THS4541_15 Datasheet, PDF (34/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
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Overview (continued)
the device and may be shared among several devices in the same area of the PCB.
• Although not always stated, make sure to tie the power disable pin to the positive supply when only an
enabled channel is desired.
• Virtually all ac characterization equipment expects a 50-Ω termination from the 50-Ω source, and a 50-Ω
single-ended source impedance from the device outputs to the 50-Ω sensing termination. This termination is
achieved in all characterizations (often with some insertion loss), but is not necessary for most applications.
Matching impedance is most often required when transmitting over longer distances. Tight layouts from a
source, through the THS4541, and on to an ADC input do not require doubly-terminated lines or filter designs;
the exception is if the source requires a defined termination impedance for correct operation (for example, a
SAW filter source).
• The amplifier signal path is flexible for single or split-supply operation. Most applications are intended to be
single supply, but any split-supply design can be used, as long as the total supply across the TH4541 is less
than 5.5 V and the required input, output, and common-mode pin headrooms to each supply are observed.
Left open, the Vocm pin defaults to near midsupply for any combination of split or single supplies used. The
disable pin is negative-rail referenced. Using a negative supply requires the disable pin to be pulled down to
within 0.7 V of the negative supply to disable the amplifier.
• External element values are normally assumed to be accurate and matched. In an FDA, match the feedback
resistor values and also match the (dc and ac) impedance from the summing junctions to the source on one
side and the reference or ground on the other side. Unbalancing these values introduces nonidealities in the
signal path. For the signal path, imbalanced resistor ratios on the two sides create a common-mode to
differential conversion. Also, mismatched Rf values and feedback ratios create some added differential output
error terms from any common-mode dc, ac signal, or noise terms. Snapping to standard 1% resistor values is
a typical approach and generally leads to some nominal feedback ratio mismatch. Mismatched resistors or
ratios do not in themselves degrade harmonic distortion. If there is meaningful CM noise or distortion coming
in, those errors are converted to a differential error through element or ratio mismatch.
9.2 Functional Block Diagram
(RGT Package) FB+
IN±
IN+
(RGT Package) FB±
Vs+
±
High-Aol +
Differential I/O
Amplifier ±
+
2.5 k
2.5 k
Vs+
100 k
OUT+
OUT±
PD
CMOS
Buffer
±
Vcm
Error
Amplifier
+
Vocm
100 k
Vs±
34
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