Analog Tip New Approach for Narrowband Matching of Fast RF ADCs

With a little simulation to help you and a few simple calculations, you can get on with developing your next high-speed RF converter in no time at all.

The development of narrowband matching for A/D converters can be particularly challenging in systems in which high intermediate frequencies are digitized for further processing. The following article outlines how the performance of an ADC can be maximized without having to invest an excessive amount of time in simulations.

The first step is to understand the properties of the analog input of the selected A/D converter. The parallel R||C value or the S parameters of the device can be found in the data sheet. For the ADC of type ADC3669 used here as an example, an R value of 100 Ω and a capacitance of 1.85 pF are specified.

The next step is to find the right transformer or balun for the A/D converter. In this case, the choice fell on the TCM2-33WX+ balun from Mini-Circuits with an impedance ratio of 1:2 and a bandwidth of 3 GHz, which proved to be the perfect complement for the ADC3669, as it covers the full input range of the A/D converter with a relatively low drive signal. Figure 1 shows the input interface.

RCL Adjustment: Determine Resistance Value

For the RCL adjustment, the resistance value of the front end must first be determined. The frequency-dependent losses and the parasitic effects of the balun must be taken into account. In order to obtain a more accurate R value for the terminating resistor, the RL value of the balun at the center frequency of 940 MHz selected here is used to calculate the characteristic impedance (Z₀) to which the balun must be adapted for optimized signal performance.

The result (36.72 Ω) is used to calculate the necessary terminating resistor on the secondary side. The result is 136.1 Ω. As the A/D converter has a built-in 100 Ω termination, both lines on the secondary side are provided with a 33 Ω resistor.

Determine the Inductance Value

In the next step, the value of the inductance required to compensate for the internal capacitance of the A/D converter must be determined. There are two methods for this. On the one hand, the equivalent circuit of the A/D converter specified in the data sheet can be used to determine the total bundled capacitance at the input of the ADC (in this case 1.85 pF). On the other hand, the S-parameters, which can be found on the ADC3669 website, can be used for this purpose.

Using the first method, an L value of 15.5 nH is obtained for 940 MHz. Using the second, more complex method, on the other hand, gives a result of 18.1 nH. The second variant provides a more precise capacitance value at the frequency of interest (here 1.62 pF). Both have the same order of magnitude and therefore the internal parasitic inductance of the A/D converter and the (layout-related) external parasitic inductance must be taken into account.

If the entire front end is simulated in the ADS simulator using the S parameters of the balun and the A/D converter, the result (Fig. 2) shows a very good reflection coefficient. The determined inductance value of 18 nH at 940 MHz thus results in a good match.

Determine the Value of the Capacity

Finally, to further improve the narrowband matching, the value of a capacitor is determined which, when connected in parallel with the inductor, results in a resonant circuit. It may seem absurd at first to add another capacitance after the internal capacitance of the A/D converter has already been compensated with the inductance, but this additional capacitance actually improves the matching.

It can be seen that narrowing the bandwidth adjustment to 350 MHz by selecting a larger external C value leads to better results. By varying the L and/or C value, the bandwidth can be precisely adapted to the respective application. (kr)

Rob Reeder is an application engineer for high-speed data converters at Texas Instruments.