THS4541_16 Datasheet, PDF (40/61 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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After being designed, this circuit can also be ac coupled by adding blocking caps in series with the two 120-Ω Rg
resistors. This active input impedance has the advantage of increasing the apparent load to the prior stage using
lower resistors values, leading to lower output noise for a given gain target.
9.4.2 Differential-Input to Differential-Output Operation
In many ways, this method is a much simpler way to operate the FDA from a design equations perspective.
Again, assuming the two sides of the circuit are balanced with equal Rf and Rg elements, the differential input
impedance is now just the sum of the two Rg elements to a differential inverting summing junction. In these
designs, the input common-mode voltage at the summing junctions does not move with the signal, but must be
dc biased in the allowable range for the input pins with consideration given to the voltage headroom required
from each supply. Slightly different considerations apply to ac- or dc-coupled, differential-in to differential-out
designs, as described in the following sections.
9.4.2.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
There are two typical ways to use the THS4541 with an ac-coupled differential source. In the first method, the
source is differential and can be coupled in through two blocking capacitors. The second method uses either a
single-ended or a differential source and couples in through a transformer (or balun). Figure 73 shows a typical
blocking capacitor approach to a differential input. An optional input differential termination resistor (Rm) is
included in this design. This Rm element allows the input Rg resistors to be scaled up while still delivering lower
differential input impedance to the source. In this example, the Rg elements sum to show a 200-Ω differential
impedance, while the Rm element combines in parallel to give a net 100-Ω, ac-coupled, differential impedance to
the source. Again, the design proceeds ideally by selecting the Rf element values, then the Rg to set the
differential gain, then an Rm element (if needed) to achieve a target input impedance. Alternatively, the Rm
element can be eliminated, the Rg elements set to the desired input impedance, and Rf set to the get the
differential gain (= Rf / Rg).
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
402
C1
100 nF
Rg1
100
Downconverter
Differential
Output
C2
100 nF
Vocm
Rm
200
Rg2
100
Vcc
±
+
FDA
±
+
PD
Vcc
Rf2
402
R1
500
Output
Measurement
Point
Figure 73. Example Down-Converting Mixer Delivering an AC-Coupled Differential Signal to the THS4541
The dc biasing here is very simple. The output Vocm is set by the input control voltage and, because there is no
dc current path for the output common-mode voltage, that dc bias also sets the input pins common-mode
operating points.
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