplies can be used with the THS4541-Q1, as long as
the total value across the device remains less than 5.5 V (absolute maximum). The thermal pad on the package
is electrically isolated; connect the thermal pad to any power or ground plane for heat spreading.
Using a negative supply to deliver a true swing to ground output in driving SAR ADCs may be desired. While the
THS4541-Q1 quotes a rail-to-rail output, linear operation requires approximately a 200-mV headroom to the
supply rails. One easy option for extending the linear output swing to ground is to provide the small negative
supply voltage required using the LM7705 fixed –230-mV, negative-supply generator. This low-cost, fixed
negative-supply generator accepts the 3-V to 5-V positive supply input used by the THS4541-Q1 and provides a
–230-mV supply for the negative rail. Using the LM7705 provides an effective solution, as shown in the TI
Designs TIDU187, Extending Rail-to-Rail Output Range for Fully Differential Amplifiers to Include True Zero
Volts.
12 Layout
12.1 Layout Guidelines
Similar to all high-speed devices, best system performance is achieved with a close attention to board layout.
The THS4541-Q1 evaluation module (EVM) shows a good example of high frequency layout techniques as a
reference. This EVM includes numerous extra elements and features for characterization purposes that may not
apply to some applications. General high-speed, signal-path layout suggestions include:
• Continuous ground planes are preferred for signal routing with matched impedance traces for longer runs;
however, open up both ground and power planes around the capacitive sensitive input and output device
pins. After the signal is sent into a resistor, parasitic capacitance becomes more of a bandlimiting issue and
less of a stability issue.
• Use good, high-frequency decoupling capacitors (0.1 µF) on the ground plane at the device power pins.
Higher value capacitors (2.2 µF) are required, but may be placed further from the device power pins and
shared among devices. For best high-frequency decoupling, consider X2Y supply-decoupling capacitors that
offer a much higher self-resonance frequency over standard capacitors.
• When using differential signal routing over any appreciable distance, use microstrip layout techniques with
matched impedance traces.
• Higher-speed FDAs, such as the THS4541-Q1, include a duplicate of the output pins on the input feedback
side of the package. This duplication is intended to allow the external feedback resistors to be connected with
virtually no trace length on the input side of the package. Use this layout approach with no extra trace length
on this critical feedback path.
• The input summing junctions are very sensitive to parasitic capacitance. Connect any Rg elements into the
summing junction with minimal trace length to the device pin side of the resistor. The other side of the Rg
elements can have more trace length if needed to the source or to ground.
Copyright © 2015, Texas Instruments Incorporated
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