THS4521 datasheet(41/66 Pages) TI2

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8.4.2 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.

8.4.2.1 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 8-2 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).

Rf1

402

500

Rg1

100

Rg2

100

Output

Measurement

Point

THS4541 Wideband,

Fully-Differential Amplifier

Rf2

402

100 nF

Downconverter

Differential

Output

200

Vcc

Vocm

Vcc

FDA

Figure 8-2. 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.

Transformer input coupling allows either a single-ended or differential source to be coupled into the THS4541;

possibly also improving the input-referred noise figure. These designs assume a source impedance that must be

matched in the balun interface. The simplest approach is shown in Figure 8-3, where an example 1:2 turns ratio

step-up transformer is used from a 50-Ω source.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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