9.4.2.1 AC-Coupled, Differential-Input to Differential-Output Design Issues

There are two typical ways to use the THS4541-Q1 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).

Figure 73. Example Down-Converting Mixer Delivering an AC-Coupled Differential Signal to the THS4541-Q1

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-Q1; 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 74, where an example 1:2 turns ratio step-up transformer is used from a 50-Ω source.

Figure 74. Input Balun Interface Delivers a Differential Input to the THS4541-Q1

In this example, this 1:2 turns ratio step-up transformer provides a source and load match from the 50-Ω source if the secondary is terminated in 200 Ω (turns-ratio squared is the impedance ratio across a balun). The two Rg elements provide that termination as they sum to the differential virtual ground at the FDA summing junctions. The input blocking cap (C1) is optional and included only to eliminate DC shorts to ground from the source. This solution often improves the input-referred noise figure more so than just the FDA using this passive (zero power dissipation) input balun. Defining a few ratios allows a noise figure expression to be written as Equation 14:

Equation 14.

where

• n ≡ turns ratio (the ohms ratio is then n²)
• α ≡ differential gain in the FDA = Rf / Rg
• β ≡ transformer insertion loss in V/V (from a dB insertion loss, convert to linear attenuation = β)
• kT = 4e-21J at 290 K (17°C)

One way to use Equation 14 is to fix the input balun selection, and then sweep the FDA gain by stepping up the Rf value. The lowest-noise method uses just the two Rg elements for termination matching (no Rm element, such as in Figure 74) and sweep the Rf values up to assess the resulting input-referred noise figure. While this method can be used with all FDAs and a wide range of input baluns, relatively low-frequency input baluns are an appropriate choice here because the THS4541-Q1 holds exceptional SFDR for less than 40-MHz applications. Two representative selections, with their typical measured spans and resulting model elements, are shown in Table 7. For these two selections, the critical inputs for the noise figures are the turns ratio and the insertion loss (the 0.2 dB for the CX2014LNL becomes a β = 0.977 in the NF expression).

Using the typical input referred noise terms for the THS4541-Q1 (e_ni = 2.2 nV and i_n = 1.9 pA) and sweeping the total gain from the input of the balun to the differential output over a 10-dB to 24-dB span, gives the input noise figure shown in Figure 75.

Figure 75. Noise Figure versus Total Gain with the Two Input Baluns of Table 7

The 50-Ω referred noise figure estimates show a decreasing input-referred noise for either balun as the gain increases through 24 dB. The only elements changing in these sweeps are the feedback-resistor values, in order to achieve the total target gain after the step up from the input balun. The example of Figure 74 is a gain of
7.86 V/V, or a 17.9-dB gain where a 9.0-dB input noise figure is predicted from Figure 75. Another advantage for this method is that the effective noise gain (NG) is reduced by the source impedance appearing as part of the total Rg element in the design. The example of Figure 74 operates with a NG = 1 + 402 / (100 + 100) = 3 V/V, giving greater than 300-MHz SSBW in the THS4541-Q1 portion of the design. Combining that with the 372 MHz in the balun itself gives greater than 200 MHz in this 18-dB gain stage; or an equivalent greater than 1.6-GHz gain bandwidth product in a low-power, high dynamic range interface.

Added features and considerations for the balun input of Figure 74 include:

• Many of these baluns offer a secondary centertap. Leave the centertap unconnected for the best HD2 suppression and DC biasing (do not include a capacitor from this centertap to ground).
• With a floating secondary centertap, the input pins common-mode voltage again equals the output Vocm setting because there is no DC path for the output common-mode voltage to create a common-mode current (I_CM).