9.1.1 Drive Circuit Requirements

For optimum performance, the analog inputs must be driven differentially. This technique improves the common-mode noise immunity and even-order harmonic rejection. A small resistor (5 Ω to 10 Ω) in series with each input pin is recommended to damp out ringing caused by package parasitics.

Figure 83 and Figure 84 show the differential impedance (Z_IN = R_IN || C_IN) between the ADC analog input pins INP and INM. The presence of the analog input buffer results in an almost constant input capacitance up to 1 GHz.

Figure 83. ADC Analog Input Resistance (R_IN) Across Frequency

Figure 84. ADC Analog Input Capacitance (C_IN) Across Frequency

9.1.2 Driving Circuit

Two example driving circuit configurations are shown in Figure 85 and Figure 86—one optimized for low input frequencies and the other optimized for high input frequencies. Notice in both cases that the board circuitry is simplified compared to the non-buffered ADS4149.

In Figure 85, a single transformer is used and is suited for low input frequencies. To optimize even-harmonic performance at high input frequencies (greater than the first Nyquist), the use of back-to-back transformers is recommended (see Figure 86). Note that both drive circuits have been terminated by 50 Ω near the ADC side. The ac-coupling capacitors allow the analog inputs to self-bias around the required common-mode voltage.

Figure 85. Drive Circuit for Low Input Frequencies

Figure 86. Drive Circuit for High Input Frequencies

The mismatch in the transformer parasitic capacitance (between the windings) results in degraded even-order harmonic performance. Connecting two identical RF transformers back-to-back helps minimize this mismatch and good performance is obtained for high-frequency input signals. An additional termination resistor pair may be required between the two transformers, as shown in Figure 85 and Figure 86. The center point of this termination is connected to ground to improve the balance between the P (positive) and M (negative) sides. The values of the terminations between the transformers and on the secondary side must be chosen to obtain an effective 50 Ω (for a 50-Ω source impedance).